For the Week Two assignment, class members will investigate how adaptations are a product of natural selection. Select one of the options and then follow the instructions below to develop a three-to four-page paper discussing the adaptations and selective pressure of your chosen species or group of species.The paper should not include direct quotes but instead be written in your own words, paraphrasing academic sources as needed to develop content. There are several videos in this week’s required and recommended reading list to help you get started. If you need clarification or additional guidance, feel free to post questions in the Ask Your Instructor Forum or email me.Options: Select one of the following five choices:Cheetahs can run faster than 60 miles per hour when chasing prey. How would a biologist explain how cheetahs evolved to run this fast, assuming their ancestors could only run 20 miles per hour?Bee pollinators are known to prefer yellow and blue flowers with nectar guides and a landing platform. How would a biologist explain the evolution of yellow and blue flowers, given that the first flowering plants where likely white and pink?Polar bears have white fur that blends in well with their arctic environment. The white fur is a type of camouflage that helps bears hunt seals. The polar bear ancestor is thought to have evolved from bears that had brown fur. How would a biologist explain the change from brown fur to white fur in polar bears?Molecular evidence shows that dogs are descended from gray wolves that were domesticated about 130,000 years ago. Suppose you wanted to turn a pack of wolves into a pack of poodles? Describe your strategy and explain why it would work.Plants with the C4 photosynthetic pathway have the ability to use water efficiently in warm, dry climates. C4 plants have evolved from C3 plants and first arose in grasses, probably during the Oligocene epoch (24–35 million yrs. ago). How can a biologist explain how C4 plants evolved?Your paper should include the following points in your response to the above selection.Define natural selection and describe the two general conditions that lead to natural selection. Explain what conditions could have led to natural selection in your species or group of species.Explain the difference between your species genotype and phenotype and the role each has in natural selection.Describe processes other than natural selection that can alter the patterns of genetic variation in the species you selected.Explain how your species adaptations reflect trade-offs and constraints imposed by the physical environment and/or species interactions.Predict how increasing global temperatures will affect the adaptations of your chosen species. If you selected an animal, be specific with regards to body size and thermoregulation. If you selected a group of plant species, be specific to photosynthetic rates as well as water demand and use.The paper should include an introductory paragraph briefly describing your selection followed by a one-to two-sentence overview of the topics that you will discuss.The paper should end with a summary by reviewing the main points discussed and then drawing a conclusion.Use the following terminology in your paper: dominant allele, phenotype, phenotypic plasticity, genetic variation, selective agent, genetic differentiation, genotype, mutations, gene flow, assortative mating, random and non-random mating.The Theory of Evolution by Natural SelectionMust be double-spaced and formatted according to APA style as outlined in the Ashford Writing Center (Links to an external site.)Links to an external site..Must include a separate title page with the following:Title of paperStudent’s nameCourse name and numberInstructor’s nameDate submittedMust reference at least three academic journal articles from the library databases (EBSCOHost, JSTOR, ProQuest) in addition to the course text. You may also use any of the assigned or recommended reading materials from the course.MUST USE THE REQUIRED RESOURCES: THE TEXT AND 3 OTHER RESOURCES. ATTACHED ARE THE RESOURCES THAT MAY BE USED AND THE CHAPTERES NEEDED FOR THE ASSIGNMENT. MUST BE DONE ON TIME SO AS TO HAVE TIME FOR REVISIONS IF NEEDED. MUST DO ALL PARTS OF THE ASSIGNMENT !!!!!! ATTACHED IS THE GRADING RUBICS SO YOU KNOW EXATLY WHAT MUST BE INCLUDED!!! USE IT!!!!
Theory of Evolution by Natural Selection
Theory of Evolution by Natural Selection
GRADING RUBICS Total Possible Score: 15.00 Provides an Introductory Paragraph That Describes the Chosen Species and an Overview of the Topics That Will be Discussed Total: 0.50 Distinguished – Provides a comprehensive introductory paragraph that thoroughly describes the chosen species and a complete overview of the topics that will be discussed. Defines Natural Selection and Describes the Two General Conditions that Lead to Natural Selection Total: 1.50 Distinguished – Accurately defines natural selection and thoroughly describes the two general conditions that lead to natural selection. Explains the Specific Conditions That Could Have Led to Natural Selection in the Chosen Species or Group of Species Total: 2.00 Distinguished – Comprehensively and accurately explains the specific conditions that could have led to natural selection in the chosen species or group of species. Explains the Difference between Species Genotype and Phenotype, and the Role Each Has in Natural Selection Total: 2.75 Distinguished – Comprehensively and accurately explains the difference between species genotype and phenotype, and the role each has in natural selection. Describes Processes Other Than Natural Selection that Can Alter the Patterns of Genetic Variation in the Chosen Species or Group of Species Total: 1.00 Distinguished – Comprehensively and accurately describes processes other than natural selection that can alter the patterns of genetic variation in the chosen species or group of species. Explains how the Chosen Species, or Group of Species, Adaptations Reflect Trade-offs and Constraints Imposed by the Physical Environment, and/or Species Interactions Total: 1.25 Distinguished – Comprehensively and accurately explains how the chosen species, or group of species, adaptations reflect trade-offs and constraints imposed by the physical environment, and/or species interactions. Predicts How Increasing Global Temperatures Will Affect the Adaptations of the Chosen Species Total: 1.50 Distinguished – Accurately predicts how increasing global temperatures will affect the adaptations of the chosen species. Provides a Summary Paragraph That Reviews the Main Points Discussed in the Paper and Then Draws a Conclusion Total: 0.50 Distinguished – Provides a comprehensive summary paragraph that reviews all of the main points in the paper and then draws a logical conclusion. Written Communication: Control of Syntax and Mechanics Total: 1.00 Distinguished – Displays meticulous comprehension and organization of syntax and mechanics, such as spelling and grammar. Written work contains no errors and is very easy to understand. Written Communication: APA Formatting Total: 0.50 Distinguished – Accurately uses APA formatting consistently throughout the paper, title page, and reference page. Written Communication: Page Requirement Total: 0.50 Distinguished – The length of the paper is equivalent to the required number of correctly formatted pages. Written Communication: Resource Requirement Total: 2.00 Distinguished – Uses more than the required number of scholarly sources, providing compelling evidence to support ideas. All sources on the reference page are used and cited correctly within the body of the assignment.
Theory of Evolution by Natural Selection
CHAPTER 5 Smith, T. M., & Smith, R. L. (2015). Elements of Ecology (9th ed.). Boston, MA: Pearson. 5.1 Adaptations Are a Product of Natural Selection Stated more precisely, natural selection is the differential success (survival and reproduction) of individuals within the population that results from their interaction with their environment. As outlined by Darwin, natural selection is a product of two conditions: (1) that variation occurs among individuals within a population in some “heritable” characteristic, and (2) that this variation results in differences among individuals in their survival and reproduction as a result of their interaction with the environment. Natural selection is a numbers game. Darwin wrote: Among those individuals that do reproduce, some will leave more offspring than others. These individuals are considered more fit than the others because they contribute the most to the next generation. Organisms that leave few or no offspring contribute little or nothing to the succeeding generations and so are considered less fit. The fitness of an individual is measured by the proportionate contribution it makes to future generations. Under a given set of environmental conditions, individuals having certain characteristics that enable them to survive and reproduce are selected for, eventually passing those characteristics on to the next generation. Individuals without those traits are selected against, failing to pass their characteristics on to future generations. In this way, the process of natural selection results in changes in the properties of populations of organisms over the course of generations, by a process known as evolution. An adaptation is any heritable behavioral, morphological, or physiological trait of an organism that has evolved over a period of time by the process of natural selection such that it maintains or increases the fitness (long-term reproductive success) of an organism under a given set of environmental conditions. The concept of adaptation by natural selection is central to the science of ecology. The study of the relationship between organisms and their environment is the study of adaptations. Adaptations represent the characteristics (traits) that enable an organism to survive, grow, and reproduce under the prevailing environmental conditions. Adaptations likewise govern the interaction of the organism with other organisms, both of the same and different species. How adaptations enable an organism to function in the prevailing environment—and conversely, how those same adaptations limit its ability to successfully function in other environments—is the key to understanding the distribution and abundance of species, the ultimate objective of the science of ecology. 5.2 Genes Are the Units of Inheritance By definition, adaptations are traits that are inherited—passed from parent to offspring. So to understand the evolution of adaptations, we must first understand the basis of inheritance: how characteristics are passed from parent to offspring and what forces bring about changes in those same characteristics through time (from generation to generation). At the root of all similarities and differences among organisms is the information contained within the molecules of DNA (deoxyribonucleic acid). You will recall from basic biology that DNA is organized into discrete subunits—genes—that form the informational units of the DNA molecule. A gene is a stretch of DNA coding for a functional product (ribonucleic acid: RNA). The product is usually messenger RNA (mRNA) and mRNA ultimately results in the synthesis of a protein. The alternate forms of a gene are called alleles (derived from the term allelomorphs, which in Greek means “different form”). The process is called gene expression in which DNA is used in the synthesis of products such as proteins. All of the DNA in a cell is collectively called the genome. Genes are arranged in linear order along microscopic, threadlike bodies called chromosomes. The position occupied by a gene on the chromosome is called the locus (Latin for place). In most multicellular organisms, each individual cell contains two copies of each type of chromosome (termed homologous chromosomes). In the process of asexual reproduction, both chromosomes are inherited from the single parent. In sexual reproduction, one is inherited from its mother through the ovum and one inherited from its father through the sperm. At any locus, therefore, every diploid individual contains two copies of the gene—one at each corresponding position in the homologous chromosomes. These two copies are the alleles of the gene in that individual. If the two copies of the gene are the same, then the individual is homozygous at that given locus. If the two alleles at the locus are different, then the individual is heterozygous at the locus. The pair of alleles present at a given locus defines the genotype of an individual; therefore, homozygous and heterozygous are the two main categories of genotypes. 5.3 The Phenotype Is the Physical Expression of the Genotype The outward appearance of an organism for a given characteristic is its phenotype. The phenotype is the external, observable expression of the genotype. When an individual is heterozygous, the two different alleles may produce an individual with intermediate characteristics or one allele may mask the expression of the other ( Figure 5.2). In the case in which one allele masks the expression of the other, the allele that is expressed is referred to as the dominant allele, whereas the allele that is masked is called the recessive allele. If the allele is recessive, it will only be expressed if the individual is homozygous for that allele (homozygous recessive). If the physical expression of the heterozygous individual is intermediate between those of the homozygotes, the alleles are said to be incomplete dominance, and each allele has a specific value (proportional effect) that it contributes to the phenotype. Phenotypic characteristics that fall into a limited number of discrete categories, such as the example of flower color presented in Figure 5.2, are referred to as qualitative traits. Even though all genetic variation is discrete (in the form of alleles), most phenotypic traits have a continuous distribution. These traits, such as height or weight, are referred to as quantitative traits. The continuous distribution of most phenotypic traits occurs for two reasons. First, most traits have more than one gene locus affecting them. For example, if the phenotypic characteristic of flower color illustrated in Figure 5.2 is controlled by two loci rather than a single locus (each with two alleles—A:a and B:b), there are nine possible genotypes ( Figure 5.3). In contrast to the three distinct flower colors (phenotypes) produced in the case of a single locus, there is now a range of flower colors varying in hue between dark red and white depending on the number of alleles coding for the production of red pigment (see Figure 5.2). The greater the number of loci, the greater is the range of possible phenotypes. The second factor influencing phenotypic variation is the environment. 5.4 The Expression of Most Phenotypic Traits Is Affected by the Environment The expression of most phenotypic traits is influenced by the environment; that is to say, the phenotypic expression of the genotype is influenced by the environment. Because environmental factors themselves usually vary continuously—temperature, rainfall, sunlight, level of predation, and so on—the environment can cause the phenotype produced by a given genotype to vary continuously. To illustrate this point, we can use the example of flower color controlled by two loci presented previously (and in Figure 5.3). Pigment production during flower development can be affected by temperature. If temperatures below some optimal value or range function to reduce the expression of the A and B alleles in the production of red pigment, fluctuations in temperatures over the period of flower development in the population of plants will function to further increase the range of flower colors (shades between red and white) produced by the nine genotypes. Interpreting Ecological Data Q1. Which of the two genotypes (G1 or G2) exhibits the greater norm of reaction? Q2. What would the line look like for a genotype that did not exhibit phenotypic plasticity? Q3. Is there any environment in which the two genotypes will express the same phenotype? Q4. Is it possible for the two genotypes to exhibit the same phenotype? The ability of a genotype to give rise to different phenotypic expressions under different environmental conditions is termed phenotypic plasticity. The set of phenotypes expressed by a single genotype across a range of environmental conditions is referred to as the norm of reaction ( Figure 5.4). Note that we are not talking about different genotypes adapted to different environmental conditions, but about a single genotype (set of alleles) capable of altering the development or expression of a phenotypic trait in response to the conditions encountered by the individual organism. The result is the improvement of the individual’s ability to survive, grow, and reproduce under the prevailing environmental conditions (i.e., increase fitness). For example, the bodies of many species of insects change in color in response to the prevailing temperature during development ( Figure 5.5). Development under colder temperatures typically results in darker coloration. Darker coloration most likely facilitates increased absorption of solar radiation, allowing them to compensate for the lower temperature (see Chapter 7 for discussion of thermoregulation in animals). Some of the best examples of phenotypic plasticity occur among plants. The size of the plant, the ratio of reproductive tissue to vegetative tissue, and even the shape of the leaves may vary widely at different levels of nutrition, light, moisture, and temperature. An excellent illustration of phenotypic plasticity in plants is the work of Sonia Sultan of Wesleyan University. Sultan’s research focuses on phenotypic plasticity in plant species in response to resource availability. In a series of greenhouse experiments, she examined the developmental response of the herbaceous annual Polygonum lapathifolium (common name curlytop knotweed) to different light environments. Sultan grew different individuals of the same genotype for eight weeks at two light levels: low light (20 percent available photosynthetically active radiation [PAR]) and high light (100 percent available PAR). Individuals of the same genotype grown under low-light conditions produced less biomass (slower growth rate), but produced far more photosynthetic leaf area per unit of biomass through changes in biomass allocation, morphology, and structure ( Figure 5.6). Individuals grown under low-light conditions produced large, thin leaves and few branches. In contrast, the larger high-light plants grew narrow leaves on many more branches. This response is referred to as developmental plasticity. As such, these changes are irreversible. After the adult plant develops, these patterns of biomass allocation (proportions of leaf, stem, and root) will remain largely unchanged, regardless of any changes in the light environment. In contrast to developmental plasticity, other forms of phenotypic plasticity in response to prevailing environmental conditions are reversible. For example, fish have an upper and lower limit of tolerance to temperature (see Chapter 7). They cannot survive at water temperatures above and below these limits. However, these upper and lower limits change seasonally as water temperatures warm and cool. This pattern of seasonal change in temperature tolerance is illustrated in the work of Nann Fangu and Wayne Bennett of the University of West Florida. Fangu and Bennett measured seasonal changes in the temperature tolerances of Atlantic stingrays (Dasyatis sabina) that inhabit shallow bays of the Florida coast. Their data for individuals inhabiting St. Josephs Bay on the Gulf Coast of Florida show a systematic shift in the critical minimum and maximum temperatures with seasonal changes in the ambient environmental (water) temperature ( Figure 5.7). As water temperatures change seasonally, shifts in enzyme and membrane structure allow the individual’s physiology to adjust slowly over a period of time, influencing heart rate, metabolic rate, neural activity, and enzyme reaction rates. These reversible phenotypic changes in an individual organism in response to changing environmental conditions are referred to as acclimation. Acclimation is a common response in both plant and animal species involving adjustments relating to biochemical, physiological, morphological, and behavioral traits. 5.5 Genetic Variation Occurs at the Level of the Population Adaptations are the characteristics of individual organisms—a reflection of the interaction of the genes and the environment. They are the product of natural selection. Although the process of natural selection is driven by the success or failure of individuals, the population—the collective of individuals and their alleles—changes through time, as individuals either succeed or fail to pass their genes to successive generations. For this reason, to understand the process of adaptation through natural selection, we must first understand how genetic variation is organized within the population. A species is rarely represented by a single, continuous interbreeding population. Instead, the population of a species is typically composed of a group of subpopulations—local populations of interbreeding individuals, linked to each other in varying degrees by the movement of individuals (see Sections 8.2 and 19.7 for discussion of metapopulations). Thus, genetic variation can occur at two hierarchical levels, within subpopulations and among subpopulations. When genetic variation occurs among subpopulations of the same species, it is called genetic differentiation. Interpreting Ecological Data Q1. What type of data do the original measures of beak depth represent? (See Chapter 1, Quantifying Ecology 1.1.) Q2. How have the original measurements of beak depth been transformed for presentation purposes in Figure 5.8? Q3. What is the range (maximum – minimum values) of beak depths observed for the sample of individuals presented in Figure 5.8? (Categories are in units of 0.2 mm.) The sum of genetic information (alleles) across all individuals in the population is referred to as the gene pool. The gene pool represents the total genetic variation within a population. Genetic variation within a population can be quantified in several ways. The most fundamental measures are allele frequency and genotype frequency . The word frequency in this context refers to the proportion of a given allele or genotype among all the alleles or genotypes present at the locus in the population. 5.6 Adaptation Is a Product of Evolution by Natural Selection We have defined evolution as changes in the properties of populations of organisms over the course of generations (Section 5.1). More specifically, phenotypic evolution can be defined as a change in the mean or variance of a phenotypic trait across generations as a result of changes in allele frequencies. In favoring one phenotype over another, the process of natural selection acts directly on the phenotype. But in doing so, natural selection changes allele frequencies within the population. Changes in allele frequencies from parental to offspring generations are a product of differences in relative fitness (survival and reproduction) of individuals in the parental generation. The work of Peter Grant and Rosemary Grant provides an excellent documented example of natural selection. The Grants have spent more than three decades studying the birds of the Galápagos Islands, the same islands whose diverse array of animals so influenced the young Darwin when he was a naturalist aboard the expeditionary ship HMS Beagle. Among other events, the Grants’ research documented a dramatic shift in a physical characteristic of finches inhabiting some of these islands during a period of extreme climate change. Recall from our initial discussion in Section 5.1 that natural selection is a product of two conditions: (1) that variation occurs among individuals within a population in some heritable characteristic and (2) that this variation results in differences among individuals in their survival and reproduction. Figure 5.8 shows variation in beak size in Darwin’s medium ground finch (Geospiza fortis) on the 40-hectare islet of Daphne Major, one of the Galápagos Islands off the coast of Ecuador. Heritability of beak size in this species was established by examining the relationship between the beak size of parents and their offspring ( Figure 5.9). (After Boag and Grant 1984.) During the early 1970s, the island received an average rainfall of between 127 and 137 millimeters (mm) per year, supporting an abundance of seeds and a large finch population (1500 birds). In 1977, however, a periodic shift in the climate of the eastern Pacific Ocean—called La Niña—altered weather patterns over the Galápagos, causing a severe drought (see Chapter 2, Section 2.9). That season, only 24 mm of rain fell. During the drought, seed production declined drastically. Small seeds declined in abundance faster than large seeds did, increasing the average size and hardness of seeds available ( Figure 5.11). The decline in food (seed) resources resulted in an 85 percent decline in the finch population as a result of mortality and possible emigration ( Figure 5.12a). Mortality, however, was not equally distributed across the population (Figure 5.12b). Small birds had difficulty finding food, whereas large birds, especially males with large beaks, had the highest rate of survival because they were able to crack large, hard seeds. The graph in Figure 5.12b represents a direct measure of the differences in fitness (as measured by survival) among individuals in the population as a function of differences in phenotypic characteristics (beak size), the second condition for natural selection. The phenotypic trait that selection acts directly upon is referred to as the target of selection; in this example, it is beak size. The selective agent is the environmental cause of fitness differences among organisms with different phenotypes, or in this case, the change in food resources (abundance and size distribution of seeds). The increased survival rate of individuals with larger beaks resulted in a shift in the distribution of beak size (phenotypes) in the population (Figure 5.13). This type of natural selection, in which the mean value of the trait is shifted toward one extreme over another (Figure 5.14a), is called directional selection. In other cases, natural selection may favor individuals near the population mean at the expense of the two extremes; this is referred to as stabilizing selection (Figure 5.14b). When natural selection favors both extremes simultaneously, although not necessarily to the same degree, it can result in a bimodal distribution of the characteristic(s) in the population (Figure 5.14c). Such selection, known as disruptive selection, occurs when members of a population are subject to different selection pressures. Interpreting Ecological Data Q1. Figure 5.12b shows the survival of ground finches as a function of beak size during the period of drought. How does the graph in Figure 5.12b relate to this figure? Q2. How do the patterns of relative fitness shown in the graphs on the left-hand column give rise to the corresponding patterns of selection illustrated by the arrows in the graphs shown in the right-hand column? The work of Beren Robinson of Guelph University in Canada provides an excellent example of disruptive selection. In studying the species of threespine stickleback (Gasterosteus aculeatus), which occupies Cranby Lake in the coastal region of British Columbia, Robinson found that individuals sampled from the open-water habitat (limnetic habitat) differed morphologically from individuals sampled from the shallower nearshore waters (benthic habitat). In a series of experiments, Robinson established that these individuals represented distinct phenotypes that are products of natural selection promoting divergence within the population. He initially established that morphological differences between the two forms were heritable, rather than an expression of phenotypic plasticity in response to the two different habitats or diets. He reared offspring of the two forms under identical laboratory conditions (environmental conditions and diet) and although there was some degree of phenotypic plasticity, differences in most characteristics remained between the two forms. On average, the benthic form (BF) had (1) shorter overall body length, (2) deeper body, (3) wider mouth, (4) more dorsal spines, and (5) fewer gill rakers than did the limnetic form (LF) ( Figure 5.15a). The two habitats in the lake—benthic and limnetic—provide different food resources; so to determine the agent of selection that caused divergence within the population, Robinson conducted feeding trials in the laboratory to test for trade-offs in the foraging efficiency of the two forms on food resources found in the two habitats. The foraging success of individual fish was assessed in two artificial habitats, mimicking conditions in the limnetic and benthic environments. Two food types were used in the trials. Brine shrimp larvae (Artemia), a common prey found in open water, were placed in the artificial limnetic habitats. Larger amphipods, fast-moving arthropods with hard exoskeletons that forage on dead organic matter on the sediment surface, were placed in the artificial benthic habitats. Results of the foraging trials revealed distinct differences in the foraging success of the two morphological forms (phenotypes; Figure 5.15b). The LF individuals were most successful at foraging on the brine shrimp larvae. They had a higher consumption rate and required only half the number of bites to consume as compared to the BF individuals. In contrast, BF individuals had a higher intake rate for amphipods and on average consumed larger amphipods than did LF individuals. Robinson was able to determine that the higher intake rate of brine shrimp larvae by LF individuals was related to this form’s greater number of gill rakers, and greater mouth width was related to the higher intake rate of amphipods by BF individuals. Therefore, he found that foraging efficiency was related to morphological differences between the two forms, suggesting that divergent selection in the two distinct phenotypes represents a trade-off in characteristics related to the successful exploitation of these two distinct habitats and associated food resources. 5.7 Several Processes Other than Natural Selection Can Function to Alter Patterns of Genetic Variation within Populations Natural selection is the only process that leads to adaptation because it is the only one in which the changes in allele frequency from one generation to the next are a product of differences in the relative fitness (survival and reproduction) of individuals in the population. Yet not all phenotypic characteristics represent adaptations, and processes other than natural selection can be important factors influencing changes in genetic variation (allele and genotype frequencies) within populations. For example, mutation is the ultimate source of the genetic variation that natural selection acts upon. Mutations are heritable changes in a gene or a chromosome. The word mutation refers to the process of altering a gene or chromosome as well as to the product, the altered state of the gene or chromosome. Mutation is a random force in evolution that produces genetic variation. Any altered phenotypic characteristic resulting from mutation may be beneficial, neutral, or harmful. Whether a mutation is beneficial depends on the environment. A mutation that enhances an organism’s fitness in one environment could harm it in another. Most of the mutations that have significant effect, however, are harmful, but the harmful mutations do not survive long. Natural selection eliminates most deleterious genes from the gene pool, leaving behind only genes that enhance (or at least do not harm) an organism’s ability to survive, grow, and reproduce in its environment. Another factor that can directly influence patterns of genetic variation within a population is a change in allele frequencies as a result of random chance—a process known as genetic drift. Recall from basic biology that the recombination of alleles in sexual reproduction is a random process. The offspring produced in sexual reproduction, however, represent only a subset of the parents’ alleles. If the parents have only a small number of offspring, then not all of the parents’ alleles will be passed on to their progeny as a result of the random assortment of chromosomes at meiosis (the process of recombination). In effect, genetic drift is the evolutionary equivalent of sampling error, with each successive generation representing only a subset or sample of the gene pool from the previous generation. In a large population, genetic drift will not affect each generation much because the effects of the random nature of the process will tend to average out. But in a small population, the effect could be rapid and significant. To illustrate this point, we can use the analogy of tossing a coin. With a single toss of the coin, the probability of each of the two possible outcomes, heads or tails, is equal, or 50 percent. Likewise, with a series of four coin tosses, the probability of the outcome being two heads and two tails is 50 percent. But each individual outcome in the coin tosses is independent; therefore, in a series of four coin tosses, there is also a probability of 0.0625, or 6.25 percent, that the outcome will be four heads. The probability of the outcome being all heads drops to 9.765 × 10−4 if the number of tosses is increased to 10, and this probability drops to 8.88 × 10−16 for 50 tosses. Likewise, the probability of heterozygous (Aa) individuals in the population producing only homozygous (either aa or AA) offspring under a system of random mating decreases with increasing population size. Patterns of genetic variation within a population can also be influenced by the movement of individuals into, or out of, the population. Recall from the discussion of genetic variation in Section 5.5 that the population of a species is typically composed of a group of subpopulations—local populations of interbreeding individuals that are linked to one another in varying degrees by the movement of individuals (see Chapter 8). Migration is defined as the movement of individuals between local populations, whereas gene flow is the movement of genes between populations (see Chapter 8). Because individuals carry genes, the terms are often used synonymously; however, if an individual immigrates into a population but does not successfully reproduce, the new genes are not established in the population. Migration is a potent force in reducing the level of population differentiation (genetic differences among local populations; see Section 5.5). One of the most important principles of genetics is that under conditions of random mating, and in the absence of the factors discussed thus far—natural selection, mutation, genetic drift, and migration—the frequency of alleles and genotypes in a population remains constant from generation to generation. In other words, no evolutionary change occurs through the process of sexual reproduction itself. This principle, referred to as the Hardy–Weinberg principle, is named for Godfrey Hardy and Wilhelm Weinberg, who each independently published the model in 1908 (see Quantifying Ecology 5.1). Mating is random when the chance that an individual mates with another individual of a given genotype is equal to the frequency of that genotype in the population. When individuals choose mates nonrandomly with respect to their genotype—or more specifically, select mates based on some phenotypic trait—the behavior is referred to as assortative mating. Perhaps the most recognized and studied form of assortative mating is female mate choice. Female mate choice is the behavior in which females exhibit a bias toward certain males as mates based on specific phenotypic traits (often secondary sex characteristics), such as body size or coloration (see Chapter 10, Section 10.11). Positive assortative mating occurs when mates are phenotypically more similar to each other than expected by chance. Positive assortative mating is common, and one of the most widely reported examples relates to the timing of reproduction. Plants mate assortatively based on flowering time. In populations of plants with an extended flowering time, early flowering plants are often no longer flowering when late flowering plants are in bloom. The genetic effect of positive assortative mating is an increase in the frequency of homozygotes with a decrease in the frequency of heterozygotes in the population. Think of a locus where AA individuals tend to be larger than Aa, which in turn are larger than aa individuals. With positive assortative mating, AA will mate with AA, and aa with other aa. All of these matings will produce only homozygous offspring. Even mating between Aa individuals will result in half of the offspring being homozygous. The genetic effects of positive assortative mating are only at the loci that affect the phenotypic characteristic by which the organisms are selecting mates. Negative assortative mating occurs when mates are phenotypically less similar to each other than expected by chance. Though not as common as positive assortative mating, negative assortative mating results in an increase in the frequency of heterozygotes. A special case of nonrandom mating is inbreeding. Inbreeding is the mating of individuals in the population that are more closely related than expected by random chance. Unlike positive assortative mating, inbreeding increases homozygosity at all loci. Inbreeding affects all loci equally because related individuals are genetically similar by common ancestry, and they are therefore more likely to share alleles throughout the genome than unrelated individuals. Inbreeding can be detrimental. Offspring are more likely to inherit rare, recessive, deleterious genes. These genes can cause decreased fertility, loss of vigor, reduced fitness, reduced pollen and seed fertility in plants, and even death. These consequences are referred to as inbreeding depression. As we have seen from the preceding discussion, nonrandom mating changes genotypic frequencies from one generation to the next, but assortative mating does not directly result in a change of allele frequencies within a population. The other three processes discussed—mutation, migration, and genetic drift, together with natural selection—alter the allele frequencies, and therefore result in a shift in the distribution of genotypes (and potentially phenotypes) within the population. As such, all four processes function as agents of evolution. However, natural selection is special among the four evolutionary processes because it is the only one that leads to adaptation. The other three can only speed up or slow the development of adaptations. Quantifying Ecology 5.1 Hardy–Weinberg Principle The Hardy–Weinberg principle states that both allele and genotype frequencies will remain the same in successive generations of a sexually reproducing population if certain criteria are met: (1) mating is random, (2) mutations do not occur, (3) the population is large, so that genetic drift is not a significant factor, (4) there is no migration, and (5) natural selection does not occur. If we have only two alleles at a locus, designated as A and a, then the usual symbols for designating their frequencies are p and q, respectively. Because frequencies (proportions) must sum to 1, then: p+q=1orq=1−pp+q=1 or q=1 −p Genotypic frequencies are typically designated by uppercase letters. In the case of a locus with two alleles, P is the frequency of AA, H is the frequency of Aa, and Q is the frequency of aa. As with gene frequencies, genotype frequencies must sum to 1: P+H+Q=1P+H+Q=1 Given a population having the genotypic frequencies of P=0.64,H=0.32,and Q =0.04P=0.64,H=0.32, and Q =0.04 we can calculate the allele frequencies as follows: P=P+H/2=0.64+(0.32/2)=0.8P=P+H/2=0.64+(0.32/2)=0.8 q=Q+H/2=0.04+(0.32/2)=0.2q=Q+H/2=0.04+(0.32/2)=0.2 With a population consisting of the three genotypes just described (AA, Aa, and aa), there are six possible types of mating ( Table 1 ). For example, the mating AA × AA occurs only when an AA female mates with an AA male, with the frequency of occurrence being P × P (or P 2) under the conditions of random mating. Similarly, an AA × Aa mating occurs when an AA female mates with an Aa male (proportion P × H) or when an Aa female mates with an AA male (proportion H × P). Therefore, the overall proportion of AA × Aa matings is PH + HP = 2PH. The frequencies of these and the other four types of matings are given in the second column of Table 1. To calculate the offspring genotypes produced by these matings, we must first examine the offspring produced by each of the six possible pairings of parental genotypes (Table 1). Because homozygous AA genotypes produce only A-bearing gametes (egg or sperm), and homozygous aa genotypes produce only a-bearing gametes, the mating of AA × AA individuals will produce only AA offspring, and likewise, the mating of aa × aa individuals will produce only offspring with genotype aa. In addition, the mating of AA × aa individuals will produce only heterozygous offspring (Aa). In contrast to homozygous individuals, heterozygous individuals produce both A- and a-bearing gametes. Therefore, the mating of a heterozygous individual with a homozygous (either AA or aa) or another heterozygous individual will produce offspring of all three possible genotypes (AA, Aa, and aa). The relative frequencies of offspring genotypes depend on the specific combination of parents (Figure 1). The offspring frequencies presented in Figure 1 are based on the assumption that an Aa heterozygote individual produces an equal number of A- and a-bearing gametes (referred to as Mendelian segregation). Using the data presented in Figure 1 and the frequencies of the different types of matings in column 2 of Table 1, the genotype frequencies of the offspring, denoted as P’ (AA), H’ (Aa), and Q’ (aa), are presented in column 3 of Table 1. The new genotype frequencies are calculated as the sum of the products shown at the bottom of Table 1. For each genotype, the frequency of each mating producing the genotype is multiplied by the fraction of the genotypes produced by that mating. We can now calculate the allele frequencies (p’) for the generation of offspring (designated as q’) using the formula presented previously: p′=p′+H′/2=0.64+(0.32/2)=0.8p’=p’+H’/2=0.64+(0.32/2)=0.8 q′+Q′+H′/2=0.04+(0.32/2)=0.2q’+Q’+H’/2=0.04+(0.32/2)=0.2 Note that both the genotype and allele frequencies of the offspring generation are the same as those of the parental generation. of these matings are shown. In natural populations the assumptions of the Hardy–Weinberg principle are never fully met. Mating is not random, mutations do occur, individuals move between local populations, and natural selection does occur. All of these circumstances change the frequencies of genotypes and alleles from generation to generation, acting as evolutionary forces in a population. The beauty of the Hardy–Weinberg principle is that it functions as a null model, where deviations from the expected frequencies can provide insight into the evolutionary forces at work within a population. How would the frequency of heterozygotes in the population change if the frequency for the A allele in the example described was p = 0.5? When the frequency of an allele is greater than 0.8, most of these alleles are contained in homozygous individuals (as illustrated in the preceding example). When the frequency of an allele is less than 0.1, in which genotype are most of these alleles? 5.8 Natural Selection Can Result in Genetic Differentiation The example of natural selection in the population of medium ground finches as described previously represents a shift in the distribution of phenotypes in the population inhabiting the island of Daphne Major in response to environmental changes that occurred over time (period of drought). This shift in the mean phenotype (beak size) reflects a change in genetic variation (allele and genotype frequencies) within the population. Natural selection can also function to alter genetic variation among local populations as a result of local differences in environmental conditions—the process of genetic differentiation (see Section 5.5). Species having a wide geographic distribution often encounter a broader range of environmental conditions than do those species whose distribution is more restricted. The variation in environmental conditions can give rise to a corresponding variation in morphological, physiological, and behavioral characteristics (phenotypes). Significant differences often exist among local populations of a single species inhabiting different regions. The greater the distance between populations, the more pronounced the differences often become as each population adapts to the locality it inhabits. The changes in phenotype across the landscape therefore reflect the changing nature of natural selection operating under the different local environmental conditions. Geographic variation within a species in response to changes in environmental conditions can result in the evolution of clines, ecotypes, and geographic isolates or subspecies. A cline is a measurable, gradual change over a geographic region in the average of some phenotypic character, such as size and coloration. Clines are usually associated with an environmental gradient that varies in a continuous manner across the landscape, such as changes in temperature or moisture with elevation or latitude. Continuous variation in the phenotypic character across the species distribution results from gene flow from one population to another along the gradient. Because environmental constraints influencing natural selection vary along the gradient, any one population along the gradient will differ genetically to some degree from another—the difference increasing with the distance between the populations. Clinal differences exist in size, body proportions, coloration, and physiological adaptations among animals. For example, the fence lizard (Sceloporus undulatus) is one of the most widely distributed species of lizards in North America, ranging throughout the eastern two-thirds of the United States and into northern Mexico. Across its range, the fence lizard exhibits a distinct gradient of increasing body size with latitude ( Figure 5.16). Lizards from northern latitudes are larger than lizards from southern latitudes. Furthermore, lizards from higher elevations in geographically proximal areas exhibit larger body size than lizards from lower elevations. Thus, mean body size increases along an environmental gradient of decreasing mean annual temperature. Similar clines are observed in plant species. Alicia Montesinos-Navarro of the University of Pittsburgh and colleagues examined phenotypic variation in Arabidopsis thaliana, a small annual flowering plant species that is native to Europe, Asia, and northwestern Africa. The researchers examined 17 natural populations that occupy an altitudinal gradient in the region of northeastern Spain. Along the gradient, precipitation increases, but maximum spring temperature and minimum winter temperature decrease with altitude. Examination of the local populations revealed a systematic variation in a variety of phenotypic characteristics. Aboveground mass, number of rosette leaves at bolting (a measure of size at reproduction), developmental time, and number of seeds and seed weight increased with altitude (Figure 5.17). Although these changes in phenotypic characteristics are clearly in response to the gradient of environmental conditions with altitude, how can the researchers be sure that the changes in phenotype represent changes in allele and genotype frequencies between populations rather than phenotypic plasticity? Recall from Section 5.4 that phenotypic plasticity is the ability of a single genotype to produce different phenotypes under different environmental conditions (norms of reaction; see Figure 5.4). A common approach used to determine if observed phenotypic differences between local populations represent differences in allele frequencies (genetic differentiation) or phenotypic plasticity is the common garden experiment. In this experiment individuals (genotypes) from the different populations are grown under controlled environmental conditions—a common garden. If the phenotypic differences observed in the local populations are maintained in individuals grown in the common garden, the differences in phenotype represent genetic differences between the populations (genetic differentiation). If the individuals from the different populations no longer exhibit differences in phenotypic characteristics, then the differences observed in the local populations in their natural environments are a function of phenotypic plasticity. When Montesinos-Navarro and her colleagues grew genotypes from the 17 local populations under uniform controlled conditions (the common garden experiment), the phenotypic differences were maintained, revealing that the A. thaliana cline represents adaptations to local environmental conditions along the altitudinal gradient. Clinal variation may show marked discontinuities. Such abrupt changes, or step clines, often reflect abrupt changes in local environments. Such variants are called ecotypes. An ecotype is a population adapted to its unique local environmental conditions (see this chapter, Field Studies: Hopi Hoekstra). For example, a population inhabiting a mountaintop may differ from a population of the same species in the valley below. This is the case with the weedy herbaceous annual Diodia teres (with the common name poorjoe) that occurs in a wide variety of habitats in eastern North America (Figure 5.18). In the southeastern United States, there are two distinct ecotypes: one occurs in inland agricultural fields and the other in coastal sand-dune habitats. The populations differ strikingly in morphology. Among many differences, the coastal population has heavier stem pubescence (covered with short, soft, erect hairs), a more flattened growth habit, and larger seed size relative to the inland population. To understand the patterns of local adaptation acting on these two distinct ecotypes, Nicholas Jordan of the University of Minnesota undertook a series reciprocal transplant studies. In these studies, seeds from the two ecotypes were planted in each of the two distinct habitats (inland and coastal). By comparing patterns of survival, growth, and reproduction of the two ecotypes in the two different habitats, Jordan was able to analyze selection for and against native and introduced individuals. Results of the study reveal two important facts regarding the two ecotypes. First, phenotypic differences between the two ecotypes were maintained in both environments indicating that the ecotypes represent genetic differences between the two populations rather than phenotypic plasticity. Second, in each of the two habitats, the native ecotype performed better than the introduced ecotype in comparisons of survival, growth, and seed production. Each of the two ecotypes exhibited a greater relative fitness in its native habitat indicating that the phenotypic differences represent adaptations to the two different local environments (see Figure 5.18). Although ecotypes typically represent distinct genetic populations (with respect to the phenotypic characteristics that relate to the local adaptations), gene flow occurs to varying degrees between adjacent populations, and often, zones of hybridization (mating between ecotypes) can be found. In some cases, however, geographic features such as rivers or mountain ranges that impede the movement of individuals (or gametes) can restrict gene flow between adjacent populations. For example, the southern Appalachian Mountains are noted for their diversity of salamanders. This diversity is fostered in part by a rugged terrain, an array of environmental conditions, and the limited ability of salamanders to disperse ( Figure 5.19). Populations become isolated from one another, preventing a free flow of genes. One species of salamander, Plethodon jordani, formed a number of semi-isolated populations, each characteristic of a particular part of the mountains. These subpopulations make up geographic isolates, in which some extrinsic barrier—in the case of the salamanders, rivers and mountain ridges—prevents the free flow of genes among subpopulations. The degree of isolation depends on the efficiency of the extrinsic barrier, but rarely is the isolation complete. These geographic isolates are often classified as subspecies, a taxonomic term for populations of a species that are distinguishable by one or more characteristics. Unlike clines, for subspecies we can draw a geographic line separating the subpopulations into subspecies. Nevertheless, it is often difficult to draw the line between species and subspecies. Field Studies Hopi Hoekstra Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts A key focus of evolutionary ecology is on identifying traits (phenotypic characteristics) that are ecologically important and determining how those traits affect the relative fitness of individuals in different environments. Color is one phenotypic characteristic that has been shown to have a major influence on the way in which organisms interact with their environment. Color plays a central role in a wide variety of ecological processes relating to survival and reproduction and can therefore directly affect an individual’s fitness. One of the more widely studied adaptive roles of color is cryptic coloration, that is, coloring that allows an animal to blend into the surrounding environment and therefore avoid detection by potential predators (see Chapter 14, Section 14.10). Geographic variation in habitat, such as the background color of surface substrate (soils, rocks, or snow cover), vegetation, or water, can present different adaptive environments and selective pressures resulting in localized differences in patterns of body color. In mammals, some of the most extreme variations in coat color occur in deer mice (genus Peromyscus). These mice occur throughout most of continental North America. One species of deer mouse that exhibits a large degree of variation in coat color over short geographic distances is Peromyscus polionotus. This species of Peromyscus occurs throughout the southeastern United States where it is commonly referred to as “oldfield mouse” because it inhabits abandoned agricultural fields. Understanding the evolution of variations in coat color within this species has been a focus of studies by the evolutionary ecologists Hopi Hoekstra and her students at Harvard University. In the southeastern United States, oldfield mice (P. polionotus) typically occupy overgrown fields with dark soil and have a dark brown coat, which serves to camouflage the mice from predators (Figure 1, left). In the last few thousand years, however, these mice have colonized the sand dunes of Florida’s coasts. Here the sands are lighter in color (white sands) than the inland soil, and there is much less vegetation cover. These beach-dwelling mice, known as “beach mice,” have reduced pigmentation and have a much lighter color (compared to the inland populations) that blends well into the light-colored sand (Figure 1, right). Using a combination of field studies, classic genetics, and modern molecular biology, Hoekstra and her students are working to understand how, through changes in pigmentation genes, these mice have adapted to this new environment. Although it may seem straightforward that being light colored would be a selective advantage over darker pigmentation on the white sand dunes of coastal Florida, evidence is needed to establish that differences in pigmentation among local populations represent adaptations. To accomplish this task, Hoekstra and colleague Sacha Vignieri undertook a series of field experiments to quantify the selective advantage of having pigmentation patterns that better match the surrounding environment. The experimental approach involved placing darker-colored mice from an inland, oldfield population (Lafayette Creek Wildlife Area, Florida) into the coastal dune environment (Topsail Hill State Park, Florida) and lighter-colored individuals from the dune environment into the inland environment. Rates of survival for the transplanted individuals could then be compared with those of local populations whose coat color better matched the surrounding environment. Using live mice for such an experiment, however, can present a number of major problems, including capturing hundreds of mice, transporting and releasing them into new locations, and then the difficulty of determining the fate of the mice over the experimental period. To avoid these problems, the researchers used a unique approach of creating model mice using nonhardening plasticine (a type of modeling clay). Although simple, this method has several advantages over using live mice. First, because plasticine preserves evidence of predation attempts (tooth, beak, or claw imprints), it is possible to quantify both predation rate and predator type. Additionally, using models, they were able to deploy a large number of individuals within a given environment. Finally, this experimental approach allowed them to focus on variation in a single trait of interest—coat color—controlling for other traits such as behavioral differences. Some 250 models of P. polionotus were made, half of which were painted to mimic the coat color and pattern of the darker oldfield mouse and half the light-colored beach mouse (Figure 2). Each afternoon, the researchers set out the light and dark models in a straight line and in random order about 10 m apart in a habitat known to be occupied by either beach or mainland mice (and hence their natural predators). To determine the difference in color (brightness) between the model and the substrate on which it was placed, soil samples from around each deployed model were collected and measured for brightness (light reflection). The researchers would then return the following day and record which models showed evidence of predation. The shape of the imprint of the model left by the predator (beak or tooth marks) and the surrounding tracks gave clues as to the type of predator. By documenting predation events in both habitats, the researchers could determine how differences in color between the model and the background environment (soil) influenced rates of predation. Results of the experiment revealed that models that were both lighter and darker than their local environment experienced a lower rate of survival (greater rate of predation) than models that were better matched in color to the soil on which they were placed (see Figure 2). Seventy five percent of all predation events occurred on mice that did not match their substrate, representing a large selective disadvantage. In the light-substrate beach environment, most attacked mice were dark, but some light models also were attacked. By analyzing the soil samples that were collected at the location where each model was placed, the researchers found that these light-colored models were all much lighter than their local substrate. In other words, selection acts against mice that are either too dark or too light relative to their background. This result demonstrates that in addition to predation acting as an agent of selection resulting in significant differences in pigmentation between inland and beach populations, there is also selection for subtle color phenotypes within a habitat. In addition to establishing the role of natural selection in the evolution of phenotypic variation in color among local populations of P. polionotus, Hoekstra and her colleagues have also identified the genetic basis for these observed differences. Their work has revealed several interesting patterns. First, they have found that most of the differences in mouse fur color are caused by changes in just a handful of genes; this means that adaptation can sometimes occur via a few large mutational steps. For example, they identified a single DNA base-pair mutation in a pigment receptor, the presence or absence of which accounts for about 30 percent of the color differences between dark mainland mice and light-colored beach mice on the Florida coast. To date, this is one of the few examples of how a single DNA change can have a profound effect on the survival of individuals in nature. Second, they have shown that the same adaptive characteristics can evolve by several different genetic pathways. Beach mice are not just restricted to Florida’s Gulf Coast but are also found some 200 miles away on the Atlantic coast. They have shown that mice on the eastern coastal dunes have also evolved to possess light-colored fur but through different mechanisms. The pigment receptor mutation causing light color in the Gulf Coast mice is absent in the East Coast beach mice. Thus, similar evolutionary changes can sometimes follow different paths. Bibliography Hoekstra, H. E. 2010. From mice to molecules: “The genetic basis of color adaptation.” In In the light of evolution: Essays from the laboratory and field. (Ed. J. B. Losos). Greenwood Village, CO: Roberts and Co. Publishers. Hoekstra, H. E., R. J. Hirschmann, R. A. Bundey, P. A. Insel, and J. P. Crossland. 2006. A single amino acid mutation contributes to adaptive beach mouse color pattern. Science 313:101–104. Mullen, L. M., S. N. Vignieri, J. A. Gore, and H. E. Hoekstra. 2009. Adaptive basis of geographic variation: Genetic, phenotypic and environmental differentiation among beach mouse populations. Proceedings of the Royal Society B 276:3809–3818. Vignieri, S. N., J. Larson, and H. E. Hoekstra. 2010. The selective advantage of cryptic coloration in mice. Evolution 64:2153–2158. Natural selection acts on phenotypic variation within a species or population. In the case of Peromyscus polionotus, what is the cause of this variation? What is the “selective agent” in this example of natural selection? 5.9 Adaptations Reflect Trade-offs and Constraints If Earth were one large homogeneous environment, perhaps a single phenotype, a single set of characteristics might bestow upon all living organisms the ability to survive, grow, and reproduce. But this is not the case. Environmental conditions that directly influence life vary in both space and time (Part One, The Physical Environment). Patterns of temperature, precipitation, and seasonality vary across Earth’s surface, producing a diversity of unique terrestrial environments (Chapter 2). Likewise, variations in depth, salinity, pH, and dissolved oxygen define an array of freshwater and marine habitats (Chapter 3). Each combination of environmental conditions presents a unique set of constraints on the organisms that inhabit them—constraints on their ability to maintain basic metabolic processes that are essential to survival and reproduction. Therefore, as features of the environment change, so will the set of traits (phenotypic characteristics) that increase the ability of individuals to survive and reproduce. Natural selection will favor different phenotypes under different environmental conditions. This principle was clearly illustrated by the example of Darwin’s medium ground finch in Section 5.6, in which a change in the resource base (abundance, size, and hardness of seeds) over time resulted in a shift in the distribution of phenotypes within the population, as well as the example of the two distinct phenotypes of the threespine stickleback adapted to the limnetic and benthic zones of lake ecosystems in the lakes of the Pacific Northwest of North America (also see this chapter, Field Studies: Hopi Hoekstra). Simply stated, the fitness of any phenotype is a function of the prevailing environmental conditions; the characteristics that maximize the fitness of an individual under one set of environmental conditions generally limit its fitness under a different set of conditions. The limitations on the fitness of a phenotype under different environmental conditions are a function of trade-offs imposed by constraints that can ultimately be traced to the laws of physics and chemistry. This general but important concept of adaptive trade-offs is illustrated in the example of natural selection for beak size in the population of Darwin’s medium ground finch (G. fortis) presented in Section 5.6. Recall from Figure 5.10 that the ability to use different seed resources (size and hardness) is related to beak size. Individuals with small beaks feed on the smallest and softest seeds, and individuals with larger beaks feed on the largest and hardest seeds. These differences in diet as a function of beak size reflect a trade-off in morphological characteristics (the depth and width of the beak) that allow for the effective exploitation of different seed resources. This pattern of trade-offs is even more apparent if we compare differences in beak morphology and the use of seed resources for the three most common species of Darwin’s ground finch that inhabit Santa Cruz island in the Galápagos. The distributions of beak size (phenotypes) for individuals of the three most common species of Darwin’s ground finches are shown in Figure 5.20a . As their common names suggest, the mean value of beak size increases from the small (Geospiza fuliginosa) to the medium (G. fortis) and large (Geospiza magnirostris) ground finch. In turn, the proportions of various seed sizes in their diets (Figure 5.20b) reflect these differences in beak size, with the average size and hardness of seeds in the diets of these three populations increasing as a function of beak size. Small beak size restricts the ability of the smaller finch species (G. fuliginosa) to feed on larger, harder seed resources. In contrast, large beak size allows individuals of the largest species, G. magnirostris, to feed on a range of seed resources from small, softer seeds to larger, harder seeds. However, because they are less efficient at exploiting the smaller seed resources, these larger-beaked individuals restrict their diet to the larger, harder seeds. The profitability—defined as the quantity of food energy gained per unit of time spent handling these small seeds (see Section 14.7)—is extremely low for the larger birds and makes feeding on smaller seeds extremely inefficient for these individuals. This inefficiency is directly related to the greater metabolic (food energy) demands of the larger birds, which illustrates a second important concept regarding the role of constraints and trade-offs in the process of natural selection: individual phenotypic characteristics (such as beak size) often are components of a larger adaptive complex involving multiple traits and loci. The phenotypic trait of beak size in Darwin’s ground finches is but one in a complex of interrelated morphological characteristics that determine the foraging behavior and diet of these birds. Larger beak size is accompanied by increased body size (length and weight) as well as specific changes in components of skull architecture and head musculature ( Figure 5.21), all of which are directly related to feeding functions. In summary, the beak size of a bird sets the potential range of seed types in the diet. This relationship between morphology and diet represents a basic trade-off that constrains the evolution of adaptations in Darwin’s finches relating to their acquisition of essential food resources. In addition, the example of Darwin’s ground finches illustrates how natural selection operates on genetic variation at the three levels we initially defined in Section 5.5: within a population, among subpopulations of the same species, and among different species. Natural selection operated in the local population of medium ground finches on the island of Daphne Major during the period of drought in the mid-1970s, increasing the mean beak size of birds in this population in response to the shift in the abundance and quality of seed resources. In addition, natural selection has resulted in differences in mean beak size between populations of the medium ground finch inhabiting the islands of Daphne Major and Santa Cruz (see Chapter 13, Figure 13.17). The larger mean beak size for the population on Santa Cruz is believed to be a result of competition from the population of small ground finch present on the island (G. fuliginosa does not occupy Daphne Major). The presence of the smaller species on the island has the effect of reducing the availability of smaller, softer seeds (see Figure 5.20) and increasing the relative fitness of G. fortis individuals with larger beaks that can feed on the larger, harder seeds (see Figure 5.10). Population genetic studies have also shown that natural selection is the evolutionary force that has resulted in the genetic differentiation of various species of Darwin’s finches inhabiting the Galápagos Islands ( Figure 5.22). The process in which one species gives rise to multiple species that exploit different features of the environment, such as food resources or habitats, is called adaptive radiation. The different features of the environment exert the selection pressures that push the populations in various directions, and reproductive isolation, the necessary condition for speciation to occur, is often a by-product of the changes in morphology, behavior, or habitat preferences that are the actual targets of selection. In this way, the differences in beak size and diet among the three species of ground finch are magnified versions of the differences observed within a population, or among populations inhabiting different islands. (From Patel 2006.) In the chapters that follow, we will examine this basic principle of trade-offs as it applies to the adaptation of species and explore how the nature of adaptations changes with changing environmental conditions. We will explore various adaptations of plant and animal species, respectively, to key features of the physical environment that directly influence the basic processes of survival and assimilation in Chapters 6 and 7, and trade-offs involved in the evolution of life history characteristics (adaptations) relating to reproduction in Chapter 10. The role of species’ interactions as a selective agent in the process of natural selection will be examined later in Part Four (Species Interactions). Throughout our discussion, adaptation by natural selection is a unifying concept, a mechanism for understanding the distribution and abundance of species. We will explore the selective forces giving rise to the adaptations that define the diversity of species as well as the advantages and constraints arising from those adaptations under different environmental conditions. Finally, we will examine how the trade-offs in adaptations to different environmental conditions give rise to the patterns and processes observed in communities and ecosystems as environmental conditions change in space and time. Ecological Issues & Applications Genetic Engineering Allows Humans to Manipulate a Species’ DNA For a millennia, humans have been using the process of selective breeding to modify the characteristics of plant and animal species. By selecting individuals that exhibit a desired trait, and mating them with individuals exhibiting the same trait (or traits), breeders produce populations with specific physical and behavioral characteristics (phenotypes). This process of selective breeding is analogous to natural selection—the differential fitness of individuals within the population resulting from differences in some heritable characteristic(s). Unlike natural selection, however, humans function as the agent of selection rather than the environment. Darwin referred to selective breeding as “artificial selection,” and his understanding of this process was instrumental in his development of the idea of natural selection. Although the process of selective breeding has provided us with the diversity of domesticated plants and animals upon which we depend for food, the process has one major limitation. The array of characteristics that can be selected for are limited to the genetic variation (alleles) that exists within the population (species). For example, red flower color can only be selected for if the allele coding for the production of red pigment exists within the plant population (species). Modern genetic techniques, however, have removed this fundamental constraint. It is now possible to transfer DNA (genes) from one species to another. The process of directly altering an organism’s genome is referred to as genetic engineering. The primary technology used in genetic engineering is genetic recombination; the development of recombinant DNA or rDNA by combining the genetic material from one organism into the genome of another organism (generally of a different species). The resulting modified gene is called a transgene, and the recipient of the recombinant DNA is called a transgenic organism. The process of genetic engineering using rDNA requires the successful completion of a series of steps (Figure 5.23). DNA extraction is the first step in the process. To work with DNA, scientists must extract it from the donor organism (the organism that has the desired trait). During the process of DNA extraction, the complete sequence of DNA from the donor organism is extracted at once. The next task is to separate the single gene of interest from the rest of the DNA using specific enzymes that “cut” the desired segment of DNA from the larger strand. Copies of the gene can then produced using cloning techniques. Once the gene has been cloned, genetic engineers begin the third step, designing the gene to work once it is inside a recipient organism. This is done by using other enzymes that are capable of adding new segments called promoters (which start a sequence) and terminators (which stop a sequence). A promoter is a region of DNA that initiates the transcription of a particular gene (the first step of gene expression, in which a particular segment of DNA is copied into RNA). The terminator is a section of genetic sequence that marks the end of gene. The next step is to insert the new gene into the cell of the recipient organism. The process in which changes in a cell or organism are brought about through the introduction of new DNA is called transformation. Transformation is accomplished through a variety of techniques, but two main approaches are used for plant species: the “gene gun” method and the Agrobacterium method. The gene gun method fires gold particles carrying the foreign DNA into plant cells. Some of these particles pass through the plant cell wall and enter the cell nucleus, where the transgene integrates itself into the plant chromosome. The Agrobacterium method involves the use of soil-dwelling bacteria known as Agrobacterium tumefaciens that cause crown gall disease in many plant species. This bacterium has a plasmid, or loop of nonchromosomal DNA, that contains tumor-inducing genes (T-DNA), along with additional genes that help the T-DNA integrate into the host genome. When the bacteria infect the plant, the plasmid is integrated into the plant’s chromosomes, becoming part of the plant’s genome. For genetic engineering purposes, the tumor-inducing part of the plasmid is removed so that it will not harm the plant. The desired gene from the donor organisms is then inserted into the bacteria’s plasmid. The bacteria can now be used as a delivery system that will transfer the transgene into the plant. An organism whose genetic material has been altered using genetic engineering techniques (including transgenic organisms) are commonly referred to as genetically modified organisms (GMOs). Genetic engineering has been used to produce a wide variety of GMOs. Organisms that have been genetically modified include microorganisms such as bacteria and yeast, insects, plants, fish, and mammals. GMOs are used in biological and medical research, production of pharmaceutical drugs, experimental medicine (e.g., gene therapy), but perhaps their most widespread application has been in agriculture (Figure 5.24). In agriculture, genetically engineered crops have been created that possess desirable traits such as resistance to pests or herbicides, increased nutritional value, or production of pharmaceuticals. In addition to the ethical and health concerns that genetically modified crop species have raised, the practice of genetic engineering and the production of transgenic species (GMOs) has raised considerable concern among ecologists. There is little concern about gene transfer between major agricultural species such as corn, soybean, and rice and native plant populations as a result of the lack of close relatives capable of cross-pollination. However, other crop species, such as members of the genus Brassica (member of the mustard family) are represented by a variety of domesticated and wild (native) species and subspecies that are capable of cross-pollination. For example, Brassica napus (rapeseed) used in the production of rapeseed oil has been genetically modified to tolerate herbicides (used to kill weeds in agricultural fields), and the transfer of herbicide-tolerant traits by pollen to weedy relatives (other members of the genus Bassica) has been recorded. Brassica includes a number of important crop species such as turnips, cabbage, and broccoli. A major application of genetic engineering in agriculture is the development of insect-resistant crop strains. Perhaps the most widely grown genetically modified crop plant is Bt corn. Bacillus thuringiensis, or Bt, is a common soil bacterium whose genome contains genes for several proteins toxic to insects. For decades, Bt has been sprayed on fields as an organic pesticide. Starting in the mid-1990s, several varieties of corn were genetically engineered to incorporate Bt genes’ encoding proteins, which are toxic to various insect pests. Some strains of Bt produce proteins that are selectively toxic to caterpillars, such as the southwestern corn borer, whereas others target mosquitoes, root worms, or beetles. The insecticide proteins are contained within the plant tissues, which are fatal to the pest species when ingested. Concerns have been raised over the potential impacts of Bt corn and other insect-resistant genetically modified crop species on nontarget insect species or to predators that feed on these insects. The use of transgenes to confer disease resistance to crops represents another possible ecological risk. If genes that code for viral resistance are transferred to crops, there is a potential for transfer to wild plants, creating the potential for the natural development of new plant viruses of increased severity. Summary Adaptation 5.1 Characteristics that enable an organism to thrive in a given environment are called adaptations. Adaptations are a product of natural selection. Natural selection is the differential fitness of individuals within the population that results from their interaction with their environment, where the fitness of an individual is measured by the proportionate contribution it makes to future generations. The process of natural selection results in changes in the properties of populations of organisms over the course of generations by a process known as evolution. Genes 5.2 The units of heredity are genes, which are linearly arranged on threadlike bodies called chromosomes. The alternative forms of a gene are alleles. The pair of alleles present at a given locus defines the genotype. If both alleles at the locus are the same, the individual is homozygous. If the alleles are different, the individual is heterozygous. The sum of heritable information carried by the individual is the genome. Phenotype 5.3 The phenotype is the physical expression of the genotype. The manner in which the genotype affects the phenotype is termed the mode of gene action. When heterozygous individuals exhibit the same phenotype as one of the homozygotes, the allele that is expressed is termed dominant and the masked allele is termed recessive. If the physical expression of the heterozygote is intermediate between the homozygotes, the alleles are said to be codominant. Even though all genetic variation is discrete, most phenotypic traits have a continuous distribution because (1) most traits are affected by more than one locus, and (2) most traits are affected by the environment. Phenotypic Plasticity 5.4 The ability of a genotype to give rise to a range of phenotypic expressions under different environmental conditions is termed phenotypic plasticity. The range of phenotypes expressed under different environmental conditions is termed the norm of reaction. If the phenotypic plasticity occurs during the growth and development of the individual and represents an irreversible characteristic, it is referred to as developmental plasticity. Reversible phenotypic changes in an individual organism in response to changing environmental conditions are referred to as acclimation. Genetic Variation 5.5 Genetic variation occurs at three levels: within subpopulations, among subpopulations of the same species, and among different species. The sum of genetic information across all individuals in the population is the gene pool. The fundamental measures of genetic variation within a population are allele frequency and genotype frequency. Natural Selection 5.6 Natural selection acts on the phenotype, but in doing so it alters both genotype and allele frequencies within the population. There are three general types of natural selection: directional selection, stabilizing selection, and disruptive selection. The target of selection is the phenotypic trait that natural selection acts upon, whereas the selective agent is the environmental cause of fitness differences among individuals in the population. Processes Influencing Genetic Variation 5.7 Natural selection is the only evolutionary process that can result in adaptations; however, some processes can function to alter patterns of genetic variation from generation to generation. These include mutation, migration, genetic drift, and nonrandom mating. Mutations are heritable changes in a gene or chromosome. Migration is the movement of individuals between local populations. This movement results in the transfer of genes between local populations. Genetic drift is a change in allele frequency as a result of random chance. Nonrandom mating on the basis of phenotypic traits is referred to as assortative mating. Assortative mating can be either positive (mates are more similar than expected by chance) or negative (dissimilar). A special case of nonrandom mating is inbreeding—the mating of individuals that are more closely related than expected by chance. Genetic Differentiation 5.8 Natural selection can function to alter genetic variation between populations; this result is referred to as genetic differentiation. Species having a wide geographic distribution often encounter a broader range of environmental conditions than do species whose distribution is more restricted. The variation in environmental conditions often gives rise to a corresponding variation in many morphological, physiological, and behavioral characteristics as a result of different selective agents in the process of natural selection. Trade-offs and Constraints 5.9 The environmental conditions that directly influence life vary in both space and time. Likewise, the objective of selection changes with environmental circumstances in both space and time. The characteristics enabling a species to survive, grow, and reproduce under one set of conditions limit its ability to do equally well under different environmental conditions. Genetic Engineering Ecological Issues & Applications Genetic engineering is the process of directly altering an organism’s genome. The primary technology used in genetic engineering is genetic recombination—the combining of genetic material from one organism into the genome of another organism, generally of a different species. The result of this process is recombinant DNA (rDNA). The resulting modified gene is called a transgene, and the recipient of the rDNA is called a transgenic organism.
Theory of Evolution by Natural Selection
CHAPTER 6 Smith, T. M., & Smith, R. L. (2015). Elements of Ecology (9th ed.). Boston, MA: Pearson. 6.1 Photosynthesis Is the Conversion of Carbon Dioxide into Simple Sugars Photosynthesis is the process by which energy from the Sun, in the form of shortwave radiation, is harnessed to drive a series of chemical reactions that result in the fixation of CO2 into carbohydrates (simple sugars) and the release of oxygen (O2) as a by-product. The portion of the electromagnetic spectrum that photosynthetic organisms use is between 400 and 700 nanometers (nm; roughly corresponding to the visible portion of the spectrum) and is referred to as photosynthetically active radiation (PAR). The process of photosynthesis can be expressed in the simplified form shown here: 6CO2+12H2O→C6H12O+6O2+6H2O6CO2+12H2O→C6H12O + 6O2 +6H2O The net effect of this chemical reaction is the use of six molecules of water (H2O) and the production of six molecules of oxygen (O2) for every six molecules of CO2 that are transformed into one molecule of sugar C6H12O6. The synthesis of various other carbon-based compounds—such as complex carbohydrates, proteins, fatty acids, and enzymes—from these initial products occurs in the leaves as well as other parts of the plant. Photosynthesis, a complex sequence of metabolic reactions, can be separated into two processes, often referred to as the light-dependent and light-independent reactions. The light-dependent reactions begin with the initial photochemical reaction in which chlorophyll (light-absorbing pigment) molecules within the chloroplasts absorb light energy. The absorption of a photon of light raises the energy level of the chlorophyll molecule. The excited molecule is not stable, and the electrons rapidly return to their ground state, thus releasing the absorbed photon energy. This energy is transferred to another acceptor molecule, resulting in a process called photosynthetic electron transport. This process results in the synthesis of adenosine triphosphate (ATP) from adenosine diphosphate (ADP) and of NADPH (the reduced form of nicotinamide adenine dinucleotide phosphate [NADP]) from NADP+. The high-energy substance ATP and the strong reductant NADPH produced in the light-dependent reactions are essential for the second step in photosynthesis—the light-independent reactions. In the light-independent reactions, CO2 is biochemically incorporated into simple sugars. The light-independent reactions derive their name from the fact that they do not directly require the presence of sunlight. They are, however, dependent on the products of the light-dependent reactions and therefore ultimately depend on the essential resource of sunlight. The process of incorporating CO2 into simple sugars begins in most plants when the five-carbon molecule ribulose biphosphate (RuBP) combines with CO2 to form two molecules of a three-carbon compound called phosphoglycerate (3-PGA). CO2+RuBP→23−PGACO2+ RuBP → 2 3-PGA 1-carbon5-carbon3-carbonmoleculemoleculemolecule1-carbon5-carbon3-carbonmoleculemoleculemolecule This reaction, called carboxylation, is catalyzed by the enzyme rubisco (ribulose biphosphate carboxylase-oxygenase). The plant quickly converts the 3-PGA formed in this process into the energy-rich sugar molecule glyceraldehyde 3-phosphate (G3P). The synthesis of G3P from 3-PGA requires both ATP and NADPH—the high-energy molecule and reductant that are formed in the light-dependent reactions. Some of this G3P is used to produce simple sugars (C6H12O6), starches, and other carbohydrates required for plant growth and maintenance; the remainder is used to synthesize new RuBP to continue the process. The synthesis of new RuBP from G3P requires additional ATP. In this way, the availability of light energy (solar radiation) can limit the light-independent reactions of photosynthesis through its control on the production of ATP and NADPH required for the synthesis of G3P and the regeneration of RuBP. This photosynthetic pathway involving the initial fixation of CO2 into the three-carbon PGAs is called the Calvin–Benson cycle, or C3 cycle, and plants employing it are known as C3 plants (Figure 6.1). The C3 pathway has one major drawback. The enzyme rubisco that drives the process of carboxylation also acts as an oxygenase; rubisco can catalyze the reaction between O2 and RuBP. The oxygenation of RuBP results in the eventual release of CO2 and is referred to as photorespiration (not to be confused with the process of cellular respiration discussed herein). This competitive reaction to the carboxylation process reduces the efficiency of C3 photosynthesis by as much as 25 percent. Some of the carbohydrates produced in photosynthesis are used in the process of cellular respiration—the harvesting of energy from the chemical breakdown of simple sugars and other carbohydrates. The process of cellular respiration (also referred to as aerobic respiration) occurs in the mitochondria of all living cells and involves the oxidation of carbohydrates to generate energy in the form of ATP. C6H12O6+6O2→6CO2+6H2O+ATPC6H12O6+6O2→6CO2+6H2O +ATP Because leaves both use CO2 during photosynthesis and produce CO2 during respiration, the difference in the rates of these two processes is the net gain of carbon, referred to as net photosynthesis . Net photosynthesis=Photosynthesis − RespirationNet photosynthesis = Photosynthesis − Respiration The rates of photosynthesis and respiration, and therefore net photosynthesis, are typically measured in moles CO2 per unit leaf area (or mass) per unit time (μmol/m2/s). 6.2 The Light a Plant Receives Affects Its Photosynthetic Activity Solar radiation provides the energy required to convert CO2 into simple sugars. Thus, the availability of light (PAR) to the leaf directly influences the rate of photosynthesis (Figure 6.2). At night, in the absence of PAR, only respiration occurs and the net uptake of CO2 is negative. The rate of CO2 loss when the value of PAR is zero provides an estimate of the rate of respiration. As the Sun rises and the value of PAR increases, the rate of photosynthesis likewise increases, eventually reaching a level at which the rate of CO2 uptake in photosynthesis is equal to the rate of CO2 loss in respiration. At that point, the rate of net photosynthesis is zero. The light level (value of PAR) at which this occurs is called the light compensation point (LCP). As light levels exceed the LCP, the rate of net photosynthesis increases with PAR. Eventually, photosynthesis becomes light saturated. The value of PAR, above which no further increase in photosynthesis occurs, is referred to as the light saturation point. In some plants adapted to extremely shaded environments, photosynthetic rates decline as light levels exceed saturation. This negative effect of high light levels, called photoinhibition, can be the result of “overloading” the processes involved in the light-dependent reactions. 6.3 Photosynthesis Involves Exchanges between the Plant and Atmosphere The process of photosynthesis occurs in specialized cells within the leaf called mesophyll cells (see Figure 6.1). For photosynthesis to take place within the mesophyll cells, CO2 must move from the outside atmosphere into the leaf. In terrestrial (land) plants, CO2 enters the leaf through openings on its surface called stomata (Figure 6.3) through the process of diffusion. Diffusion is the movement of a substance from areas of higher to lower concentration. CO2 diffuses from areas of higher concentration (the air) to areas of lower concentration (the interior of the leaf). When the concentrations are equal, an equilibrium is achieved and there is no further net exchange. diffusion. Two factors control the diffusion of CO2 into the leaf: the diffusion gradient and stomatal conductance. The diffusion gradient is defined as the difference between the concentration of CO2 in air adjacent to the leaf and the concentration of CO2 in the leaf interior. Concentrations of CO2 are often described in units of parts per million (ppm) of air. A CO2 concentration of 400 ppm would be 400 molecules of CO2 for every 1 million molecules of air. Stomatal conductance is the flow rate of CO2 through the stomata (generally measured in units of μmol/m2/s) and has two components: the number of stoma per unit leaf surface area (stomatal density) and aperture (the size of the stomatal openings). Stomatal aperture is under plant control, and stomata open and close in response to a variety of environmental and biochemical factors. As long as the concentration of CO2 in the air outside the leaf is greater than that inside the leaf and the stomata are open, CO2 will continue to diffuse through the stomata into the leaf. So as CO2 diffuses into the leaf through the stomata, why do the concentrations of CO2 inside and outside the leaf not come into equilibrium? The concentration inside the leaf declines as CO2 is transformed into sugar during photosynthesis. As long as photosynthesis occurs, the gradient remains. If photosynthesis stopped and the stomata remained open, CO2 would diffuse into the leaf until the internal CO2 equaled the outside concentration. When photosynthesis and the demand for CO2 are reduced for any reason (such as a reduction in light), the stomata tend to close, thus reducing flow into the leaf. The stomata close because they play a dual role. As CO2 diffuses into the leaf through the stomata, water vapor inside the leaf diffuses out through the same openings. This water loss through the stomata is called transpiration. As with the diffusion of CO2 into the leaf, the rate of water diffusion out of the leaf will depend on the diffusion gradient of water vapor from inside to outside the leaf and the stomatal conductance (flow rate of water). Like CO2, water vapor diffuses from areas of high concentration to areas of low concentration—from wet to dry. The relative humidity (see Section 2.5, Figure 2.15) inside a leaf is typically greater than 99 percent, therefore there is usually a large difference in water vapor concentration between the inside and outside of the leaf, resulting in the diffusion of water out of the leaf. The lower the relative humidity of the air, the larger the diffusion gradient and the more rapidly the water inside the leaf will diffuse through the stomata into the surrounding air. The leaf must replace the water lost to the atmosphere, otherwise it will wilt and die. 6.4 Water Moves from the Soil, through the Plant, to the Atmosphere The force exerted outward on a cell wall by the water contained in the cell is called turgor pressure. The growth rate of plant cells and the efficiency of their physiological processes are highest when the cells are at maximum turgor—that is, when they are fully hydrated. When the water content of the cell declines, turgor pressure drops and water stress occurs, ranging from wilting to dehydration. For leaves to maintain maximum turgor, the water lost to the atmosphere in transpiration must be replaced by water taken up from the soil through the root system of the plant and transported to the leaves. You may recall from basic physics that work—the displacement of matter, such as transporting water from the soil into the plant roots and to the leaves—requires the transfer of energy. The measure of energy available to do work is called Gibbs energy (G), named for the U.S. physicist Willard Gibbs, who first developed the concept in the 1870s. In the process of active transport, such as transporting water from the ground to an elevated storage tank using an electric pump, the input of energy to the system is in the form of electricity to the pump. The movement of water through the soil–plant–atmosphere continuum, however, is an example of passive transport, a spontaneous reaction that does not require an input of energy to the system. The movement of water is driven by internal differences in the Gibbs energy of water at any point along the continuum between the soil, plant, and atmosphere. The Second Law of Thermodynamics states that the transfer of energy (through either heat or work) always proceeds in the direction from higher to lower energy content (e.g., from hot to cold). Therefore, a gradient of decreasing energy content of the water between any two points along the continuum must exist to enable the passive movement of water between the soil, plant, and atmosphere. The measure used to describe the Gibbs energy of water at any point along the soil–plant–atmosphere continuum is called water potential (ψ). Water potential is the difference in Gibbs energy per mole (the energy available to do work) between the water of interest and pure water (at a standard temperature and pressure). Plant physiologists have chosen to express water potential in terms of pressure, which has the dimensions of energy per volume, and is expressed in terms of Pascals (Pa = 1 Newton/m2). By convention, pure water at atmospheric pressure has a water potential of zero and the addition of any solutes or the creation of suction (negative hydrostatic pressure) will function to lower the water potential (more negative values). We can now examine the movement of water through the soil–plant–atmosphere continuum as a function of the gradient in water potential. As previously stated, the transfer of energy will always proceed from a region of higher energy content to a region of lower energy content, or in the case of water potential, from areas of higher water potential to areas of lower water potential. We can start with the exchange of water between the leaf and the atmosphere in the process of transpiration. When relative humidity of the atmosphere is 100 percent, the atmospheric water potential (ψatm) is zero. As values drop below 100 percent, the value of Gibbs energy declines, and ψatm becomes negative (Figure 6.4). Under most physiological conditions, the air within the leaf is at or near saturation (relative humidity ∼ 99 percent). As long as the relative humidity of the air is below 99 percent, a steep gradient of water potential between the leaf (ψleaf) and the atmosphere (ψatm) will drive the process of diffusion. Water vapor will move from the region of higher water potential (interior of the leaf) to the region of lower water potential (atmosphere)—that is, from a state of high to low Gibbs energy. As water is lost to the atmosphere through the stomata, the water content of the cells decreases (turgor pressure drops) and in turn increases the concentration of solutes in the cell. This decrease in the cell’s water content (and corresponding increase in solute concentration) decreases the water potential of the cells. Unlike the water potential of the atmosphere, which is determined only by relative humidity, several factors determine water potential within the plant. Turgor pressure (positive pressure) in the cell increases the plant’s water potential. Therefore, a decrease in turgor pressure associated with water loss functions to decrease water potential. The component of plant water potential as a result of turgor pressure represents hydrostatic pressure and is represented as ψp. Increasing concentrations of solutes in the cells are associated with water loss and will lower the water potential. This component of plant water potential is termed osmotic potential (ψπ) because the difference in solute content inside and outside the cell results in the movement of water through the process of osmosis. The surfaces of larger molecules, such as those in the cell walls, exert an attractive force on water. This tendency for water to adhere to surfaces reduces the Gibbs energy of the water molecules, reducing water potential. This component of water potential is called matric potential (ψm). The total water potential ψ at any point in the plant, from the leaf to the root, is the sum of these individual components: ψ = ψp + ψp + ψmψ = ψp + ψp + ψm The osmotic and matric potentials will always have a negative value, whereas the turgor pressure (hydrostatic pressure) component can be either positive or negative. Thus, the total potential can be either positive or negative, depending on the relative values of the individual components. Values of total water potential at any point along the continuum (soil, root, leaf, and atmosphere), however, are typically negative and the movement of water proceeds from areas of higher (zero or less negative) to lower (more negative) potential (from the region of higher energy to the region of lower energy). Therefore, the movement of water from the soil to the root, from the root to the leaf, and from the leaf to the atmosphere depends on maintaining a gradient of increasingly negative water potential at each point along the continuum (Figure 6.4). ψatm < ψleaf < ψroot < ψsoilψatm < ψleaf < ψroot < ψsoil Drawn by the low water potential of the atmosphere, water from the surface of and between the mesophyll cells within the leaf evaporates and escapes through the stomata. This gradient of water potential is transmitted into the mesophyll cells and on to the water-filled xylem (hollow conducting tubes throughout the plant) in the leaf veins. The gradient of increasingly negative water potential extends down to the fine rootlets in contact with soil particles and pores. As water moves from the root and up through the stem to the leaf, the root water potential declines so that more water moves from the soil into the root. Water loss through transpiration continues as long as (1) the amount of energy striking the leaf is enough to supply the necessary latent heat of evaporation (see Section 2.5), (2) moisture is available for roots in the soil, and (3) the roots are capable of maintaining a more negative water potential than that of the soil. At field capacity, water is freely available, and soil water potential (ψsoil) is at or near zero (see Section 4.8). As water is drawn from the soil, the water content of the soil declines, and the soil water potential becomes more negative. As the water content of the soil declines, the remaining water adheres more tightly to the surfaces of the soil particles, and the matric potential becomes more negative. For a given water content, the matric potential of soil is influenced strongly by its texture (see Figure 4.10). Soils composed of fine particles, such as clays, have a higher surface area (per soil volume) for water to adhere to than sandy soils do. Clay soils, therefore, are characterized by more negative matric potentials for the same water content. As soil water potential becomes more negative, the root and leaf water potentials must decline (become more negative) if the potential gradient is to be maintained. If precipitation does not recharge soil water, and soil water potentials continue to decline, eventually the gradient between the soil, root, and leaf cannot be maintained, and at that point, the stomata close to stop further water loss through transpiration. However, this closure also results in stopping further uptake of CO2. The value of leaf water potential at which stomata close and net photosynthesis ceases varies among plant species ( Figure 6.5 ) and reflects basic differences in their biochemistry, physiology, and morphology. The rate of water loss varies with daily environmental conditions, such as humidity and temperature, and with the characteristics of plants. Opening and closing the stomata is probably the plant’s most important means of regulating water loss. The trade-off between CO2 uptake and water loss through the stomata results in a direct link between water availability in the soil and the plant’s ability to carry out photosynthesis. To carry out photosynthesis, the plant must open its stomata; but when it does, it loses water, which it must replace to live. If water is scarce, the plant must balance the opening and closing of the stomata, taking up enough CO2 while minimizing the loss of water. The ratio of carbon fixed (photosynthesis) per unit of water lost (transpiration) is called the water-use efficiency. We can now appreciate the trade-off faced by terrestrial plants. To carry out photosynthesis, the plant must open the stomata to take up CO2. But at the same time, the plant loses water through the stomata to the outside air—water that must be replaced through the plant’s roots. If its access to water is limited, the plant must balance the opening and closing of stomata to allow for the uptake of CO2 while minimizing water loss through transpiration. This balance between photosynthesis and transpiration is an extremely important constraint that has governed the evolution of terrestrial plants and directly influences the productivity of ecosystems under differing environmental conditions (see Chapter 20). 6.5 The Process of Carbon Uptake Differs for Aquatic and Terrestrial Autotrophs A major difference in CO2 uptake and assimilation by aquatic autotrophs (submerged plants, algae, and phytoplankton) versus terrestrial plants is the lack of stomata in aquatic autotrophs. CO2 diffuses from the atmosphere into the surface waters and is then mixed into the water column. Once dissolved, CO2 reacts with the water to form bicarbonate (HCO3 −). This reaction is reversible, and the concentrations of CO2 and bicarbonate tend toward a dynamic equilibrium (see Section 3.7). In aquatic autotrophs, CO2 diffuses directly from the waters across the cell membrane. Once the CO2 is inside the cell, photosynthesis proceeds in much the same way as outlined previously for terrestrial plants. One difference between terrestrial and aquatic autotrophs is that some aquatic species can also use bicarbonate as a carbon source. However, the organism must first convert it to CO2 using the enzyme carbonic anhydrase. This conversion can occur in two ways: (1) active transport of bicarbonate into the cell followed by conversion to CO2 or (2) excretion of the enzyme into adjacent waters and subsequent uptake of converted CO2 across the membrane. As CO2 is taken up, its concentration in the waters adjacent to the organism decline. Because the diffusion of CO2 in water is 104 times slower than in the air, it can easily become depleted (low concentrations) in the waters adjacent to the organism, reducing rates of uptake and photosynthesis. This constraint can be particularly important in still waters such as dense seagrass beds or rocky intertidal pools. 6.6 Plant Temperatures Reflect Their Energy Balance with the Surrounding Environment Both photosynthesis and respiration respond directly to variations in temperature ( Figure 6.6 ). As temperatures rise above freezing, both photosynthesis and respiration rates increase. Initially, photosynthesis increases faster than respiration. As temperatures continue to rise, the photosynthetic rate reaches a maximum related to the temperature response of the enzyme rubisco. As temperatures continue to rise, photosynthetic rate declines and respiration rate continues to increase. As temperatures rise further, even respiration declines as temperatures reach critical levels. The temperature response of net photosynthesis is the difference between the rate of carbon uptake in photosynthesis and the rate of carbon loss in respiration (see Figure 6.6). Three values describe the temperature response curve: Tmin, Topt, and Tmax. The values Tmin and Tmax are, respectively, the minimum and maximum temperatures at which net photosynthesis approaches zero (meaning no net carbon uptake). Topt is the temperature, or range of temperatures, over which net carbon uptake is at its maximum. The temperature of the leaf, not the air, controls the rate of photosynthesis and respiration; and leaf temperature depends on the exchange of thermal energy between the leaf and its surrounding environment. Plants absorb both shortwave (solar) and longwave (thermal) radiation (see Section 2.1). Plants reflect some of this solar radiation and emit longwave radiation back to the atmosphere. The difference between the radiation a plant receives and the radiation it reflects and emits back to the surrounding environment is the net radiation balance of the plant (Rn). The net radiation balance of a plant is analogous to the concept of the radiation balance of Earth (see Chapter 2, Figure 2.3). Of the net radiation absorbed by the plant, some is used in metabolic processes and stored in chemical bonds—namely, in the processes of photosynthesis and respiration. This quantity is quite small, typically less than 5 percent of Rn. The remaining energy raises the temperature of the leaves and the surrounding air. On a clear, sunny day, the amount of energy plants absorb can raise internal leaf temperatures well above ambient (air or water temperature). Internal leaf temperatures may go beyond the optimum for photosynthesis and possibly reach critical levels (see Figure 6.6). To maintain internal temperatures within the range of tolerance (positive net photosynthesis), plants must exchange thermal energy (heat) with the surrounding environment. The transfer of heat between the plant and the surrounding air (or water) is governed by the Second Law of Thermodynamics—thermal energy flows in only one direction, from areas of higher temperature to areas of lower temperature. The primary means by which terrestrial plants dissipate heat are evaporation and convection; aquatic plants do so primarily by convection. Evaporation occurs in the process of transpiration. Recall from Chapter 3 that the phase change of water from a liquid to a gas (evaporation) requires an input of thermal energy (540 calories or 2260 joules per gram [g] of water). As waters transpires from the leaves of plants to the surrounding atmosphere through the stomata, the leaves lose thermal energy and their temperature declines through evaporative cooling (see Section 3.2). The ability of terrestrial plants to dissipate heat by evaporation is dependent on the rate of transpiration. The transpiration rate is in turn influenced by the relative humidity of the air and by the availability of water to the plant (see Section 6.3). Convection is the transfer of heat energy through the circulation of fluids (Chapter 2), whereas conduction is the transfer of thermal energy through direct contact (between two objects). For convection to occur, the surface of the leaf must first transfer thermal energy between the adjacent molecules of air or water through conduction. The direction of this conductive exchange depends on the difference between the temperature of the leaf and the surrounding air. If the leaf temperature is higher than that of the surrounding air, there is a net transfer of heat from the leaf to the surrounding air. Thermal energy is then transported from the air adjacent to the surface of the leaf to the surrounding air through the process of convection, the circulation of fluids. The transfer of heat from the plant to the surrounding environment is influenced by the existence of the boundary layer , which is a layer of still air (or water) adjacent to the surface of each leaf. The environment of the boundary layer differs from that of the surrounding environment (air or water) because it is modified by the diffusion of heat, water, and CO2 from the plant surface. As water is transpired from the stomata, the humidity of the air within the boundary layer increases, reducing further transpiration. Likewise, as thermal energy (heat) is transferred from the leaf surface to the boundary layer, the air temperature of the boundary layer increases, reducing further heat transfer from the leaf surface. Under still conditions (no air or water flow), the boundary layer increases in thickness reducing the transfer of heat and materials (water and CO2) between the leaf and the atmosphere (or water). Wind or water flow functions to reduce the size of the boundary layer, allowing for mixing between the boundary layer and the surrounding air (or water) and reestablishing the diffusion or temperature gradient between the leaf surface and the bulk air. Leaf size and shape also influence the thickness and dynamics of the boundary layer, and therefore, the ability of plants to exchange heat through convection. Air tends to move more smoothly (laminar flow) over a larger surface than a smaller one, and as a result, the boundary layer tends to be thicker and more intact in larger leaves. Deeply lobed leaves, and small, compound leaves (Figure 6.7) tend to disrupt the flow of air, causing turbulence that functions to reduce the boundary layer and increase the exchange of heat and water. The relative importance of evaporation and convection to the maintenance of leaf temperatures (dissipation of heat) is dependent on the physical environment. In locations where water is available, such as regions of high mean annual precipitation, most of the dissipation of heat can occur through transpiration (evaporation) as plants open stomata to support the uptake of CO2. As conditions become drier, however, transpiration becomes limited and the average leaf size of species decreases, enhancing heat loss through convection (see Figure 6.18b 6.7 Constraints Imposed by the Physical Environment Have Resulted in a Wide Array of Plant Adaptations We have explored variation in the physical environment over Earth’s surface: the salinity, depth, and flow of water; spatial and temporal patterns in climate (precipitation and temperature); variations in geology and soils (Part One). In all but the most extreme of these environments, autotrophs harness the energy of the Sun to fuel the conversion of CO2 into glucose in the process of photosynthesis. To survive, grow, and reproduce, plants must maintain a positive carbon balance, converting enough CO2 into glucose to offset the expenses of respiration (photosynthesis > respiration). To accomplish this, a plant must acquire the essential resources of light, CO2, water, and mineral nutrients as well as tolerate other features of the environment that directly affect basic plant processes such as temperature, salinity, and pH. Although often discussed—and even studied as though they are independent of each other—the adaptations exhibited by plants to these features of the environment are not independent, for reasons relating to the physical environment and to the plants themselves. Many features of the physical environment that directly influence plant processes are interdependent. For example, the light, temperature, and moisture environments are all linked through a variety of physical processes (Chapters 2– 4). The amount of solar radiation not only influences the availability of light (PAR) required for photosynthesis but also directly influences the temperature of the leaf and its surroundings. In addition, air temperature directly affects the relative humidity, a key feature influencing the rate of transpiration and evaporation of water from the soil (see Section 2.5, Figure 2.15). For this reason, we see a correlation in the adaptations of plants to variations in these environmental factors. Plants adapted to dry, sunny environments must be able to deal with the higher demand for water associated with higher temperatures and lower relative humidity, and they tend to have characteristics such as smaller leaves and increased production of roots. In other cases, there are trade-offs in the ability of plants to adapt to limitations imposed by multiple environmental factors, particularly resources. One of the most important of these trade-offs involves the acquisition of above- and belowground resources. Allocating carbon to the production of leaves and stems increases the plant’s access to the resources of light and CO2, but it is at the expense of allocating carbon to the production of roots. Likewise, allocating carbon to the production of roots increases access to water and soil nutrients but limits carbon allocation to the production of leaves. The set of characteristics (adaptations) that allow a plant to successfully survive, grow, and reproduce under one set of environmental conditions inevitably limits its ability to do equally well under different environmental conditions. We explore the consequences of this simple premise in the following sections. 6.8 Species of Plants Are Adapted to Different Light Environments The amount of solar radiation reaching Earth’s surface varies diurnally, seasonally, and geographically (Chapter 2, Section 2.3). However, a major factor influencing the amount of light (PAR) a plant receives is the presence of other plants through shading (see Section 4.2 and Chapter 4, Quantifying Ecology 4.1). Although the amount of light that reaches an individual plant varies continuously as a function of the area of leaves above it, plants live in one of two qualitatively different light environments—sun or shade—depending on whether they are overtopped by other plants. Plants have evolved to possess a range of physiological and morphological adaptations that allow individuals to survive, grow, and reproduce in these two different light environments (see this chapter, Field Studies: Kaoru Kitajima). Plant species adapted to high-light environments are called shade-intolerant species, or sun-adapted species. Plant species adapted to low-light environments are called shade-tolerant species, or shade-adapted species. Shade-tolerant and shade-intolerant species differ across a wide variety of phenotypic characteristics that represent adaptations to sun and shade environments. One of the most fundamental differences between shade-intolerant and shade-tolerant plant species lies in their patterns of photosynthesis in response to varying levels of light availability. Shade-tolerant species tend to have a lower light saturation point and a lower maximum rate of photosynthesis than shade-intolerant species (Figure 6.8). These differences relate in part to lower concentrations of the photosynthetic enzyme rubisco (see Section 6.1) found in shade-tolerant plants. Plants must expend a large amount of energy and nutrients to produce rubisco and other components of the photosynthetic apparatus. In shaded environments, low light, not the availability of rubisco to catalyze the fixation of CO2, limits the rate of photosynthesis. Shade-tolerant (shade-adapted) species produce less rubisco as a result. By contrast, production of chlorophyll, the light-harvesting pigment in the leaves, often increases. The reduced energy cost of producing rubisco and other compounds involved in photosynthesis lowers the rate of leaf respiration. Because the LCP is the value of PAR necessary to maintain photosynthesis at a rate that exactly offsets the loss of CO2 in respiration (net photosynthesis = 0), the lower rate of respiration can be offset by a lower rate of photosynthesis, requiring less light. The result is a lower LCP in shade-tolerant species. However, the same reduction in enzyme concentrations that is associated with lower rates of respiration limits the maximum rate at which photosynthesis can occur when light is abundant (high PAR; see Figure 6.8), lowering both the light saturation point and the maximum rate of photosynthesis. The lower maximum rates of photosynthesis inevitably result in a lower rate of net carbon gain and growth rate by shade-tolerant species as compared to shade-intolerant species when growing under high light levels (see Figure 6.8). Field Studies Kaoru Kitajima Department of Botany, University of Florida, Gainesville, Florida A major factor influencing the availability of light to a plant is its neighbors. By intercepting light, taller plants shade individuals below, influencing rates of photosynthesis, growth, and survival. Nowhere is this effect more pronounced than on the forest floor of a tropical rain forest, where light levels are often less than 1 percent of those recorded at the top of the canopy (see Section 4.2). With the death of a large tree, however, a gap is created in the canopy, giving rise to an “island” of light on the forest floor. With time, these gaps in the canopy eventually close because individuals grow up to the canopy from below or neighboring trees expand their canopies, which once again shades the forest floor. How these extreme variations in availability of light at the forest floor have influenced the evolution of rain forest plant species has been the central research focus of plant ecologist Kaoru Kitajima of the University of Florida. Kitajima’s work in the rain forests of Barro Colorado Island in Panama presents a story of plant adaptations to variations in the light environment that includes all life stages of the individual, from seed to adult. Within the rain forests of Barro Colorado Island, the seedlings of some tree species survive and grow only in the high-light environments created by the formation of canopy gaps, whereas the seedlings of other species can survive for years in the shaded conditions of the forest floor. In a series of experiments designed to determine shade tolerance based on patterns of seedling survival in sun and shade environments (see Figure 6.10), Kitajima noted that seed mass (weight) is negatively correlated with seedling mortality rates. Interestingly, large-seeded species not only had higher rates of survival in the shade but also exhibited slower rates of growth after germination. Intuitively, one might think that larger reserves of energy and nutrients within the seed (larger mass) would allow for a faster rate of initial development, but this was not the case. What role does seed size play in the survival and growth of species in different light environments? An understanding of these relationships requires close examination of how seed reserves are used. The storage structure or structures within a seed are called the cotyledon. Upon germination, cotyledons transfer reserve materials (lipids, carbohydrates, and mineral nutrients) into developing shoots and roots. The cotyledons of some species serve strictly as organs to store and transfer seed reserves throughout their life span and are typically positioned at or below the ground level (Figure 1a). The cotyledons of other species develop a second function—photosynthetic carbon assimilation. In these species, the cotyledons function as “seed leaves” and are raised above the ground (Figure 1b). As Kitajima’s research has revealed, the physiological function of cotyledons is crucial in determining the growth response of seedlings to the light environment. The smaller seeds of the shade-intolerant species had photosynthetic cotyledons and developed leaves earlier than did shade-tolerant species with their larger storage cotyledons. These differences reflect two different “strategies” in the use of initial seed reserves. Shade-intolerant species invested reserves in the production of leaves to bring about a rapid return (carbon uptake in photosynthesis), whereas shade-tolerant species kept seed reserves as storage for longer periods at the expense of growth rate. Having used their limited seed reserves for the production of leaves, the shade-intolerant species responded to light availability earlier than did the shade-tolerant species. And without sufficient light, mortality was generally the outcome. So the experiments revealed that larger seed storage in shade-tolerant species does not result in a faster initial growth under shaded conditions (Figure 2). Rather, these species (shade-tolerant) exhibit a conservative strategy of slow use of reserves over a prolonged period. In a later study, Kitajima established that the lower relative growth rate of shade tolerant seedlings is associated with an increased storage of nonstructural carbohydrates (sugars and starches) in the stem and roots (Figure 3a), enabling them to cope with periods of biotic (herbivory and disease) and abiotic (shade) stress. Results of the study show a significant positive relationship between seedling survival during the first year under shaded conditions and the storage of nonstructural carbohydrates (Figure 3b). Whether shade tolerant or intolerant, once seedlings use up seed reserves, they must maintain a positive net carbon gain as a prerequisite for survival (see Section 6.7). What suites of seedling traits allow some species to survive better than others in the shade? To answer this question, Kitajima grew seedlings in the experimental sun and shade environments for an extended period beyond the reserve phase. Under both sun and shade conditions, shade-tolerant species had a greater proportional allocation to roots (relative to leaves), thicker leaves (lower SLA), and as a result, a lower photosynthetic surface area than did shade-intolerant species. As a result, the relative growth rates of shade-intolerant species were consistently greater than those of the shade-tolerant species, both in sun and shaded conditions (see Figure 2). Whereas the characteristics exhibited by the shade-intolerant species reflect strong natural selection for fast growth within light gaps, shade-tolerant species appear adapted to survive for many years in the understory, where their ability to survive attacks by pathogens and herbivores is enhanced by their well-developed reserves within the root system. Bibliography Kitajima, K. 1994. “Relative importance of photosynthetic and allocation traits as correlates of seedling shade tolerance of 15 tropical tree species.” Oecologia 98: 419–428. Kitajima, K. 1996. Ecophysiology of tropical tree seedlings. In Tropical forest plant ecophysiology (S. Mulkey, R. L. Chazdon, and A. P. Smith, eds.), 559–596. New York: Chapman & Hall. Kitajima, K. 2002. “Do shade-tolerant tropical tree seedlings depend longer on seed reserves?” Functional Ecology 16:433–444. Myers, J. A., and Kitajima, K. 2007. “Carbohydrate storage enhances seedling shade and stress tolerance in a neotropical forest.” Journal of Ecology 95:383–395. What processes might create gaps in the forest canopy? How might seed size influence the method of seed dispersal from the parent plant? The variations in photosynthesis, respiration, and growth rate that characterize plant species adapted to different light environments are illustrated in the work of plant ecologist Peter Reich and colleagues at the University of Minnesota. They examined the characteristics of nine tree species that inhabit the cool temperate forests of northeastern North America (boreal forest). The species differ widely in shade tolerance from very tolerant of shaded conditions to very intolerant. Seedlings of the nine species were grown in the greenhouse, and measurements of maximum net photosynthetic rate at light saturation, leaf respiration rate, and relative growth rate (growth rate per unit plant biomass; see Quantifying Ecology 6.1)) were made over the course of the experiment (Figure 6.9). Species adapted to lower light environments (shade-tolerant) are characterized by lower maximum rates of net photosynthesis, leaf respiration, and relative growth rate than are species adapted to higher light environments (shade-intolerant). The difference in the photosynthetic characteristics between shade-tolerant and shade-intolerant species influences not only rates of net carbon gain and growth but also ultimately the ability of individuals to survive in low-light environments. This relationship is illustrated in the work of Caroline Augspurger of the University of Illinois. She conducted a series of experiments designed to examine the influence of light availability on seedling survival and growth for a variety of tree species, both shade-tolerant and shade-intolerant, that inhabit the tropical rain forests of Panama. Augspurger grew tree seedlings of each species under two levels of light availability. These two treatments mimic the conditions found either under the shaded environment of a continuous forest canopy (shade treatment) or in the higher light environment in openings or gaps in the canopy caused by the death of large trees (sun treatment). She continued the experiment for a year, monitoring the survival and growth of seedlings on a weekly basis. Figure 6.10 presents the results for two contrasting species, shade-tolerant and shade-intolerant. The shade-tolerant species (Myroxylon balsamum) showed little difference in survival and growth rates under sunlight and shade conditions. In contrast, the survival and growth rates of the shade-intolerant species (Ceiba pentandra) were dramatically reduced under shade conditions. These observed differences are a direct result of the difference in the adaptations relating to photosynthesis and carbon allocation discussed previously. The higher rate of light-saturated photosynthesis resulted in a high growth rate for the shade-intolerant species in the high-light environment. The associated high rate of leaf respiration and LCP, however, reduced rates of survival in the shaded environment. By week 20 of the experiment, all individuals had died. In contrast, the shade-tolerant species was able to survive in the low-light environment. The low rates of leaf respiration and light-saturated photosynthesis that allow for the low LCP, however, limit rates of growth even in high-light environments. Interpreting Ecological Data Q1. In general, how do net photosynthesis and leaf respiration vary with increasing shade tolerance for the nine boreal tree species? What does this imply about the corresponding pattern of gross photosynthesis with increasing shade tolerance for these species? Q2. Based on the data presented in graphs (a) and (b), how would you expect the light compensation point to differ between Populus tremuloides and Picea glauca? In addition to the differences in photosynthesis and growth rate, shade-tolerant and shade-intolerant species also exhibit differences in patterns of leaf morphology. The ratio of surface area (measured in centimeters squared [cm2]) to weight (g) for a leaf is called the specific leaf area (SLA; cm2/g). The value of SLA represents the surface area of leaf produced per gram of biomass (or carbon) allocated to the production of leaves. Shade-tolerant species typically produce leaves with a greater specific leaf area. This difference in leaf structure effectively increases the surface area for the capture of light (the limiting resource) per unit of biomass allocated to the production of leaves. Marc Abrams and Mark Kubiske of Pennsylvania State University examined leaf morphology of 31 tree species inhabiting the forests of central Wisconsin. The researchers measured both SLA and leaf thickness for individuals of each species growing in full sunlight and in the shaded conditions of the understory. Shade-tolerant species show a consistent pattern of higher SLA and lower leaf thickness than shade-intolerant species (Figure 6.11). Their data also illustrate a second important point regarding plant adaptations: plant species exhibit phenotypic plasticity in response to the light environment. Individuals of both shade-tolerant and shade-intolerant species exhibit an increase in SLA and a reduction in leaf thickness when growing under shaded conditions compared to open, sunny conditions (see Figure 6.11). The increased surface area of leaves in the shade functions to increase the photosynthetic surface area, partially offsetting the reduced rates of photosynthesis. The dichotomy in adaptations between shade-tolerant and shade-intolerant species reflects a fundamental trade-off between characteristics that enable a species to maintain high rates of net photosynthesis and growth under high-light conditions and the ability to continue survival and growth under low-light conditions. The differences in biochemistry, physiology, and leaf morphology exhibited by shade-tolerant species reduce the amount of light required to survive and grow. However, these same characteristics limit their ability to maintain high rates of net photosynthesis and growth when light levels are high. In contrast, plants adapted to high-light environments (shade-intolerant species) can maintain high rates of net photosynthesis and growth under high-light conditions but at the expense of continuing photosynthesis, growth, and survival under shaded conditions. Quantifying Ecology 6.1 Relative Growth Rate When we think of growth rate, what typically comes to mind is a measure of change in size during some period of time, such as change in weight during the period of a week (grams weight gain/week). However, this conventional measure of growth is often misleading when comparing individuals of different sizes or tracking the growth of an individual through time. Although larger individuals may have a greater absolute weight gain when compared with smaller individuals, this may not be the case when weight gain is expressed as a proportion of body weight (proportional growth). A more appropriate measure of growth is the mass-specific or relative growth rate. Relative growth rate (RGR) expresses growth during an observed period of time as a function of the size of the individual. This calculation is performed by dividing the increment of growth during some observed time period (grams [g] weight gain) by the size of the individual at the beginning of that time period (g weight gain/total g weight at the beginning of observation period) and then dividing by the period of time to express the change in weight as a rate (g/g/time). Using RGR to evaluate the growth of plants has an additional value; RGR can be partitioned into components reflecting the influences of assimilation (photosynthesis) and allocation on growth—the assimilation of new tissues per unit leaf area (g/centimeters squared [cm2]/time) called the net assimilation rate (NAR) , and the leaf area per unit of plant weight (cm2/g), called the leaf area ratio (LAR). The NAR is a function of the total gross photosynthesis of the plant minus the total plant respiration. It is the net assimilation gain expressed on a per-unit leaf area basis. The LAR is a function of the amount of that assimilation that is allocated to the production of leaves—more specifically, leaf area—expressed on a per-unit plant weight basis. The LAR can be further partitioned into two components that describe the allocation of net assimilation to leaves, the leaf weight ratio (LWR) , and a measure of leaf density or thickness, the specific leaf area (SLA) . The LWR is the total weight of leaves expressed as a proportion of total plant weight (g leaves/g total plant weight), whereas the SLA is the area of leaf per gram of leaf weight. For the same tissue density, a thinner leaf would have a greater value of SLA. The real value of partitioning the estimate of RGR is to allow for comparison, either among individuals of different species or among individuals of the same species grown under different environmental conditions. For example, the data presented in Table 1 are the results of a greenhouse experiment in which seedlings of Acacia tortilis (a tree that grows on the savannas of southern Africa) were grown under two different light environments: full sun and shaded (50 percent full sun). Individuals were harvested at two times (at four and six weeks), and the total plant weight, total leaf weight, and total leaf area were measured. The mean values of these measures are shown in the table. From these values, estimates of RGR, LAR, LWR, and SLA were calculated. The values of RGR are calculated using the total plant weights at four and six weeks. NAR was then calculated by dividing the RGR by LAR. Because LAR varies through time (between weeks four and six), the average of LAR at four and six weeks was used to characterize LAR in estimating RGR. Note that the average size (weight and leaf area) of seedlings grown in the high-light environment is approximately twice that of seedlings grown in the shade. Despite this difference in size, and the lower light levels to support photosynthesis, the difference in RGR between sun- and shade-grown seedlings is only about 20 percent. By examining the components of RGR, we can see how the shaded individuals are able to accomplish this task. Low-light conditions reduced rates of photosynthesis, subsequently reducing NAR for the individuals grown in the shade. The plants compensated, however, by increasing the allocation of carbon (assimilates) to the production of leaves (higher LWR) and producing thinner leaves (higher SLA) than did the individuals grown in full sun. The result is that individuals grown in the shade have a greater LAR (photosynthetic surface area relative to plant weight), offsetting the lower NAR and maintaining comparable RGR. These results illustrate the value of using the RGR approach for examining plant response to varying environmental conditions, either among individuals of the same species or among individuals of different species adapted to different environmental conditions. By partitioning the components of plant growth into measures directly related to morphology, carbon allocation, and photosynthesis, we can begin to understand how plants both acclimate and adapt to differing environmental conditions. 1When plants are grown under dry conditions (low water availability), there is an increase in the allocation of carbon to the production of roots at the expense of leaves. How would this shift in allocation influence the plant’s LAR? 2understory. Shade-tolerant species show a consistentNitrogen availability can directly influence the rate of net photosynthesis. Assuming no change in the allocation of carbon or leaf morphology, how would an increase in the rate of net photosynthesis resulting from an increase in nitrogen availability influence RGR? Which component of RGR would be influenced by the increase in net photosynthesis? 6.9 The Link between Water Demand and Temperature Influences Plant Adaptations As with the light environment, a range of adaptations has evolved in terrestrial plants in response to variations in precipitation and soil moisture. As we saw in the previous discussion of transpiration (see Section 6.3), however, the demand for water is linked to temperature. As air temperature rises, the saturation vapor pressure will likewise rise, increasing the gradient of water vapor between the inside of the leaf and the outside air and subsequently the rate of transpiration (see Section 2.5). As a result, the amount of water required by the plant to offset losses from transpiration will likewise increase with temperature. Plants exhibit both acclimation and developmental plasticity (both forms of phenotypic plasticity; see Section 5.4 ) in response to changes in water availability and demand on a variety of timescales. When the atmosphere or soil is dry, plants respond by partially closing the stomata and opening them for shorter periods of time. In the early period of water stress, a plant closes its stomata during the hottest part of the day when relative humidity is the lowest (Figure 6.12). It resumes normal activity in the afternoon. As water becomes scarcer, the plant opens its stomata only in the cooler, more humid conditions of morning. Closing the stomata reduces the loss of water through transpiration, but it also reduces CO2 diffusion into the leaf and the dissipation of heat through evaporative cooling. As a result, the photosynthesis rate declines and leaf temperatures may rise. Some plant species, such as evergreen rhododendrons, respond to moisture stress by an inward curling of the leaves. Others show it in a wilted appearance caused by a lack of turgor in the leaves. Leaf curling and wilting allow leaves to reduce water loss and heat gain by reducing the surface area exposed to solar radiation. Interpreting Ecological Data Q1. How does specific leaf area (cm2 of leaf area per gram of leaf weight) change for leaves grown in the shade as compared to the open (full sun)? Does the relationship differ for shade-tolerant and shade-intolerant species (same direction of change)? Q2. How does the observed change in leaf thickness for leaves growing in the shade relate to the changes in specific leaf area (think about what a higher specific leaf area implies about leaf thickness)? Q3. Are the observed changes in leaf morphology under shaded conditions an example of phenotypic plasticity? Interpreting Ecological Data Q1. How does relative humidity change with temperature? Why (see Section 2.5, Figure 2.15)? Q2. How does stomatal conductance (the opening and closing of stomata) respond to changes in relative humidity? Q3. What is the advantage to the plant of partially closing the stomata during the middle of the day? How would the decline in stomatal conductance influence net photosynthesis? Prolonged moisture stress inhibits the production of chlorophyll, causing the leaves to turn yellow or, later in the summer, to exhibit premature autumn coloration. As conditions worsen, deciduous trees may prematurely shed their leaves—the oldest ones dying first. Such premature shedding can result in dieback of twigs and branches. Plants also exhibit developmental plasticity in response variations in the availability of water to meet the demands of transpiration. During development, plants respond to low soil water availability by increasing the allocation of carbon to the production of roots while decreasing the production of leaves (Figure 6.13a). By increasing its production of roots, the plant can explore a larger volume and depth of soil for extracting water. The reduction in leaf area decreases the amount of solar radiation the plant intercepts as well as the surface area that is losing water through transpiration. The combined effect is to increase the uptake of water per unit leaf area while reducing the total amount of water that is lost to the atmosphere through transpiration. The decline in leaf area with decreasing water availability is actually a combined effect of reduced allocation of carbon to the production of leaves (Figure 6.13b) and changes in leaf morphology (size and shape). The leaves of plants grown under reduced water conditions tend to be smaller and thicker (lower specific leaf area; see Section 6.8) than those of individuals growing in more mesic (wet) environments (Figure 6.13c). On an evolutionary timescale, a wide array of adaptations has evolved in plant species in response to variations in the availability of water relative to demand. In some species of plants, referred to as C4 plants and CAM plants, a modified form of photosynthesis has evolved that increases water-use efficiency in warmer and drier environments. The modification involves an additional step in the conversion (fixation) of CO2 into sugars. In C3 plants, the capture of light energy and the transformation of CO2 into sugars occur in the mesophyll cells (see Section 6.1). The products of photosynthesis move into the vascular bundles, part of the plant’s transport system, where they can be transported to other parts of the plant. In contrast, plants possessing the C4 photosynthetic pathway have a leaf anatomy different from that of C3 plants (see Figure 6.3). C4 plants have two distinct types of photosynthetic cells: the mesophyll cells and the bundle sheath cells. The bundle sheath cells surround the veins or vascular bundles (Figure 6.14). C4 plants divide photosynthesis between the mesophyll and the bundle sheath cells. In C4 plants, CO2 reacts with phosphoenolpyruvate (PEP), a three-carbon compound, within the mesophyll cells. This is in contrast to the initial reaction with RuBP in C3 plants. This reaction is catalyzed by the enzyme PEP carboxylase , producing oxaloacetate (OAA) as the initial product. The OAA is then rapidly transformed into the four-carbon molecules of malic and aspartic acids, from which the name C4 photosynthesis is derived. These organic acids are then transported to the bundle sheath cells (see Figure 6.14). There, enzymes break down the organic acids to form CO2, reversing the process that is carried out in the mesophyll cells. In the bundle sheath cells, the CO2 is transformed into sugars using the C3 pathway involving RuBP and rubisco. The extra step in the fixation of CO2 gives C4 plants certain advantages. First, PEP does not react with oxygen, as does RuBP. This eliminates the inefficiency that occurs in the mesophyll cells of C3 plants when rubisco catalyzes the reaction between O2 and RuBP (photorespiration), leading to the production of CO2 and a decreased rate of net photosynthesis (see Section 6.1). Second, the conversion of malic and aspartic acids into CO2 within the bundle sheath cells acts to concentrate CO2. The CO2 within the bundle sheath cells can reach much higher concentrations than in either the mesophyll cells or the surrounding atmosphere. The higher concentrations of CO2 in the bundle sheath cells increase the efficiency of the reaction between CO2 and RuBP catalyzed by rubisco. The net result is generally a higher maximum rate of photosynthesis in C4 plants than in C3 plants. To understand the adaptive advantage of the C4 pathway, we must go back to the trade-off in terrestrial plants between the uptake of CO2 and the loss of water through the stomata. Resulting from the higher photosynthetic rate, C4 plants exhibit greater water-use efficiency (CO2 uptake/H2O loss; see Section 6.4). That is, for a given degree of stomatal opening and associated water loss in transpiration, C4 plants typically fix more carbon in photosynthesis. This increased water-use efficiency can be a great advantage in hot, dry climates where water is a major factor limiting plant growth. However, it comes at a price. The C4 pathway has a higher energy expenditure because of the need to produce PEP and the associated enzyme, PEP carboxylase. The C4 photosynthetic pathway is not found in algae, bryophytes, ferns, gymnosperms (includes conifers, cycads, and ginkgos), or the more primitive flowering plants (angiosperms). C4 plants are mostly grasses native to tropical and subtropical regions and some shrubs characteristic of arid and saline environments, such as Larrea (creosote bush) and Atriplex (saltbush) that dominate regions of the desert southwest in North America. The distribution of C4 grass species in North America reflects the advantage of the C4 photosynthetic pathway under warmer and drier conditions (Figure 6.15). The proportion of grass species that are C4 increases from north to south, reaching a maximum in the southwest. In the hot deserts of the world, environmental conditions are even more severe. Solar radiation is high, and water is scarce. To counteract these conditions, a small group of desert plants, mostly succulents in the families Cactaceae (cacti), Euphorbiaceae, and Crassulaceae, use a third type of photosynthetic pathway—crassulacean acid metabolism (CAM). The CAM pathway is similar to the C4 pathway in that CO2 initially reacts with PEP and is transformed into four-carbon compounds using the enzyme PEP carboxylase. The four-carbon compounds are later converted back into CO2, which is transformed into glucose using the C3 cycle. Unlike C4 plants, however, in which these two steps are physically separate (in mesophyll and bundle sheath cells), both steps occur in the mesophyll cells but at separate times (Figure 6.16). CAM plants open their stomata at night, taking up CO2 and converting it to malic acid using PEP, which accumulates in large quantities in the mesophyll cells. During the day, the plant closes its stomata and reconverts the malic acid into CO2, which it then fixes using the C3 cycle. Relative to both C3 and C4 plants, the CAM pathway is slow and inefficient in the fixation of CO2. But by opening their stomata at night when temperatures are lowest and relative humidity is highest, CAM plants dramatically reduce water loss through transpiration and increase water-use efficiency. In addition to adaptations relating to modifications of the photosynthetic pathway, plants adapted to different soil moisture environments exhibit a variety of physiological and morphological characteristics that function to allow them to either tolerate or avoid drought conditions. Plant species adapted to xeric conditions typically have a lower stomatal conductance (lower number and size of stomata) than species adapted to more mesic conditions. This results in a lower rate of transpiration but also functions to decrease rates of photosynthesis. Because of the higher diffusion gradient of water relative to CO2, the reduction in stomatal conductance functions to increase water-use efficiency. (Data from Mokany et al. 2006.) Plant species adapted to drier conditions tend to have a greater allocation of carbon to the production of roots relative to aboveground tissues (greater ratio of roots to shoots), particularly leaves (Figure 6.17). This pattern of carbon allocation allows the plant to explore a larger volume and depth of soil for extracting water. The decline in leaf area in more xeric environments is actually a combined effect of reduced allocation of carbon to the production of leaves and changes in leaf morphology (size and shape). The leaves of plant species adapted to xeric conditions tend to be smaller and thicker (lower specific leaf area; see Section 6.9) than those of species adapted to more mesic environments (Figure 6.18). In some plants, the leaves are small, the cell walls are thickened, the stomata are tiny, and the vascular system for transporting water is dense. Some species have leaves covered with hairs that scatter incoming solar radiation, whereas others have leaves coated with waxes and resins that reflect light and reduce its absorption. All these structural features function to reduce the amount of energy striking the leaf, enhance the dissipation of heat through convection (see Section 6.6, Figure 6.7), and thus, reduce the loss of water through transpiration. In tropical regions with distinct wet and dry seasons, some species of trees and shrubs have evolved the characteristic of dropping their leaves at the onset of the dry season (see Section 2.6). These plants are termed drought deciduous. In these species, leaf senescence occurs as the dry season begins, and new leaves are grown just before the rainy season begins. Although the decrease in leaf area and corresponding increase in biomass allocated to roots observed for plant species adapted to reduced water availability functions to reduce transpiration and increase the plant’s ability to acquire water from the soil, this shift in patterns of allocation has consequences for plant growth. The reduced leaf area decreases carbon gain from photosynthesis resulting in a reduction in plant growth rate. 6.10 Plants Exhibit Both Acclimation and Adaptation in Response to Variations in Environmental Temperatures As sessile organisms, terrestrial plants are subject to wide variations in temperature on a number of spatial scales and timescales. As we discussed in Chapter 2, at a continental to global scale, temperatures vary with latitude (see Section 2.2). At a local to regional scale, temperatures vary with elevation, slope, and aspect. Seasonal changes in temperature are influenced by both latitude and position relative to the coast (large bodies of water; see Section 2.7 , whereas diurnal (daily) changes in temperature occur everywhere. These patterns of temperature variation are consistent and predictable, and evolution has resulted in a variety of adaptations that enable plants to cope with these variations. When examined across a range of plant species inhabiting different thermal environments, Tmin, Topt, and Tmax (see Figure 6.6) tend to match the prevailing environmental temperatures. Species adapted to cooler environments typically have a lower Tmin, Topt, and Tmax than species that inhabit warmer climates (Figure 6.19). These differences in the temperature response of net photosynthesis are directly related to a variety of biochemical and physiological adaptations that act to shift the temperature responses of photosynthesis and respiration toward the prevailing temperatures in the environment. These differences are most pronounced between plants using the C3 and C4 photosynthetic pathways (see Section 6.9). C4 plants inhabit warmer, drier environments and exhibit higher optimal temperatures for photosynthesis (generally between 30°C and 40°C) than do C3 plants (Figure 6.20). This is in large part because of the higher Topt for PEP carboxylase as compared to rubisco (see Section 6.9). Although species from different thermal habitats exhibit different temperature responses for photosynthesis and respiration, these responses are not fixed. When individuals of the same species are grown under different thermal conditions in the laboratory or greenhouse, divergence in the temperature response of net photosynthesis is often observed (Figure 6.21). In general, the range of temperatures over which net photosynthesis is at its maximum shifts in the direction of the thermal conditions under which the plant is grown. That is to say, individuals grown under cooler temperatures exhibit a lowering of Topt, whereas those individuals grown under warmer conditions exhibit an increase in Topt. This same shift in the temperature response can be observed in individual plants in response to seasonal shifts in temperature (Figure 6.22). These modifications in the temperature response of net photosynthesis are a result of the process of acclimation—reversible phenotypic changes in response to changing environmental conditions (see Section 5.4). In addition to the influence of temperature on plant carbon balance, periods of extreme heat or cold can directly damage plant cells and tissues. Plants that inhabit seasonally cold environments, where temperatures drop below freezing for periods of time, have evolved several adaptations for survival. The ability to tolerate extreme cold, referred to as frost hardening, is a genetically controlled characteristic that varies among species as well as among local populations of the same species. In seasonally changing environments, plants develop frost hardening through the fall and achieve maximum hardening in winter. Plants acquire frost hardiness—the turning of cold-sensitive cells into hardy ones—through the formation or addition of protective compounds in the cells. Plants synthesize and distribute substances such as sugars, amino acids, and other compounds that function as antifreeze, lowering the temperature at which freezing occurs. Once growth starts in spring, plants lose this tolerance quickly and are susceptible to frost damage in late spring. Producing the protective compounds that allow leaves to survive freezing temperatures requires a significant expenditure of energy and nutrients. Some species avoid these costs by shedding their leaves before the cold season starts. These plants are termed winter deciduous, and their leaves senesce during the fall. The leaves are replaced during the spring, when conditions are once again favorable for photosynthesis. In contrast, needle-leaf evergreen species—such as pine (Pinus spp.) and spruce (Picea spp.) trees—contain a high concentration of these protective compounds, allowing the needles to survive the freezing temperatures of winter. Although evolution has resulted in an array of physiological and morphological mechanisms that enable plant species to adjust to the prevailing environmental temperatures, these adaptations have a cost. Most mechanisms (particularly biochemical) for both acclimation and adaptation to temperature involve trade-offs between performance at higher temperatures and performance at lower temperatures. For example, shifts of enzymes and membranes (both acclimation and adaptation) to low temperatures generally result in poor performance (or maladaptation) to high temperatures, that is, shifts in Tmin are associated with a corresponding shift in Tmax . In addition, reductions in Topt are typically associated with a decline in maximum rates of net photosynthesis and growth. 6.11 Plants Exhibit Adaptations to Variations in Nutrient Availability Plants require a variety of chemical elements to carry out their metabolic processes and to synthesize new tissues (Table 6.1). Thus, the availability of nutrients has many direct effects on plant survival, growth, and reproduction. Some of these elements, known as macronutrients, are needed in large amounts. Other elements are needed in lesser, often minute quantities. These elements are called micronutrients, or trace elements. The prefixes micro– and macro– refer only to the quantity of nutrients needed, not to their importance to the organism. If micronutrients are lacking, plants fail as completely as if they lacked nitrogen, calcium, or any other macronutrient. Table 6.1 Essential Elements in Plants Alternate View Element Major Functions Macronutrients Carbon (C) Hydrogen (H) Oxygen (O) Basic constituents of all organic matter Nitrogen (N) Used only in a fixed form: nitrates, nitrites, and ammonium; component of chlorophyll and enzymes (such as rubisco); building block of protein Calcium (Ca) In plants, combines with pectin to give rigidity to cell walls; activates some enzymes; regulates many responses of cells to stimuli; essential to root growth Phosphorus (P) Component of nucleic acids, phospholipids, adenosine triphosphate (ATP), and several enzymes Magnesium (Mg) Essential for maximum rates of enzymatic reactions in cells; integral part of chlorophyll; involved in protein synthesis Sulfur (S) Basic constituent of protein Potassium (K) Involved in osmosis and ionic balance; activates many enzymes Micronutrients Chlorine (Cl) Enhances electron transfer from water to chlorophyll in plants Iron (Fe) Involved in the production of chlorophyll; is part of the complex protein compounds that activate and carry oxygen and transport electrons in mitochondria and chloroplasts Manganese (Mn) Enhances electron transfer from water to chlorophyll and activates enzymes in fatty-acid synthesis Boron (B) Fifteen functions are ascribed to boron in plants, including cell division, pollen germination, carbohydrate metabolism, water metabolism, maintenance of conductive tissue, and translation of sugar Copper (Cu) Concentrates in chloroplasts, influences photosynthetic rates, and activates enzymes Molybdenum (Mo) Essential for symbiotic relationship with nitrogen-fixing bacteria Zinc (Zn) Helps form growth substances (auxins); associated with water relationships; active in formation of chlorophyll; component of several enzyme systems Nickel (Ni) Necessary for enzyme functioning in nitrogen metabolism Of the macronutrients, carbon (C), hydrogen (H), and oxygen (O) form the majority of plant tissues. These elements are derived from CO2 and H2O and are made available to the plant as glucose through photosynthesis. The remaining six macronutrients—nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S)—exist in varying states in the soil and water, and their availability to plants is affected by different processes depending on their location in the physical environment (see Chapters 3 and 4). In terrestrial environments, plants take up nutrients from the soil. Autotrophs in aquatic environments take up nutrients from the substrate or directly from the water. The rate of nutrient absorption (uptake per unit root) depends on concentrations in the external solution (soil or water; Figure 6.23a). As the availability (concentration) of nutrients at the root surface declines, the rate of absorption declines, which eventually results in a decline in tissue nutrient concentrations (Figure 6.23b). In the case of nitrogen, the decrease in leaf concentrations has a direct effect on maximum rates of photosynthesis (Figure 6.23c) through a reduction in the production of rubisco and chlorophyll (see Section 6.1). In fact, more than 50 percent of the nitrogen content of a leaf is in some way involved directly with the process of photosynthesis, with much of it tied up in these two compounds. In response to reduced nutrient availability, carbon is allocated to root growth at the expense of shoot growth, resulting in an increase in the ratio of roots to shoots ( Figure 6.24 ). The increased production of roots is an example of phenotypic plasticity and allows the plant to compensate for the decrease in nutrient absorption per-unit root by increasing the root area and soil volume from which nutrients are absorbed. Despite the shift in patterns of carbon allocation to compensate for reduced nutrient availability, decreased rates of photosynthesis and reduced allocation to leaves resulting from increased allocation to roots inevitably lead to a reduction in growth rate. We have seen that geology, climate, and biological activity alter the availability of nutrients in the soil (see Chapter 4). Consequently, some environments are relatively rich in nutrients, and others are poor. How do plants from low-nutrient environments succeed? Species that inhabit low nutrient environments exhibit a wide array of phenotypic characteristics that enable them to survive, grow, and reproduce under reduced nutrient levels in the soil or water. Compared with species from more fertile soils, species characteristic of infertile soils usually exhibit a low absorption rate. Consequently, in comparison with species from high-nutrient environments, species from infertile soils absorb considerably less nutrient under high-nutrient conditions but similar quantities, and in some cases, even more nutrients at extremely low availability. In addition, plants adapted to low-nutrient environments generally have a greater allocation of carbon to the production of roots and subsequently have a higher ratio of roots to shoots. Because plants require nutrients for synthesizing new tissue, in physiological terms, it is growth that creates demand for nutrients. Conversely, as we have seen, the plant’s uptake rate of the nutrients directly influences its growth rate. This relationship may seem circular, but the important point is that not all plants have the same inherent (maximum potential) rate of growth. In Section 6.8 (see Figure 6.8), we saw how shade-tolerant plants have an inherently lower rate of photosynthesis and growth than shade-intolerant plants do, even under high-light conditions. This lower rate of photosynthesis and growth translates to a lower demand for resources, including nutrients. The same pattern of reduced photosynthesis occurs among plants that are characteristic of low-nutrient environments. Figure 6.25 shows the growth responses of two grass species when soil is enriched with nitrogen. The species that naturally grows in a high-nitrogen environment keeps increasing its rate of growth with increasing availability of soil nitrogen. The species native to a low-nitrogen environment reaches its maximum rate of growth at low to medium nitrogen availability. It does not respond to further additions of nitrogen. Some plant ecologists suggest that a low maximum growth rate is an adaptation to a low-nutrient environment. One advantage of slower growth is that the plant can avoid stress under low-nutrient conditions. A slow-growing plant can still maintain optimal rates of photosynthesis and other metabolic processes crucial for growth under low-nutrient availability. In contrast, a plant with an inherently high rate of growth will show signs of stress. Another hypothesized adaptation to low-nutrient environments is leaf longevity (Figure 6.26). Leaf production has a cost to the plant. This cost can be defined in terms of the carbon and other nutrients required to grow the leaf. At a low rate of photosynthesis, a leaf needs a longer time to “pay back” the cost of its production. As a result, plants inhabiting low-nutrient environments tend to have longer-lived leaves. A good example is the dominance of pine species on nutrient poor, sandy soils in the coastal region of the southeastern United States. In contrast to deciduous tree species, which shed their leaves every year, these pines have needles that live for up to three years. 6.12 Plant Adaptations to the Environment Reflect a Trade-off between Growth Rate and Tolerance As we have seen in the preceding discussion, plant adaptations to the abiotic (physical and chemical) environment represent a fundamental trade-off between phenotypic characteristics that enable high rates of photosynthesis and plant growth in high resource/energy environments and the ability to tolerate (survive, grow, and reproduce) low resource/energy conditions (Figure 6.27). The basic physiological processes, particularly photosynthesis, function optimally under warm temperatures and adequate supplies of light, water, and mineral nutrients. As environmental temperatures get colder and supplies of essential resources decline, plants respond through a variety of mechanisms that function to both increase access to the limiting resource or enhance the ability of the plant to function under the reduced resource/energy conditions. For example, species adapted to low-light environments exhibit lower rates of respiration that enable the maintenance of positive rates of photosynthesis under low-light levels (reduced LCP), but at the same time these characteristics reduce maximum rates of photosynthesis and plant growth under high-light levels. Likewise, characteristics that enable plant species to successfully grow in arid and drought-prone environments, such as increased production of root, reduced leaf area, and smaller leaf size, enhance its ability to access water and reduce rates of water loss through transpiration; however, these same characteristics limit growth rates under mesic conditions. What has emerged in our discussion is a general pattern of evolutionary constraints and trade-offs (costs and benefits) such that the set of phenotypic characteristics that enhance an organism’s relative fitness under one set of environmental conditions inevitably limit its relative fitness under different environmental conditions. The set of phenotypic characteristics that enhance a species’ carbon gain (photosynthesis and plant growth) under high resource/energy environments limit its tolerance (survival and growth) of low resource/energy conditions. Conversely, the phenotypic characteristics that enable a species to survive, grow, and reproduce under low resource/energy conditions limit its ability to maximize growth rate in high resource/energy environments. This basic concept will provide a foundation for later discussions of the interactions of plant species under different environmental conditions (e.g., competition) and how patterns of plant species distribution and abundance change across the landscape. Ecological Issues & Applications Plants Respond to Increasing Atmospheric CO2 In Chapter 2 we discussed that atmospheric concentrations of CO2 have been rising exponentially since the mid-19th century from preindustrial levels of approximately 280 ppm to current levels of 400 ppm (as of June 2013; Chapter 2, Ecological Issues & Applications). In addition to influencing the planet’s energy balance (Section 2.1) and the pH of the oceans (Chapter 3, Ecological Issues & Applications), rising atmospheric concentrations of CO2 have a direct influence on terrestrial plants. Recall that CO2 diffuses from the air into the leaf through the stomatal openings (see Section 6.3). The rate of diffusion is a function of two factors: the diffusion gradient (the difference in CO2 concentration between the air and the leaf interior) and stomatal conductance. Therefore, for a given stomatal conductance, an increase in the CO2 concentration of the air will increase the diffusion gradient, subsequently increasing the movement of CO2 into the leaf interior. In turn, the increased concentration of CO2 within the leaf (mesophyll cells) will result in a greater rate of photosynthesis. The higher rates of diffusion and photosynthesis under elevated atmospheric concentrations of CO2 have been termed the CO2 fertilization effect . The increased rate of photosynthesis under elevated CO2 is in large part a result of reduced photorespiration (Section 6.1). The higher internal concentrations of CO2 increase the affinity of rubisco to catalyze the reaction of RuBP with CO2 (photosynthesis) rather than with O2 (photorespiration). Because photorespiration can reduce photosynthetic rates by as much as 25 percent, the reduction, or even elimination of photorespiration under elevated CO2 greatly enhances potential rates of net photosynthesis. Because C4 plants avoid photorespiration (see discussion of C4 pathway in Section 6.9) they do not exhibit the same increase in photosynthesis under elevated CO2 (Figure 6.28). A second observed response of plants to elevated CO2 is a reduction in stomatal conductance. Recall that stomatal conductance has two components: the number of stoma per unit area (stomatal density) and aperture (the size of the stomatal openings). In the short term, the observed decrease in stomatal conductance under elevated CO2 is caused by a reduction in the aperture (partial closure of the stomata); in the long term, developmental plasticity has been shown to result in a decline in the stomatal density. As with the partial closure of stomata in response to decreased relative humidity (see Section 6.3 and Figure 6.12), this decrease in stomatal conductance functions to reduce rates of transpiration to a greater degree than CO2 uptake and photosynthesis, and therefore results in an increase in water-use efficiency (ratio of photosynthesis to transpiration; see Section 6.3). Most of our fundamental understanding about the response of plants to elevated CO2 has come from experiments in controlled environments, greenhouses, and open-top chambers. However, because these techniques can alter the environment surrounding the plant, the use of free-air CO2 enrichment (FACE) experiments—in which plants are grown at elevated CO2 in the field under fully open-air conditions—provide scientists with the best estimates of how plants will respond to increasing atmospheric concentrations of CO2 in natural ecosystems (Figure 6.29). Elizabeth Ainsworth and Alistair Rogers of the University of Illinois conducted a meta-analysis of the results of FACE experiments and summarized the current understanding of the response of plant species to elevated CO2 concentrations. Averaged across all plant species grown at elevated CO2 (567 ppm) in FACE experiments, stomatal conductance was reduced by 22 percent (Figure 6.30a). There was significant variation among different plant groups. On average, trees, shrubs, and forbs showed a lower percentage decrease in stomatal conductance as compared to C3 and C4 grasses and herbaceous crop species. Although studies have shown a reduction in stomatal density in a wide variety of species when grown under elevated CO2, the observed decrease in stomatal conductance in the FACE experiments was not significantly influenced by a change in stomatal density. Elevated CO2 stimulated light-saturated photosynthetic rates (see Figure 6.2) in C3 plants grown in FACE experiments by an average of 31 percent (Figure 6.30b). The magnitude of increase in photosynthetic rates, however, varied with plant type and environment. Trees showed the largest response to elevated CO2, whereas shrub species showed the smallest response. There was a surprising increase in photosynthetic rates of C4 crop species, however; this stimulation of photosynthesis at elevated CO2 was an indirect effect of reduced stomatal conductance. The reduction in stomatal conductance is associated with improved soil water status (Figure 6.31) as a result of reduced transpiration. Increased rates of photosynthesis in the C4 crops sorghum and maize (corn) were associated with improved water status or were limited to periods of low rainfall. The effects of long-term exposure to elevated CO2 on plant growth and development, however, may be more complicated. Plant ecologists Hendrik Poorter and Marta Pérez-Soba of Utrecht University in the Netherlands reviewed the results from more than 600 experimental studies examining the growth of plants at elevated CO2 levels. These studies examined a wide variety of plant species representing all three photosynthetic pathways: C3, C4, and CAM (Section 6.9). Their results revealed that C3 species respond most strongly to elevated CO2, with an average increase in biomass of 47 percent (Figure 6.32). Data on the response of CAM species were limited, but the mean response for the six species reported was 21 percent. The C4 species examined also responded positively to elevated CO2, with an average increase of 11 percent. Interpreting Ecological Data Q1. How does soil moisture at both soil depths (0.0–0.15 m [circles] and 0.15–0.30 m [squares]) differ between ambient and elevated CO2 chambers during the month of June? Q2. How is the increase in soil moisture under elevated CO2 a result of reduction in stomatal conductance and lower rates of transpiration? Q3. How might the increased soil moisture under elevated CO2 during the summer months affect net photosynthesis of plants (in addition to the direct enhancement of net photosynthesis by elevated CO2)? On average within C3 species, crop species show the highest biomass enhancement (59 percent) and wild herbaceous plants the lowest (41 percent). Most of the experiments with woody species were conducted with seedlings, therefore covering only a small part of their life cycle. The growth stimulation of woody plants was on average 49 percent. In some longer-term studies, the enhanced effects of elevated CO2 levels on plant growth have been short-lived (Figure 6.33). Some plants produce less of the photosynthetic enzyme rubisco at elevated CO2, reducing photosynthesis to rates comparable to those measured at lower CO2 concentrations; this phenomenon is known as downregulation. Other studies reveal that plants grown at increased CO2 levels allocate less carbon to producing leaves and more to producing roots. One factor that has been shown to influence the magnitude of the response of photosynthesis to elevated CO2 is the availability of nitrogen. Under elevated CO2 the ability of the plant to acquire adequate nitrogen and other essential resources to support an enhanced growth potential has been shown to lead to reductions in the production of rubisco and thereby functions to reduce rates of photosynthesis (downregulation) and plant growth. Summary Photosynthesis and Respiration 6.1 Photosynthesis harnesses light energy from the Sun to convert CO2 and H2O into glucose. A nitrogen-based enzyme, rubisco, catalyzes the transformation of CO2 into sugar. Because the first product of the reaction is a three-carbon compound, this photosynthetic pathway is called C3 photosynthesis. Cellular respiration releases energy from carbohydrates to yield energy, H2O, and CO2. The energy released in this process is stored as the high-energy compound ATP. Respiration occurs in the living cells of all organisms. Photosynthesis and Light 6.2 The amount of light reaching a plant influences its photosynthetic rate. The light level at which the rate of CO2 uptake in photosynthesis equals the rate of CO2 loss as a result of respiration is called the light compensation point. The light level at which a further increase in light no longer produces an increase in the rate of photosynthesis is the light saturation point. CO2 Uptake and Water Loss 6.3 Photosynthesis involves two key physical processes: diffusion and transpiration. CO2 diffuses from the atmosphere to the leaf through leaf pores, or stomata. As photosynthesis slows down during the day and demand for CO2 lessens, stomata close to reduce loss of water to the atmosphere. Water loss through the leaf is called transpiration. The amount of water lost depends on the humidity. Water lost through transpiration must be replaced by water taken up from the soil. Water Movement 6.4 Water moves from the soil into the roots, up through the stem and leaves, and out to the atmosphere. Differences in water potential along a water gradient move water along this route. Plants draw water from the soil, where the water potential is the highest, and release it to the atmosphere, where it is the lowest. Water moves out of the leaves through the stomata in transpiration, and this reduces water potential in the roots so that more water moves from the soil through the plant. This process continues as long as water is available in the soil. This loss of water by transpiration creates moisture conservation problems for plants. Plants need to open their stomata to take in CO2, but they can conserve water only by closing the stomata. Aquatic Plants 6.5 A major difference between aquatic and terrestrial plants in CO2 uptake and assimilation is the lack of stomata in submerged aquatic plants. In aquatic plants, there is a direct diffusion of CO2 from the waters adjacent to the leaf across the cell membrane. Plant Energy Balance 6.6 Leaf temperatures affect both photosynthesis and respiration. Plants have optimal temperatures for photosynthesis beyond which photosynthesis declines. Respiration increases with temperature. The internal temperature of all plant parts is influenced by heat gained from and lost to the environment. Plants absorb longwave and shortwave radiation. They reflect some of it back to the environment. The difference is the plant’s net radiation balance. The plant uses some of the absorbed radiation in photosynthesis. The remainder must be either stored as heat in the plant and surrounding air or dissipated through the processes of evaporation (transpiration) and convection. Interdependence of Plant Adaptations 6.7 A wide range of adaptations has evolved in plants in response to variations in environmental conditions. The adaptations exhibited by plants to these features of the environment are not independent for reasons relating to the physical environment and to the plants themselves. Plant Adaptations to High and Low Light 6.8 Plants exhibit a variety of adaptations and phenotypic responses (phenotypic plasticity) in response to different light environments. Shade-adapted (shade-tolerant) plants have low photosynthetic, respiratory, metabolic, and growth rates. Sun plants (shade-intolerant) generally have higher photosynthetic, respiratory, and growth rates but lower survival rates under shaded conditions. Leaves in sun plants tend to be small, lobed, and thick. Shade-plant leaves tend to be large and thin. Alternative Pathways of Photosynthesis 6.9 The C4 pathway of photosynthesis involves two steps and is made possible by leaf anatomy that differs from C3 plants. C4 plants have vascular bundles surrounded by chlorophyll-rich bundle sheath cells. C4 plants fix CO2 into malate and aspartate in the mesophyll cells. They transfer these acids to the bundle sheath cells, where they are converted into CO2. Photosynthesis then follows the C3 pathway. C4 plants are characterized by high water-use efficiency (the amount of carbon fixed per unit of water transpired). Succulent desert plants, such as cacti, have a third type of photosynthetic pathway, called CAM. CAM plants open their stomata to take in CO2 at night, when the humidity is high. They convert CO2 to malate, a four-carbon compound. During the day, CAM plants close their stomata, convert malate back to CO2, and follow the C3 photosynthetic pathway. Adaptations to Temperature 6.10 Plants exhibit a variety of adaptations to extremely cold as well as hot environments. Cold tolerance is mostly genetic and varies among species. Plants acquire frost hardiness through the formation or addition of protective compounds in the cell, where these compounds function as antifreeze. The ability to tolerate high air temperatures is related to plant moisture balance. Plant Adaptations to Nutrient Availability 6.11 Terrestrial plants take up nutrients from soil through the roots. As roots deplete nearby nutrients, diffusion of water and nutrients through the soil replaces them. Availability of nutrients directly affects a plant’s survival, growth, and reproduction. Nitrogen is important because rubisco and chlorophyll are nitrogen-based compounds essential to photosynthesis. Uptake of nitrogen and other nutrients depends on availability and demand. Plants with high nutrient demands grow poorly in low-nutrient environments. Plants with lower demands survive and grow, slowly, in low-nutrient environments. Plants adapted to low-nutrient environments exhibit lower rates of growth and increased longevity of leaves. Trade-off between Growth and Tolerance 6.12 Plant adaptations to the abiotic environment represent a fundamental trade-off between phenotypic characteristics that enable high rates of photosynthesis and plant growth in high resource/energy environments and the ability to tolerate (survive, grow, and reproduce) under low resource/energy conditions. Plant Response to Elevated CO2 Ecological Issues & Applications Plants exhibit two primary responses to CO2: an increase in photosynthesis and a reduction in stomatal conductance. The increase in photosynthesis occurs primarily in C3 plant species and is a response to reduced photorespiration. The decrease in stomatal conductance functions to increase water-use efficiency. Increased rates of photosynthesis result in an increase in growth rates. (Smith 120-121) Smith, Thomas M. Smith and Robert L. Elements of Ecology, 9th Edition. Pearson Learning Solutions, 06/2016. VitalBook file. The citation provided is a guideline. Please check each citation for accuracy before use.
Theory of Evolution by Natural Selection
CHAPTER 7 Smith, T. M., & Smith, R. L. (2015). Elements of Ecology (9th ed.). Boston, MA: Pearson. 7.1 Size Imposes a Fundamental Constraint on the Evolution of Organisms Living organisms occur in a wide range of sizes (Figure 7.1). The smallest animals are around 2–10 micrograms [μg], and the largest living animals are mammals (the blue whale weighing more than 100,000 kilograms (kg) in marine environments and the African elephant at 5000 kg on land). Each taxonomic group of animals has its own particular size range, largely as a result of morphological and physiological constraints. Some groups such as Bryozoa (aquatic colonial animals) contain species all within one or two orders of magnitude, whereas mammals are hugely variable in size. The smallest mammal is a species of shrew weighing only about 2 grams (g) fully grown—or about 100 million (108) times less than the blue whale. Size has consequences for structural and functional relationships in animals, and as such, presents a fundamental constraint on adaptation. Most morphological and physiological features change as a function of body size in a predictable way—by a process known as scaling. Geometrically similar objects, such as cubes or spheres, are referred to as being isometric (Greek for “having equal measurement”). The surface area (SA) and volume (V) of isometric objects are related to their linear dimensions (length = l) to the second and third power, respectively. For example, the surface area of a square is l2, where l is the length of each side. Therefore the surface area of a cube of length l is 6l2 (six sides). In contrast, the volume of the cube is l3 (Figure 7.2a). Interpreting Ecological Data Q1. The volume of a cube of length 4 is 64 (or 43), and the surface area is 96 (or 6 × 42). Now consider a three-dimensional rectangle having the following dimensions of length (l), height (h), and width (w): l = 16, h = 2, w = 2. The volume is l × h × w = 64. The surface area is 4(l × h) + 2(w × h). Calculate the surface area. How does the SA:V ratio differ for these two objects with the same volume? Q2. Which of the two objects (cube or three-dimensional rectangle) would be a more efficient design for exchanging substances between the surface and the body interior? An important consequence of the characteristics of isometric scaling is the relationship between the surface area and volume. If the ratio of surface area to volume (SA:V) is plotted against length (l) for a square, there is an inverse relationship between SA:V and l (Figure 7.2b); smaller bodies have a larger surface area relative to their volume than do larger objects of the same shape. This relationship between surface area and volume imposes a critical constraint on the evolution of animals. The range of biochemical and physiological processes associated with basic metabolism (assimilation and respiration) requires the transfer of materials and energy between the organism’s interior and its exterior environment. For example, most organisms depend on oxygen (O2) to maintain the process of cellular respiration (see Section 6.1). Every living cell in the body, therefore, requires that oxygen diffuse into it to function and survive. Oxygen is a relatively small molecule that readily diffuses across the cell surface; in a matter of seconds, it can penetrate into a millimeter (mm) of living tissue. So the center of a spherical organism that is 1 mm in radius is close enough to the surface that as the organism uses oxygen in the process of respiration, its oxygen is replenished by a steady diffusion from the surface in contact with the external environment (air or water). Now imagine a spherical organism with the radius of a golf ball: approximately 21 mm. It would now take more than an hour for oxygen to diffuse into the center. Although the layers of cells just below the surface would receive adequate oxygen, the continuous depletion of oxygen as it diffused toward the center and the greater distance over which oxygen would have to diffuse, would result in the death of the interior cells (and eventually the organism) because of oxygen depletion. The problem is that, as the size (length or radius) of the organism increases, the surface area of the body across which oxygen diffuses into the organism decreases relative to the interior volume of the body that requires the oxygen (the SA:V ratio decreases as shown in Figure 7.2). So how can animals respond to this constraint so that an adequate flow of oxygen may reach the entire interior of the body in larger organisms? A more complex, convoluted, or wrinkled surface, as shown in Figure 7.3, functions to increase the surface area of an object having the same volume as the golf-ball-shaped organism. The difference is that now (1) no point on the interior of the organism is more than a few millimeters from the surface, and (2) the total surface area over which oxygen can diffuse is much greater. Another way of responding to the constraint is to actively transport oxygen into the interior of the body. Many of the smallest animals have a tube-like shape with a central chamber (Figure 7.4a). These animals draw water into their interior chamber (tube), allowing for the diffusion of oxygen and essential nutrients into the interior cells. Once again, the end result is the increase of the surface area for absorption (diffusion) relative to the volume (SA:V), which assures that every point (cell) in the interior is close enough to the surface to allow for the diffusion of oxygen. As body size increases, however, a more complex network of transport vessels (tubes) is needed for oxygen to reach every point in the body. worms shown in (b). Much of the shape of larger organisms is governed by the transport of oxygen and other essential substances to cells in the interior of the body. To allow for this, a complex set of anatomical structures has evolved in animals. Lungs function as interior chambers that bring oxygen close to blood vessels, where it can be transferred to molecules of hemoglobin for transport throughout the body. A circulatory system with a heart functioning as a pump assures that oxygen-containing blood is actively transported into the minute vessels or capillaries that permeate all parts of the body. These complex systems increase the surface area for exchange, assuring that all cells in the body are well within the maximum distance over which oxygen can diffuse at the rate necessary to support cellular respiration. The same body size constraints apply to the wide range of metabolic processes that require the exchange of materials and energy between the external environment and the interior of the organism. Carbon and other essential nutrients must be taken in through a surface. The food canal (digestive system) in most animals is a tube in which the process of digestion occurs and through which dissolved substances must be absorbed into the circulatory system for transport throughout the body. In the smallest of animals, such as the Bryozoa (see Figure 7.1) or tube worms (Figure 7.4b), the central chamber into which water is drawn also functions as the food canal, where digestion occurs and substances are absorbed. Waste products then exit through the opening as water is expelled. In larger animals, the food canal is a tube extending from the mouth to the anus. As food travels through the tube it is broken down, and essential nutrients and amino acids are absorbed and transported into the circulatory system. The greater the surface area of the food canal, the greater its ability to absorb food. Because surface area increases as the square of length, the larger the animal (which increases as a cube), the greater the surface area of its food canal must be to maintain a constant ratio of surface area to volume. From these simple examples, it should be clear that greater body size requires complex changes in the organism’s structure. These changes represent adaptations that maintain the relationship between the volume (or mass) of living cells that must be constantly supplied with essential resources from the outside environment and the surface area through which these exchanges occur. We will examine various adaptations relating to the ability of animals to maintain the exchange of essential nutrients (food), oxygen, water, and thermal energy (heat) with the external environment. We will also consider how those adaptations are constrained by both body size and the physical environments in which the animals live (Section 7.11). 7.2 Animals Have Various Ways of Acquiring Energy and Nutrients The diversity of potential energy sources in the form of plant and animal tissues requires an equally diverse array of physiological, morphological, and behavioral characteristics that enable animals to acquire (Figure 7.5) and assimilate these resources. There are many ways to classify animals based on the resources they use and how they exploit them. The most general of these classifications is the division based on how animals use plant and animal tissues as sources of food. Animals that feed exclusively on plant tissues are classified as herbivores. Those that feed exclusively on the tissues of other animals are classified as carnivores, and those that feed on both plant and animal tissues are called omnivores. In addition, animals that feed on dead plant and animal matter, called detritus, are detrital feeders, or detritivores (see Chapter 21). Each of these four feeding groups has characteristic adaptations that allow it to exploit its particular diet. Herbivory Because plants and animals have different chemical compositions, the problem facing herbivores is how to convert plant tissue to animal tissue. Animals are high in fat and proteins, which they use as structural building blocks. Plants are low in proteins and high in carbohydrates—many of them in the form of cellulose and lignin in cell walls, which have a complex structure and are difficult to break down (see Chapter 21). Nitrogen is a major constituent of protein. In plants, the ratio of carbon to nitrogen is about 50:1. In animals, the ratio is about 10:1. Herbivores are categorized by the type of plant material they eat. Grazers feed on leafy material, especially grasses. Browsers feed mostly on woody material. Granivores feed on seeds, and frugivores eat fruit. Other types of herbivorous animals, such as avian sapsuckers (Sphyrapicus spp.) and sucking insects such as aphids, feed on plant sap; hummingbirds, butterflies, and a variety of moth and ant species feed on plant nectar (nectivores). Grazing and browsing herbivores, with some exceptions, live on diets high in cellulose (complex carbohydrates made up of hundreds or thousands of simple sugar molecules). In doing so, they face several dietary problems. Their diets are rich in carbon but low in protein. Most of the carbohydrates are locked in indigestible cellulose, and the proteins exist in chemical compounds. Lacking the enzymes needed to digest cellulose, herbivores depend on specialized bacteria and protists living in their digestive tracts. These bacteria and protozoans digest cellulose and proteins, and they synthesize fatty acids, amino acids, proteins, and vitamins. The highest-quality plant food for herbivores, vertebrate and invertebrate, is high in nitrogen in the form of protein. As the nitrogen content of their food increases, the animals’ assimilation of plant material improves, increasing growth, reproductive success, and survival. Nitrogen is concentrated in the growing tips, new leaves, and buds of plants. Its content declines as leaves and twigs mature and become senescent. Herbivores have adapted to this period of new growth. Herbivorous insect larvae are most abundant early in the growing season, and they complete their growth before the leaves mature. Many vertebrate herbivores, such as deer, give birth to their young at the start of the growing season, when the most protein-rich plant foods are available for their growing young. Although availability and season strongly influence food selection, both vertebrate and invertebrate herbivores do show some preference for the most nitrogen-rich plants, which they probably detect by taste and odor. For example, beavers show a strong preference for willows (Salix spp.) and aspen (Populus spp.), two species that are high in nitrogen content. Chemical receptors in the nose and mouth of deer encourage or discourage consumption of certain foods. During drought, nitrogen-based compounds are concentrated in certain plants, making them more attractive and vulnerable to herbivorous insects. However, preference for certain plants means little if they are unavailable. Food selection by herbivores reflects trade-offs between quality, preference, and availability (see this chapter, Field Studies: Martin Wikelski). Carnivory Herbivores are the energy source for carnivores—the flesh eaters. Unlike herbivores, carnivores are not faced with problems relating to digesting cellulose or to the quality of food. Because the chemical composition of the flesh of prey and the flesh of predators is quite similar, carnivores encounter no problem in digesting and assimilating nutrients from their prey. Their major problem is obtaining enough food. Among the carnivores, quantity is more important than quality. Carnivores rarely have a dietary problem because they consume animals that have resynthesized and stored protein and other nutrients from plants in their tissues. Omnivory Omnivores feed on both plants and animals. The food habits of many omnivores vary with the seasons, stages in the life cycle, and their size and growth rate. The red fox (Vulpes vulpes), for example, feeds on berries, apples, cherries, acorns, grasses, grasshoppers, crickets, beetles, and small rodents. The black bear (Ursus americanus) feeds heavily on vegetation—buds, leaves, nuts, berries, tree bark—supplemented with bees, beetles, crickets, ants, fish, and small- to medium-sized mammals. The means of food resource acquisition functions as a major selective agent in the process of natural selection, directly influencing the physiology, morphology, and behavior of animal species. From the specific behaviors and morphologies necessary to locate, capture, and consume different food resources (see Figures 5.10, 5.15, 5.20, 5.21, and 7.5 for specific examples), to the different enzymes and digestive systems necessary to break down and extract essential nutrients from the plant and animal tissues upon which they feed, the means of acquiring food resources has been a major force in the evolution of animal diversity. 7.3 In Responding to Variations in the External Environment, Animals Can Be either Conformers or Regulators Some environments change little on timescales relevant to living organisms, such as the deep waters of the oceans. However, the majority of environments on our planet vary on a wide range of timescales. Regular annual, lunar, and daily cycles (see Chapters 2 and 3) present organisms with predictable changes in environmental conditions, whereas changes on a much shorter timescale of hours, minutes, or seconds as a result of weather are much less predictable. When an animal is confronted with changes in its environment, it can respond in one of two ways: conformity or regulation. In some species, changes in external environmental conditions induce internal changes in the body that parallel the external conditions (Figure 7.6a). Such animals, called conformers, are unable to maintain consistent internal conditions such as body fluid salinity or levels of tissue oxygen. Echinoderms such as the starfish, for example, are osmoconformers whose internal body fluids quickly come to equilibrium with the seawater that surrounds them. The degree to which conformers can survive in changing environments depends largely on the tolerance of their body tissues to internal changes brought about by the changes in the external environment. Conforming largely involves changes at the physiological and biochemical levels. If the internal conditions are allowed to vary widely, be it in terms of temperature, salinity, or oxygen supply, then tissues and cells will need to have biochemical systems in place that can continue to function under the new conditions. In extreme conditions, changes in these systems must be sufficient enough to keep the animal functional, even if at a low level, to avoid potentially irreversible damages, such as freezing, hypoxia (lack of oxygen), or osmotic water loss. Typically the biochemical and physiological changes that occur are simple and energetically inexpensive but carry the cost of reduced activity and growth. Regulators, as their name implies, use a variety of biochemical, physiological, morphological, and behavioral mechanisms to regulate their internal environments over a broad range of external environmental conditions (Figure 7.6b). For example, in contrast to an osmoconformer, an osmoregulator maintains the ion concentrations of its body fluids within a limited range of values when faced with changes in the ion concentration of the surrounding water. In contrast to conformity, regulation may require substantial and energetically expensive changes in biochemistry, physiology, morphology, and behavior. Behavior is often the first line of defense; however, behavior is augmented by substantial physiological and biochemical adjustments. The strategies of conformity and regulation, therefore, have different costs and benefits. The benefit of conformity is a low energetic expenditure associated with mechanisms that maintain internal environmental conditions, but it results in reduced activity, growth, and reproduction as environmental conditions deviate from those that optimize the function of cells, tissues, and organs. In contrast, regulation is generally expensive. For example, regulation of body temperature in terrestrial animals may account for as much as 90 percent of their total energy budget. The benefit, however, is in the level of performance and the greatly extended range of environmental conditions over which activity can be maintained (see Section 7.11). Although conformity and regulation represent two distinct strategies for coping with variations in the external environment, a single species may exhibit a different strategy under different environmental conditions or during different activities (Figure 7.7). Extreme environmental conditions may exceed the ability of a species to regulate internal conditions, resulting in conformity with external environmental conditions (see Section 7.12). In addition, a species may be a regulator with respect to one feature of the environment, such as oxygen, but a conformer with respect to another, such as temperature. 7.4 Regulation of Internal Conditions Involves Homeostasis and Feedback Organisms that maintain their internal environment within narrow limits need some means of regulating internal conditions relative to the external environment, including body temperature, water balance, pH, and the amounts of salts in fluids and tissues. For example, the human body must maintain internal temperatures within a narrow range around 37°C. An increase or decrease of only a few degrees from this range could prove fatal. The maintenance of a relatively constant internal environment in a varying external environment is called homeostasis. Whatever the processes involved in regulating an organism’s internal environment, homeostasis depends on negative feedback—meaning that when a system deviates from the normal or desired state, referred to as the set point, mechanisms function to restore the system to that state. All feedback systems consist of a parameter or variable that is the focus of regulation (e.g., temperature or oxygen) and three components: receptor, integrator, and effector (Figure 7.8). The receptor measures the internal environment for the variable and transfers the information to the integrator. The integrator evaluates the information from the receptor (compares to set point) and determines whether action must be taken by the effector. The effector functions to modify the internal environment (the variable being regulated). The thermostat that controls the temperature in your home is an example of a negative feedback system (see Figure 7.8). If we wish the temperature of the room to be 20°C (68°F), we set that point on the thermostat. When the temperature of the room air falls below that point, a temperature-sensitive device within the thermostat trips the switch that turns on the furnace. When the room temperature reaches the set point, the thermostat responds by shutting off the furnace. Should the thermostat fail to function properly and not shut off the furnace, then the furnace would continue to heat, the temperature would continue to rise, and the furnace would ultimately overheat, causing either a fire or a mechanical breakdown. Among animals, the control of homeostasis is both physiological and behavioral. An example is temperature regulation in humans (see Figure 7.8). The normal temperature, or set point, for humans is 37°C. When the temperature of the environment rises, sensory mechanisms in the skin detect the change. They send a message to the brain, which automatically relays the message to receptors that increase blood flow to the skin, induces sweating, and stimulates behavioral responses. Water excreted through the skin evaporates, cooling the body. When the environmental temperature falls below a certain point, another reaction takes place. This time it reduces blood flow and causes shivering, an involuntary muscular exercise that produces more heat. If the environmental temperature becomes extreme, the homeostatic system breaks down. When it gets too warm, the body cannot lose heat fast enough to maintain normal temperature. Metabolism speeds up, further raising body temperature, until death results from heatstroke. If the environmental temperature drops too low, metabolic processes slow down, further decreasing body temperature until death by freezing ensues. Field Studies Martin Wikelski Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey The isolated archipelago of the Galápagos Islands off the western coast of South America is known for its amazing diversity of animal and plant life. It was the diversity of life on these islands that so impressed the young Charles Darwin and laid the foundations for his theory of natural selection (see Chapter 5). However, one member of the Galápagos fauna has consistently been met with revulsion by historic visitors: the marine iguana, Amblyrhynchus cristatus. Indeed, even Darwin himself commented on this “hideous-looking creature.” Marine iguanas are widely distributed throughout the Galápagos Islands, and individuals of different populations vary dramatically in size (both length and weight). Because of these variations, many of the iguana populations were long considered separate species, yet modern genetic studies have confirmed that all of the populations are part of a single species. What could possibly account for the marked variation in body size among populations? This question has been central to the research of University of Konstanz ecologist Martin Wikelski. Studies by Wikelski and his colleagues during the past decade have revealed an intriguing story of the constraints imposed by variations in the environment of the Galápagos on the evolution of these amazing creatures. In a series of studies, Wikelski and his colleagues have examined differences in body size between two populations of marine iguanas that inhabit the islands of Santa Fe and Genovesa. The populations of these two islands differ markedly in body size (as measured by the snout-vent length), with an average body length of 25 cm (maximum body weight of 900 g) for adult males on Genovesa as compared to 40 cm (maximum body weight of 3500 g) for adult males on the island of Santa Fe. Wikelski hypothesized that these differences reflected energetic constraints on the two populations in the form of food supply. Marine iguanas are herbivorous reptiles that feed on submerged intertidal and subtidal algae (seaweed) along the rocky island shores, referred to as algae pastures. To determine the availability of food for iguana populations, Wikelski and colleagues measured the standing biomass and productivity of pastures in the tidal zones of these two islands. Their results show that the growth of algae pastures correlates with sea surface temperatures. Waters in the tidal zone off Santa Fe (the more southern island) are cooler than those off Genovesa, and as a result, both the length of algae plants and the productivity of pastures are five times greater off Santa Fe than Genovesa. By examining patterns of food intake and growth of marked individuals on the two islands, Wikelski was able to demonstrate that food intake limits growth rate and subsequent body size in marine iguanas, which in turn depends on the availability of algae (Figure 1). Body size differences between members of the two island populations can be explained by differences in food availability. Temporal variations in climate and sea surface temperatures also influence food availability for the marine iguanas across the Galápagos Islands. Marine iguanas can live for up to 30 years, and environmental conditions can change dramatically within an individual’s lifetime. El Niño events usually recur at intervals of three to seven years but were more prevalent in the decade of the 1990s (see Section 2.9). During El Niño years in the Galápagos, sea surface temperatures increase from an average of 18°C to a maximum of 32°C as cold ocean currents and cold-rich upwellings are disrupted. As a result, green and red algal species—the preferred food of marine iguanas—disappear and are replaced by the brown algae, which the iguanas find hard to digest. Up to 90 percent of marine iguana populations on islands can die of starvation as a result of these environmental changes. In studying patterns of mortality during the El Niño events of the 1990s, Wikelski observed the highest mortality rate among larger individuals. This higher mortality rate was directly related to observed differences in foraging efficiency with body size. Wikelski and colleagues determined that although larger individuals have a higher daily intake of food, smaller individuals have a higher food intake per unit body mass, a result of higher foraging efficiency (food intake per bite per gram body mass). Large iguanas on both islands showed a marked decline in body mass during the El Niño events. The result is a strong selective pressure against large body size during these periods of food shortage (Figure 2). Perhaps the most astonishing result of Wikelski’s research is that the marine iguanas exhibit an unusual adaptation to the environmental variations caused by El Niño. Change in body length is considered to be unidirectional in vertebrates, but Wikelski repeatedly observed shrinkage of up to 20 percent in the length of individual adult iguanas. This shrinking coincided with low food availability resulting from El Niño events. Shrinking did not occur equally across all size classes. Wikelski found an inverse relationship between the initial body size of individuals and the observed change in body length during the period of food shortage—larger individuals shrank less than smaller individuals. Shrinkage was found to influence survival. Large adult individuals that shrank more survived longer because their foraging efficiency increased and their energy expenditure decreased (Figure 3). Given the disadvantage of larger body size during periods of low resource availability, what factors were selecting for larger body size in the marine iguana? What is the advantage of being big? Marine iguanas do not compete for food, either with other iguanas or other species of marine herbivores, and their potential predators are not size specific, so these factors were discounted as selective agents influencing body size. Instead Wikelski found that larger body size benefits males in attracting mates. Male iguanas establish display territories, and females select males for mating. Wikelski found that females favor larger males, and therefore larger males have greater reproductive success and relative fitness. It appears that the evolution of body size in the marine iguana is a continuous battle (trade-off) between the advantage of large body size in reproductive success and the disadvantage of large body size during regular periods of resource shortage. Bibliography Wikelski, M. 2005. “Evolution of body size in Galápagos marine iguanas.” Proceedings of the Royal Society B 272:1985–1993. Wikelski, M., and C. Thom. 2000. “Marine iguanas shrink to survive El Niño.” Nature 403:37–38. Wikelski, M., V. Carrillo, and F. Trillmich. 1997. “Energy limits on body size in a grazing reptile, the Galápagos marine iguana.” Ecology 78:2204–2217. Wikelski, M. and F. Trillmich. 1997. “Body size and sexual size dimorphism in marine iguanas fluctuate as a result of opposing natural and sexual selection: An island comparison.” Evolution 51:922–936. Does the mortality of iguanas during El Niño events represent a case of natural selection? Which of the three models of selection best describes the pattern of natural selection? If the iguanas could not shrink during the period of resource shortage, how do you think the El Niño events would influence natural sel 7.5 Animals Require Oxygen to Release Energy Contained in Food Animals obtain their energy from organic compounds in the food they eat; and they do so primarily through aerobic respiration, which requires oxygen (see Section 6.1). Most organisms are oxygen regulators, maintaining their own oxygen consumption even when external (ambient) oxygen levels drop below normal. Oxygen conformity in which oxygen consumption decreases in proportion to decreasing ambient oxygen concentrations is found, however, in some smaller aquatic organisms. Oxygen is easily available in the atmosphere for terrestrial animals. However, for aquatic animals, oxygen may be limiting and its acquisition problematic (see Section 3.6). Differences between terrestrial and aquatic animals in the means of acquiring oxygen reflect the availability of oxygen in the two environments. Minute terrestrial organisms take in oxygen by diffusion across the body surface. With increasing body size, however, direct diffusion across the body surface is insufficient to supply oxygen throughout the body (see Section 7.1). Insects have tracheal tubes that open to the outside through openings (or spiracles) on the body wall (Figure 7.9a). The tracheal tubes carry oxygen directly to the interior of the body allowing diffusion to the cells. Unable to meet oxygen demand through the direct diffusion of oxygen across the body surface, larger terrestrial animals (mammals, birds, and reptiles) have some form of lungs (Figure 7.9b). Unlike tracheal systems that branch throughout the insect body, lungs are restricted to one location. Structurally, lungs have innumerable small sacs that increase surface area across which oxygen readily diffuses into the bloodstream. Amphibians take in oxygen through a combination of lungs and vascularized skin (containing blood vessels). Lungless salamanders are an exception; they live in a moist environment and take in oxygen directly through the skin. In aquatic environments, organisms must take in oxygen from the water or gain oxygen from the air in some way. Marine mammals such as whales and dolphins come to the surface to expel carbon dioxide and take in air containing oxygen to the lungs. Some aquatic insects rise to the surface to fill the tracheal system with air. Others, like diving beetles, carry a bubble of air with them when submerged. Held beneath the wings, the air bubble contacts the spiracles of the beetle’s abdomen. A number of smaller aquatic animals are oxygen conformers, particularly sedentary marine invertebrates, most cnidarians (corals, jellyfish, and sea anemones), and echinoderms (starfish and sea urchins). Most, however, are oxygen regulators, and as with terrestrial animals, the mechanisms controlling oxygen uptake are related to size. Minute aquatic animals, zooplankton, take up oxygen from the water by diffusion across the body surface. Larger aquatic animals have gills, that is, outfoldings of the body surface that are suspended in the water and across which oxygen can diffuse. The gills of many aquatic invertebrates, such as starfish, are simple in shape and distributed over much of their body. In others, such as the crayfish or sea scallop, gills are restricted to specific regions of the body (Figure 7.9c). Fish, the major aquatic vertebrates, pump water through their mouth. The water flows over gills and exits through the back of the gill covers (Figure 7.9d). The close contact with and the rapid flow of water over the gills allows for exchanges of oxygen and carbon dioxide between water and the gills. 7.6 Animals Maintain a Balance between the Uptake and Loss of Water Living cells, both plant and animal, contain about 75–95 percent water. Water is essential for virtually all biochemical reactions within the body, and it functions as a medium for excreting metabolic wastes and for dissipating excess heat through evaporative cooling. For an organism to stay properly hydrated, these water losses must be offset by the uptake of water from the external environment. This balance between the uptake and loss of water with the surrounding environment is referred to as an organism’s water balance (see Section 4.1). Terrestrial animals have three major ways of gaining water and solutes: directly by drinking and eating and indirectly by producing metabolic water in the process of respiration (see Section 6.1). They lose water and solutes through urine, feces, evaporation from the skin, and from the moist air they exhale. Some birds and reptiles have a salt gland, and all birds and reptiles have a cloaca—a common receptacle for the digestive, urinary, and reproductive tracts. They reabsorb water from the cloaca back into the body proper. Mammals have kidneys capable of producing urine with high ion concentrations. In arid environments, animals, like plants, face a severe problem of water balance. Survival depends on either evading the drought or by avoiding its effects. Animals of semiarid and desert regions may evade drought by leaving the area during the dry season and moving to areas where permanent water is available. Many of the large African ungulates (Figure 7.10) and many birds use this strategy. (a) Many of the large ungulate species in the semiarid regions of Africa, such as the wildebeest shown here, migrate over the course of the year, following the seasonal shift in rainfall. (b) The changing distribution of wildebeest populations in the contiguous Serengeti, Masai Mara, and Ngorongoro Conservation areas in East Africa. This seasonal pattern of migration gives these species consistent access to food (grass production) and water. Many animals that inhabit arid regions avoid the effects of drought by entering a period of physiological inactivity (dormancy) termed estivation. During hot, dry periods the spadefoot toad (Scaphiopus couchi) of the southern deserts of the United States remains below ground in a state of estivation and emerges when the rains return (Figure 7.11). Some invertebrates inhabiting ponds that dry up in summer, such as the flatworm Phagocytes vernalis, develop hardened casings and remain in them for the dry period. Other aquatic or semiaquatic animals retreat deep into the soil until they reach the level of groundwater. Many insects undergo diapause, a stage of arrested development in their life cycle from which they emerge when conditions improve. Other animals remain active during the dry season but reduce respiratory water loss. Some small desert rodents lower the temperature of the air they breathe out. Moist air from the lungs passes over cooled nasal membranes, leaving condensed water on the walls. As the rodent inhales, this water humidifies and cools the warm, dry air. There are other approaches to the problem. Some small desert mammals reduce water loss by remaining in burrows by day and emerging by night. Many desert mammals, from kangaroos to camels, extract water from the food they eat—either directly from the moisture content of the plants or from metabolic water produced during respiration—and produce highly concentrated urine and dry feces. Some desert mammals can tolerate a certain degree of dehydration. Desert rabbits may withstand water losses of up to 50 percent and camels of up to 27 percent of their body weight. Unlike terrestrial animals, aquatic animals face the constant exchange of water with the external environment through the process of osmosis. As in the discussion of passive transport of water in plants, osmotic pressure moves water through cell membranes from the side of greater water concentration to the side of lesser water concentration (see Section 6.4). Aquatic organisms living in freshwater are hyperosmotic; they have a higher salt concentration in their bodies than does the surrounding water. Consequently, water moves inward into the body, whereas salts move outward. Their problem is the prevention of uptake, or the removal of excess water, and replacement of salts lost to the external environment. Because of the large disparity between the osmotic concentration of the freshwater and body fluids (e.g., blood), osmoconformity is not an option in freshwater environments. Freshwater fish maintain osmotic balance by absorbing and retaining salts in special cells in the gills and by producing copious amounts of watery urine (Figure 7.12a). Amphibians balance the loss of salts through the skin by absorbing ions directly from the water and transporting them across the skin and gill membranes. In the terrestrial phase, amphibians store water from the kidneys in the bladder. If circumstances demand it, they can reabsorb the water through the bladder wall. The constraint imposed upon marine organisms is opposite of that faced by freshwater organisms. These organisms are hypoosmotic; they have a lower salt concentration in their bodies than does the surrounding water. When the concentration of salts is greater outside the body than within, organisms tend to dehydrate. Osmosis draws water out of the body into the surrounding environment. In marine and brackish environments, organisms have to inhibit water loss by osmosis through the body wall and prevent an accumulation of salts in the body (see Chapter 3). Marine animals have evolved a variety of mechanisms that function to regulate water balance. Some animals are isosmotic; their body fluids have the same osmotic pressure as the surrounding seawater. For example, the bodies of invertebrates such as tunicates, jellyfish, many mollusks, and sea anemones are unable to actively adjust the amount of water in their tissues. These animals are osmoconformers, and their bodies gain water and lose ions until they are isosmotic to the surrounding water. In contrast, others function as osmoregulators, employing a variety of mechanisms to maintain constant salt concentration in their body. Marine bony (teleost) fish absorb saltwater into the gut. They secrete magnesium and calcium through the kidneys and pass these ions off as a partially crystalline paste. In general, fish excrete sodium and chloride, major ions in seawater, by pumping the ions across special membranes in the gills (Figure 7.12b). This pumping process is one type of active transport, moving salts against the concentration gradient, but it has a high energy cost. Sharks and rays retain enough urea to maintain a slightly higher concentration of solute in the body than exists in surrounding seawater. Birds of the open sea and sea turtles can consume seawater because they possess special salt-secreting nasal glands. Seabirds of the order Procellariiformes (e.g., albatrosses, shearwaters, and petrels) excrete fluids in excess of 5 percent salt from these glands. Petrels forcibly eject the fluids through the nostrils; other species drip the fluids out of the internal or external nares. In marine mammals, the kidney is the main route for elimination of salt; porpoises have highly developed kidneys to eliminate salt loads rapidly. 7.7 Animals Exchange Energy with Their Surrounding Environment In principle, an animal’s energy balance is the same as that described for a plant (see Section 6.6). Animals, however, differ significantly from plants in their thermal relations with the environment. Animals can produce significant quantities of heat by metabolism, and their mobility allows them to seek out or escape heat and cold. Body structure influences the exchange of heat between animals and the external environment. Consider a simple thermal model of an animal body (Figure 7.13). The interior or core of the body must be regulated within a defined range of temperature. In contrast, the temperature of the environment surrounding the animal’s body varies. The temperature at the body’s surface, however, is not the same as the air or water temperature in which the animal lives. Rather, it is the temperature at a thin layer of air (or water) called the boundary layer, which lies at the surface just above and within hair, feathers, and scales (see Section 6.6). Therefore, body surface temperature differs from both the air (or water) and the core body temperature. Separating the body core from the body surface are layers of muscle tissue and fat, across which the temperature gradually changes from the core temperature to the body surface temperature. This layer of insulation influences the organism’s thermal conductivity; that is, the ability to conduct or transmit heat. To maintain its core body temperature, the animal must balance gains and losses of heat to the external environment. It does so through changes in metabolic rate and by heat exchange. The core area exchanges heat (produced by metabolism and stored in the body) with the surface area by conduction, that is, the transfer of heat through a solid. Influencing this exchange are the thickness and conductivity of fat and the movement of blood to the surface. The surface layer exchanges heat with the environment by conduction, convection, radiation, and evaporation, which are all influenced by the characteristics of skin and body covering. External environmental conditions heavily influence how animals confront thermal stress. Because air has a lower specific heat and absorbs less solar radiation than water does (Section 3.2), terrestrial animals face more radical and dangerous changes in their thermal environment than do aquatic animals. Incoming solar radiation can produce lethal heat. The loss of radiant heat to the air, especially at night, can result in deadly cold. Aquatic animals live in a more stable energy environment, but they have a lower tolerance for temperature changes (Section 7.9). 7.8 Animal Body Temperature Reflects Different Modes of Thermoregulation Different animal species exhibit different ranges of body temperature in their natural environments. In some, body temperature varies; these species are referred to as poikilotherms (from the Greek poikilos meaning “changeable”). In others species, termed homeotherms (from the Greek homoeo meaning “same”), body temperature is constant or nearly constant. These terms, poikilotherm and homeotherm, are not, however, synonymous with conformers and regulators discussed in Section 7.3. In fact, probably the only true thermoconformers are those animals that live in environments, such as the deep regions of the oceans, that have little to no variations in ambient temperature, and body temperature is virtually identical to the unchanging water temperature. Whether poiklotherms or homeotherms, all animals exhibit some degree of regulation. Although environmental temperatures vary widely, both temporally and spatially, in most organisms body temperature is regulated through behavior, physiology, and morphology. The term thermoregulation does not merely refer to an organism’s internal temperature differing from that of the surrounding environment; rather regulation implies maintaining the average body temperature or variations in body temperature within certain bounds. This requires mechanisms for the organism to sense and respond to its thermal environment. There are two categories of thermal regulation that emphasize the source of thermal energy used to influence body temperature: ectothermy and endothermy. Ectothermy is the process of maintaining body temperature through the exchange of thermal energy with the surrounding environment. Species that use this mechanism of thermoregulation are called ectotherms. In contrast, endothermy is the process of maintaining body temperature through internally generated metabolic heat. Species that use this mechanism of thermoregulation are called endotherms. Although in practice all animals generate some internal heat as a function of metabolic processes, and all animals use external sources of thermal energy to modify body temperatures (such as basking in the sun or seeking shade), these two categories are largely distinct. Endothermic species have the special ability to raise their metabolic activity markedly in excess of their immediate needs, using the resulting metabolic heat to maintain body temperature. In contrast, ectothermic species lack this ability and depend on external sources. So how is the classification of animals based on variations in body temperature (poikilotherm and homeotherm) related to the classification of species based on primary means of temperature regulation (ectotherm and endotherm)? Although some species that inhabit environments where the thermal environment is fairly constant, such as the cold deep waters of the ocean or the litter layer of a tropical rain forest, may exhibit little if any variations in body temperature, the term homeotherm is generally applied to endothermic animals that maintain a constant body temperature through metabolic processes (endothermy). The only animals that fall within the category of endothermic homeotherms are birds and mammals. All other animals are typically classified as poikilotherms. To simplify our proceeding discussion, we will discuss mechanisms in thermoregulation in terms of the two categories of animals based on variations in body temperature: poikilotherms, who use primarily ectothermy, and homeotherms, who primarily regulate body temperature using endothermy. 7.9 Poikilotherms Regulate Body Temperature Primarily through Behavioral Mechanisms The performance (common measures include locomotion, growth, development, fecundity, and survivorship) of poikilotherms varies as a function of body temperature. As with plants (see Section 6.6, Figure 6.6), each species has minimum and maximum temperatures at which performance approaches zero (Tmin and Tmax) and a temperature or range of temperatures over which performance is optimal (Topt; Figure 7.14). Likewise, the relationship between body temperature and performance varies among species and is correlated to the temperature characteristics of the environments they inhabit. Jonathon Stillman and George Somero of Stanford University examined the upper thermal tolerance limits (Tmax) of 20 species of porcelain crabs, genus Petrolisthes, from intertidal and subtidal habitats (see Chapter 25, Figure 25.1) throughout the eastern Pacific. The researchers found that the upper thermal tolerance limit (Tmax) was positively correlated with surface water temperature and maximum temperature in the microhabitats in which the species were found (Figure 7.15). Poikilotherms have a low metabolic rate and a high ability to exchange heat between body and environment (high thermal conductivity; see Figure 7.13). During normal activities, poikilotherms carry out aerobic respiration. Under stress and while pursuing prey, the poikilotherms’ inability to supply sufficient oxygen to the body requires that much of their energy production come from anaerobic respiration, in which oxygen is not used. This process depletes stored energy and accumulates lactic acid in the muscles. (Anaerobic respiration can occur in the muscles of marathon runners and other athletes, causing leg cramps.) Anaerobic respiration metabolism limits poikilotherms to short bursts of activity and results in rapid physical exhaustion. To maintain body temperature in the “preferred” or optimal range, terrestrial and amphibious poikilotherms rely largely on behavioral thermoregulation. They seek out appropriate microclimates where environmental temperatures allow for body temperatures to approach optimal values. Insects such as butterflies, moths, bees, dragonflies, and damselflies bask in the sun to raise their body temperature to the level necessary to become highly active. When they become too warm, these animals seek the shade. Semiterrestrial frogs, such as bullfrogs (Rana catesbeiana) and green frogs (Rana clamitans), exert considerable control over their body temperature. By basking in the sun, frogs can raise their body temperature as much as 10°C above ambient temperature. Because of associated evaporative water losses, such amphibians must be either near or partially submerged in water. By changing position or location or by seeking a warmer or cooler substrate, amphibians can maintain body temperatures within a narrow range. Lizards raise and lower their bodies and change body shape to increase or decrease heat conduction between them and the rocks or soil they rest on. They also seek sunlight or shade or burrow into the soil to adjust their temperatures. Desert beetles, locusts, and scorpions exhibit similar behavior. They raise their legs to reduce contact between their body and the ground, minimizing conduction and increasing convection by exposing body surfaces to the wind. The work of Gabriel Blouin-Demers and Patrick Weatherhead of Carleton University (Ottawa, Canada) illustrates the role that behavior plays in the thermoregulation of snakes. The researchers conducted a series of studies to examine how the body temperatures of individual black rat snakes (Elaphe obsoleta) varied on a daily basis under field conditions. Individual snakes were implanted with sensors that allowed their movement and body temperature to be monitored. Although it is relatively straightforward to monitor the temperatures of the various environments used by the snakes over the course of the day, the more relevant measure is the body temperature that occurs when the snake occupies each of these environments, referred to as the operative environmental temperature. For example, the body temperature of a snake lying on bare soil would not be the same as either the air temperature at the soil surface or the surface temperature of the soil. As presented in Section 7.7, the temperature of the snake is influenced by the physical characteristics of the snake (body shape, color, and thermal conductivity) and the exchange of heat between the snake and the surrounding environment. To better estimate the range of body temperatures that each environment represented, the researchers used physical models of a black rat snake constructed from painted copper tubing that matched the reflectance and conductance properties of the snake’s body. The preferred (selected) body temperature(s) of the black rat snakes was established using thermal gradients in the laboratory. Daily variations in average body temperature for the month of July are presented in Figure 7.16. By selecting a variety of microhabitats (rocks, bare ground, forest, fields, in the open or in the shade; Figure 7.17), individuals were able to maintain their preferred temperature during most of the active period of the day regardless of variations in the operative environmental temperatures. Both the thermal environment and the behavior of the snakes determined the daily pattern of body temperature. When faced with longer-term changes in environmental temperatures, such as seasonal changes, poikilotherms are able to undergo the process of temperature acclimation (see Chapter 5, Section 5.4). Acclimation allows an animal’s relationship between body temperature and performance to shift. For example, under acclimation, an animal’s metabolic reactions in cold temperatures are increased to a level that is closer to that of warm-acclimated individuals, even though their body temperatures are that of the environment (Figure 7.18). This type of thermal acclimation involves specific biochemical changes (such as shifts in enzyme systems). Interpreting Ecological Data Q1. How does the temperature at which the maximum sustained swimming speed occurs differ for each of the two water acclimation treatments? How do these differences relate to the water temperatures to which the crocodiles were acclimated (acclimation treatment temperatures)? Q2. How would you describe the trade-off that occurs between temperature acclimation and swimming speed for the crocodiles (costs and benefits of acclimation to the two different temperatures)? The thermal conductivity of water is approximately 25 times greater than air, meaning that heat is transferred 25 times faster than in air. For this reason, animals in water reach an equilibrium with their surrounding environment much faster than terrestrial animals. As a result, it is much more difficult for the body temperature of aquatic animals to be independent of the surrounding water temperature. Aquatic poikilotherms, when completely immersed, maintain no appreciable difference between their body temperature and the surrounding water. Aquatic poikilotherms are poorly insulated. Any heat produced in the muscles moves to the blood and on to the gills and skin, where heat transfers to the surrounding water by convection. Exceptions are sharks and tunas, which use a form of countercurrent exchange—a blood circulation system that allows them to keep internal temperatures higher than external ones. (See Section 7.13 and Figure 7.23 for discussion of countercurrent heat exchange.) Because seasonal water temperatures are relatively stable, fish and aquatic invertebrates maintain a constant temperature within any given season. They adjust seasonally to changing temperatures by acclimation or physiological adjustment to a change in environmental conditions (for an example of seasonal acclimation see Chapter 5, Section 5.4 and Figure 5.8). They undergo these physiological changes over a period of time. Because water temperature changes slowly through the year, aquatic poikilotherms may adjust slowly. The process of thermal acclimation involves changes in both the upper (Tmax) and lower (Tmin) limits of tolerance to temperature (see Figure 5.4). If they live at the upper end of their tolerable thermal range, poikilotherms’ physiologies adjust at the expense of the ability to tolerate the lower range. Similarly, during periods of cold, the animals’ physiological functions shift to a lower temperature range, which would have been debilitating before. Fish are highly sensitive to rapid change in environmental temperatures. If they are subjected to a sudden temperature change (faster than biochemical and physiological adjustments can occur), they may die of thermal shock. 7.10 Homeotherms Regulate Body Temperature through Metabolic Processes Homeothermic birds and mammals meet the thermal constraints of the environment by being endothermic. Their body temperature is maintained by the oxidization of glucose and other energy-rich molecules in the process of respiration. The process of oxidation is not 100 percent efficient, and in addition to the production of chemical energy in the form of adenosine triphosphate (ATP), some energy is converted to heat energy (see Section 6.1). Because oxygen is used in the process of respiration, an organism’s basal metabolic rate is typically measured by the rate of oxygen consumption. Recall from Section 6.1 that all living cells respire. Therefore, the rate of respiration for homeothermic animals is proportional to their body mass (grams body mass0.75; however, the exponent varies across different taxonomic groups, ranging from 0.6 to 0.9; Figure 7.19). For homeotherms, the thermoneutral zone is a range of environmental temperatures within which the metabolic rates are minimal (Figure 7.20). Outside this zone, marked by upper and lower critical temperatures, metabolic rate increases. Maintenance of a high body temperature is associated with specific enzyme systems that operate optimally within a high temperature range, with a set point of about 40°C. Because efficient cardiovascular and respiratory systems bring oxygen to their tissues, homeotherms can maintain a high level of energy production through aerobic respiration (high metabolic rates). Thus, they can sustain high levels of physical activity for long periods. Independent of external temperatures, homeotherms can exploit a wider range of thermal environments. They can generate energy rapidly when the situation demands, escaping from predators or pursuing prey. To regulate the exchange of heat between the body and the environment, homeotherms use some form of insulation—a covering of fur, feathers, or body fat (see Figure 7.13). For mammals, fur is a major barrier to heat flow, but its insulation value varies with thickness, which is greater in large mammals than in small ones. Small mammals are limited in the amount of fur they carry because a thick coat would reduce their ability to move. The thickness of mammals’ fur changes with the season, a form of acclimation (see Section 5.4). Aquatic mammals—especially of Arctic regions—and Arctic and Antarctic birds such as auklets (Alcidae) and penguins (Spheniscidae) have a heavy layer of fat beneath the skin. Birds reduce heat loss by fluffing the feathers and drawing the feet into them, making the body a round, feathered ball. Some Arctic birds, such as ptarmigan (Lagopus spp.), have feathered feet—unlike most birds, which have scaled feet that function to lose heat. Although the major function of insulation is to keep body heat in, it also keeps heat out. In a hot environment, an animal must either rid itself of excess body heat or prevent heat from being absorbed in the first place. One way is to reflect solar radiation from light-colored fur or feathers. Another way is to grow a heavy coat of fur that heat does not penetrate. Large mammals of the desert, notably the camel, use this method. The outer layers of hair absorb heat and return it to the environment. Some insects—notably moths, bees, and bumblebees—have a dense, furlike coat over the thoracic region that serves to retain the high temperature of flight muscles during flight. The long, soft hairs of caterpillars, together with changes in body posture, act as insulation to reduce convective heat exchange. When insulation fails, many animals resort to shivering, which is a form of involuntary muscular activity that increases heat production. Many species of small mammals increase heat production without shivering by burning (oxidizing) highly vascular brown fat. Found about the head, neck, thorax, and major blood vessels, brown adipose tissue (fat) is particularly prominent in hibernators, such as bats and groundhogs (Marmota monax). Many species employ evaporative cooling to reduce the body heat load. Birds and mammals lose some heat through the evaporation of moisture from the skin. When their body heat is above the upper critical temperature, evaporative cooling is accelerated by sweating and panting. Only certain mammals have sweat glands—in particular, horses and humans. Panting in mammals and gular fluttering in birds function to increase the movement of air over moist surfaces in the mouth and pharynx. Many mammals, such as pigs, wallow in water and wet mud to cool down. 7.11 Endothermy and Ectothermy Involve Trade-offs Prime examples of the trade-offs involved in the adaptations of organisms to their environment are endothermy and ectothermy, which are the two alternative approaches to regulation of body temperature in animals. Each strategy has advantages and disadvantages that enable the organisms to excel under different environmental conditions. For example, endothermy allows homeotherms to remain active regardless of variations in environmental temperatures, whereas environmental temperatures largely dictate the activity of poikilotherms (ectothermy). However, the freedom of activity enjoyed by homeotherms comes at a great energy cost. The maintenance of internal body temperature in homeotherms requires a high metabolic rate, and heat lost to the surrounding environment must be continuously replaced by additional heat generated through respiration. As a result, metabolic costs weigh heavily against homeotherms. In contrast, ectotherms, not needing to burn calories to provide metabolic heat, allocate more of their energy intake to biomass production than to metabolic needs. Ectotherms, therefore, require fewer calories (food) per gram of body weight. A homeotherm must take in some 20 times more food energy than a poikilotherm of equal body mass. Because they do not depend on internally generated body heat, ectotherms can curtail metabolic activity in times of food and water shortage and temperature extremes. Low energy demands enable some terrestrial poikilotherms to colonize areas with limited food and water. One of the most important features influencing its ability to regulate body temperature is an animal’s size. Poikilotherms (ectothermy) absorb heat across their body’s surface but must absorb enough energy to heat the entire body mass (volume). Therefore, the ratio of surface area to volume (SA:V) is a key factor controlling the uptake of heat and the maintenance of body temperature. As an organism’s size increases, the SA:V ratio decreases (see Figure 7.2b). Because the organism must absorb sufficient energy across its surface to warm the entire body mass, the amount of energy or the period of time required to raise body temperature increases. For this reason, ectothermy imposes a constraint on maximum body size for poikilotherms and restricts the distribution of the larger poikilotherms to the warmer, aseasonal regions of the subtropics and tropics. For example, large reptiles such as alligators, crocodiles, iguanas, komodo dragons, anacondas, and pythons are all restricted to warm tropical environments. The constraint that size imposes on homeotherms (endothermy) is opposite that presented earlier for poikilotherms. For homeotherms, it is the body mass (or volume) that produces heat through respiration, while heat is lost to the surrounding environment across the body surface. The smaller the organism, the larger the SA:V ratio, therefore, the greater the relative heat loss to the surrounding environment. To maintain a constant body temperature, the heat loss must be offset by increased metabolic activity (respiration). Thus, small homeotherms have a higher mass-specific metabolic rate (metabolic rate per unit body mass; Figure 7.21) and consume more food energy per unit of body weight than do large ones. Small shrews (Sorex spp.), for example, ranging in weight from 2 to 29 g (see Figure 7.1), require a daily amount of food (wet weight) equivalent to their own body weight. Therefore, small animals must spend most of their time seeking and eating food. The mass-specific metabolic rate (respiration rate per gram of body weight) of small endotherms rises so rapidly that below a certain size, they do not meet their energy demands. On average, 2 g is about as small as an endotherm may be and still maintain a metabolic heat balance; however, this minimum constraint depends on the thermal environment. Some shrews and hummingbirds undergo daily torpor (see Section 7.13) to reduce their metabolic needs. As a result of the conflicting metabolic demands of body temperature and growth, most young birds and mammals are born in an altricial state, meaning they are blind, naked, helpless, and begin life as ectotherms. They depend on the body heat of their parents to maintain their body temperature, which allows most of these young animals’ energy to be allocated to growth. 7.12 Heterotherms Take on Characteristics of Ectotherms and Endotherms Species that sometimes function as homeotherms while at other times as poikilotherms are called temporal heterotherms. At different stages of their daily and seasonal cycle or in certain situations, these animals take on characteristics of endotherms or ectotherms. They can undergo rapid, drastic, repeated changes in body temperature. Interpreting Ecological Data Q1. How does the variable plotted on the y-axis of this graph (mass-specific metabolic weight) differ from the variable plotted on the y-axis of Figure 7.19? Q2. What does the graph imply about the rates of cellular respiration for a mouse compared to a horse? Q3. How would the graph differ if the y-axis was plotted on a logarithmic scale (log10)? Insects are ectothermic and poikilothermic; yet in the adult stage, most species of flying insects are heterothermic. When flying, they have high rates of metabolism, with heat production as great as or greater than that of homeotherms. They reach this high metabolic state in a simpler way than do homeotherms because they are not constrained by the uptake and transport of oxygen through the lungs and vascular system. Insects take in oxygen by demand through openings in the body wall and transport it throughout the body in a tracheal system (see Section 7.5). Temperature is crucial to the flight of insects. Most cannot fly if the temperature of the body muscles is less than 30°C, nor can they fly if muscle temperature is higher than 44°C. This constraint means that an insect must warm up to take off, and it must get rid of excess heat in flight. With wings beating up to 200 times per second, flying insects can produce a prodigious amount of heat. Some insects, such as butterflies and dragonflies, warm up by orienting their bodies and spreading their wings to the sun. Most warm up by shivering their flight muscles in the thorax. Moths and butterflies vibrate their wings to raise thoracic temperatures above ambient temperatures. Bumblebees pump their abdomens without any external wing movements. They do not maintain any physiological set point, and they cool down to ambient temperatures when not in flight. To reduce metabolic costs during periods of inactivity, some small homeothermic animals become heterothermic and enter into torpor daily. Daily torpor is the dropping of body temperature to approximately ambient temperature for a part of each day, regardless of season. Some birds, such as the common poorwill (Phalaenoptilus nuttallii) and hummingbirds (Trochilidae), and small mammals, such as bats, pocket mice, kangaroo mice, and white-footed mice, undergo daily torpor. Such daily torpor seems to have evolved as a way to reduce energy demands over that part of the day when the animals are inactive, allowing them to save the energy that would otherwise be used to maintain a high (normal) body temperature. Nocturnal mammals, such as bats, go into torpor by day; and diurnal animals, such as hummingbirds, go into torpor by night. As the animal goes into torpor, its body temperature falls steeply and oxygen consumption drops (Figure 7.22). With the relaxation of homeothermic responses, the body temperature declines to within a few degrees of ambient temperature. Arousal returns the body temperature to normal rapidly as the animal renews its metabolic heat production. Some birds, such as the common poorwill (Phalaenoptilus nuttallii) and hummingbirds (Trochilidae), and small mammals, such as bats, pocket mice, kangaroo mice, and white-footed mice, undergo daily torpor. Such daily torpor seems to have evolved as a way to reduce energy demands over that part of the day when the animals are inactive, allowing them to save the energy that would otherwise be used to maintain a high (normal) body temperature. Nocturnal mammals, such as bats, go into torpor by day; and diurnal animals, such as hummingbirds, go into torpor by night. As the animal goes into torpor, its body temperature falls steeply and oxygen consumption drops (Figure 7.22). With the relaxation of homeothermic responses, the body temperature declines to within a few degrees of ambient temperature. Arousal returns the body temperature to normal rapidly as the animal renews its metabolic heat production. To escape the rigors of long, cold winters, some heterothermic mammals go into a long, seasonal torpor called hibernation. Hibernation is characterized by the cessation of activity and controlled hypothermia (reduction of body temperature). Homeothermic regulation is relaxed, and the body temperature is allowed to approach ambient temperature. Heart rate, respiration, and total metabolism fall, and body temperature sinks below 10°C. Associated with hibernation are high blood levels of carbon dioxide and an associated decrease in blood pH (increased acidity). This state, called acidosis, lowers the threshold for shivering and reduces the metabolic rate. Hibernating homeotherms, however, are able to rewarm spontaneously using only metabolically generated heat. Among homeotherms, entrance into hibernation is a controlled process difficult to generalize from one species to another. Some hibernators, such as the groundhog (Marmota monax), feed heavily in late summer to store large fat reserves from which they will draw energy during hibernation. Others, like the chipmunk (Tamias striatus), lay up a store of food instead. All hibernators, however, convert to a means of metabolic regulation different from that of the active state. Most hibernators rouse periodically and then drop back into torpor. The chipmunk, with its large store of seeds, spends much less time in torpor than do species that store large amounts of fat. Although popularly said to hibernate, black bears (Ursus americanus), grizzly bears (Ursus arctos), and female polar bears (Ursus maritimus) do not. Instead, they enter a unique winter sleep from which they easily rouse. They do not enter extreme hypothermia but allow body temperatures to decline only a few degrees below normal. The bears do not eat, drink, urinate, or defecate, and females give birth to and nurse young during their sleep; yet they maintain a metabolism that is near normal. To do so, the bears recycle urea, normally excreted in urine, through the bloodstream. The urea is degraded into amino acids that are reincorporated in plasma proteins. Hibernation provides selective advantages to small homeotherms. For them, the maintenance of high body temperature during periods of cold and limited food supply is too costly. It is far less expensive to reduce metabolism and allow the body temperature to drop. Doing so eliminates the need to seek scarce food resources to maintain higher body temperatures. 7.13 Some Animals Use Unique Physiological Means for Thermal Balance Because of an animal’s limited tolerance for heat, storing body heat does not seem like a sound option to maintain thermal balance in the body. But certain mammals, especially the camel, oryx, and some gazelles, do just that. The camel, for example, stores body heat by day and dissipates it by night, especially when water is limited. Its temperature can fluctuate from 34°C in the morning to 41°C by late afternoon. By storing body heat, these animals of dry habitats reduce the need for evaporative cooling and thus reduce water loss and food requirements. Many ectothermic animals of temperate and Arctic regions withstand long periods of below-freezing temperatures in winter through supercooling and developing a resistance to freezing. Supercooling of body fluids takes place when the body temperature falls below the freezing point without actually freezing. The presence of certain solutes in the body that function to lower the freezing point of water influences the amount of supercooling that can take place (see Chapter 3). Some Arctic marine fish, certain insects of temperate and cold climates, and reptiles exposed to occasional cold nights employ supercooling by increasing solutes, notably glycerol, in body fluids. Glycerol protects against freezing damage, increasing the degree of supercooling. Wood frogs (Rana sylvatica), spring peepers (Hyla crucifer), and gray tree frogs (Hyla versicolor) can successfully overwinter just beneath the leaf litter because they accumulate glycerol in their body fluids. Some intertidal invertebrates of high latitudes and certain aquatic insects survive the cold by freezing and then thawing out when the temperature moderates. In some species, more than 90 percent of the body fluids may freeze, and the remaining fluids contain highly concentrated solutes. Ice forms outside the shrunken cells, and muscles and organs are distorted. After thawing, they quickly regain normal shape. To conserve heat in a cold environment and to cool vital parts of the body under heat stress, countercurrent heat exchange has evolved in some animals (Figure 7.23). For example, the porpoise (Phocaena spp.), swimming in cold Arctic waters, is well insulated with blubber. It could experience an excessive loss of body heat, however, through its uninsulated flukes and flippers. The porpoise maintains its body core temperature by exchanging heat between arterial (coming from the lungs) and venous (returning to the lungs) blood in these structures (see Figure 7.23). Veins completely surround arteries, which carry warm blood from the heart to the extremities. Warm arterial blood loses its heat to the cool venous blood returning to the body core. As a result, little body heat passes to the environment. Blood entering the flippers cools, whereas blood returning to the deep body warms. In warm waters, where the animals need to get rid of excessive body heat, blood bypasses the heat exchangers. Venous blood returns unwarmed through veins close to the skin’s surface to cool the body core. Such vascular arrangements are common in the legs of mammals and birds as well as in the tails of rodents, especially the beaver (Castor canadensis). Many animals have arteries and veins divided into small, parallel, intermingling vessels that form a discrete vascular bundle or net known as a rete. In a rete, countercurrent heat exchange occurs as blood flows in opposite directions. Countercurrent heat exchange can also keep heat out. The oryx (Oryx beisa), an African desert antelope exposed to high daytime temperatures, experiences elevated body temperatures yet keeps the highly heat-sensitive brain cool by a rete in its head. The external carotid artery passes through a cavernous sinus filled with venous blood that is cooled by evaporation from the moist mucous membranes of the nasal passages (Figure 7.24). Arterial blood passing through the cavernous sinus cools on the way to the brain, lowering the temperature of the brain by 2°C to 3°C compared to the body core. Countercurrent heat exchangers are not restricted to homeotherms. Certain poikilotherms that assume some degree of endothermism employ the same mechanism. The swift, highly predaceous tuna (Thunnus spp.) and the mackerel shark (Isurus tigris) possess a rete in a band of dark muscle tissue used for sustained swimming effort. Metabolic heat produced in the muscle warms the venous blood, which gives up heat to the adjoining newly oxygenated blood returning from the gills. Such a countercurrent heat exchange increases the power of the muscles because warm muscles contract and relax more rapidly. Sharks and tuna maintain fairly constant body temperatures, regardless of water temperatures. 7.14 An Animal’s Habitat Reflects a Wide Variety of Adaptations to the Environment One of the most fundamental factors defining the relationship between an organism and the environment is the place where it is found, its habitat. The millions of species that inhabit our planet are not found everywhere, nor are they distributed at random across Earth’s environments. There is a correspondence between the different species of organisms and the environments in which they are found. Each species on our planet occupies a unique geographic area where its members live and reproduce. So why are some species found in one location—one habitat—but not in another? The most fundamental constraint on the distribution of species is the ability of the environment to provide essential resources and environmental conditions capable of sustaining basic life processes. For a species to persist in a given location, it must have the physiological potential to survive and reproduce in that habitat. As we have seen, physical and chemical (abiotic) features of the environment, such as pH, temperature, or salinity have a direct influence on basic physiological processes necessary to sustain life. Because physiological processes proceed at different rates under different conditions, any one organism has only a limited range of conditions under which it can survive. For a species to succeed in a given location, however, it must do more than survive. A species’ habitat must provide the wide array of resources necessary to sustain growth and reproduction. The environment must provide essential food resources, cover from potential predators, areas for successful reproduction (courtship, mating, nesting, etc.), and a substrate for a wide array of life activities. As such, a species’ habitat is a reflection of the wide variety of adaptations relating to these processes (e.g., feeding and mating behaviors). For plants and sessile animal species, there is only the hope that gametes or individuals dispersed by a variety of means (wind, water, animals, etc.) arrive at a site that is suitable for successful establishment. In mobile animal species, however, individuals actively choose a specific location to inhabit. The process of selecting a specific location to inhabit is called habitat selection. Given the importance of habitat selection on an organism’s fitness, how are organisms able to assess the suitability of an area in which to settle? What do they seek in a living place? Such questions have been intriguing ecologists for many years. Habitat selection has been most widely studied in birds, particularly in species that defend breeding territories—areas of habitat that the individual defends against other individuals and in which it carries out its life activities (feeding, mating, and rearing of offspring). (See Section 11.10 for discussion of territoriality.) The advantage of studying habitat selection in territorial species is that territories can be delineated, and the features of the habitat within the territory can be described and contrasted with the surrounding environment. Of particular importance is the contrast between those areas that have been chosen as habitats and adjacent areas that have not. Using this approach, a wide variety of studies examining the process of habitat selection in birds has demonstrated a strong correlation between the selection of an area as habitat and structural features of the vegetation. These studies suggest that habitat selection most likely involves a hierarchical approach. Birds appear initially to assess the general features of the landscape: the type of terrain; presence of lakes, ponds, streams, and wetland; gross features of vegetation such as open grassland, shrubby areas, and type and extent of forest. Once in a broad general area, the birds respond to more specific features, such as the structural configuration of the vegetation, particularly the presence or absence of various vertical layers such as shrubs, small trees, tall canopy, and degree of patchiness (Figure 7.25). Frances James, an avian ecologist at Florida State University, coined the term niche gestalt to describe the vegetation profile associated with the breeding territory of a particular species. In addition to the physical structure of the vegetation, the actual plant species present can be important. Certain species of plants might produce preferred food items, such as seeds or fruits, or influence the type and quantity of insects available as food for insectivorous birds. The structural features of the vegetation that define its suitability for a given species may be related to a variety of specific needs, such as food, cover, and nesting sites. The lack of an adequate nesting site may prevent an individual from occupying an otherwise suitable habitat. Animals require sufficient shelter to protect themselves and young from enemies and adverse weather conditions. Cavity-nesting animals require suitable dead trees or other structures in which they can access cavities (Figure 7.26). Habitat selection is a common behavioral characteristic of a wide array of vertebrates other than birds; fish, amphibians, reptiles, and mammals furnish numerous examples. Garter snakes (Thamnophis elegans) living along the shores of Eagle Lake in the sagebrush-ponderosa pine ecosystems of northeastern California select rocks of intermediate thickness (20–30 cm) over thinner and thicker rocks as their retreat sites. Shelter under thin rocks becomes lethally hot; shelter under thicker rocks does not allow the snakes to warm their bodies to preferred temperatures (see Figure 7.17). Insects, too, cue in on habitat features. Thomas Whitham of the University of Utah studied habitat selection by the gall-forming aphid Pemphigus betae, which parasitizes the narrow-leaf cottonwood (Populus angustifolia). He found that aphids select the largest leaves to colonize and discriminate against small leaves. Beyond that, they select the best positions on the leaf. Occupying this particular habitat, which provides the best food source, produces individuals with the highest fitness. Ecological Issues & Applications Increasing Global Temperature Is Affecting the Body Size of Animals Body size is one of the most important phenotypic traits of animals, influencing virtually all physiological and ecological processes (Section 7.1). Variation in body size, both geographically and through time, is a common phenomenon and assumed to be a product of adaptation through natural selection (see Section 5.6). For example, temperatures have a direct effect on an animal’s energy balance, and the relationship between body size and heat exchange (see Section 7.11) is the basis for Bergmann’s rule. Bergmann’s rule states that for endotherms, body size for a species tends to increase with decreasing mean annual temperature. The result is a cline in body size with latitude (for discussion of clines, see Section 5.8): increasing body size with increasing latitude. Similar changes in body size in response to temperature have been observed over time. For example, Ross Secord of the University of Nebraska and colleagues examined shifts in body size in the earliest known horses (family Equidae) during the Paleocene-Eocene Thermal Maximum (PETM) approximately 56 million years ago. A high-resolution record of continental climate and body size of fossil horses shows a directional body size decrease of approximately 30 percent over the first 130,000 years of the PETM (a period of warming), followed by a 76 percent increase in the recovery phase of the PETM as temperatures cooled. Given the strong selective (evolutionary) influence that temperature has on body size, could patterns of recent climate warming over the past century (0.6°C increase in mean global temperature; see discussion in Chapter 2, Ecological Issues & Applications) as a result of human activities have influenced the body size of animals? Numerous studies have documented recent changes in animal body size for local populations over the timescale of decades to a century that are correlated to changes in local temperature. For example, Celine Teplitsky of the University of Helsinki (Finland) and colleagues examined data on mean body mass of red-billed gulls (Larus novaehollandiae scopulinus) from New Zealand over a 47-year period (1958–2004). Results of their analyses show that mean body mass had decreased over time as ambient temperatures increased (Figure 7.27). Similar patterns have been observed from a wide array of bird and mammal species. Some of the most pronounced changes have occurred in animal species that inhabit the northern latitudes, where the largest changes in temperature have occurred over recent decades (see Chapter 2, Ecological Issues & Applications). Yoram Yom-Tov of Tel Aviv University (Israel) and colleagues examined variations in body size of the stone marten (Martes foina) collected in Denmark between 1858 and 1999. Analyses show that skull size (and by implication body size) had two periods of decrease and that these two periods coincided with the periods of increase in ambient temperature (of 0.7°C and 0.55°C, respectively). The changes in temperature for the region of Denmark during these periods are equivalent to the observed global rise in mean global temperature during the 20th century (approximately 0.6°C). Although numerous studies have found evidence of decreasing body size with increasing temperatures across various species of endotherms—a pattern consistent with the hypothesis that smaller body size is more energetically efficient under a warmer climate—other studies have observed increases in body size with rising temperatures. For example, in contrast to the pattern of decreasing body size in the stone marten that he observed in Denmark, Yoram Yom-Tov observed an increase in average body size with increasing ambient temperature for Eurasian otter (Lutra lutra) collected in Sweden between 1962 and 2008. To understand the apparent discrepancy in these studies, it is important to understand that temperature has direct and indirect effects on animals through a variety of processes other than thermoregulation that may complicate the story. For example, temperature can have a significant influence on both food availability and nutrition. In cold regions, the direct effect of an increase in temperature may be a reduction in the cost of body maintenance, thus enabling animals to divert energy toward growth, which results in an increase in body size. Increases in temperature can also have an effect on the availability of food resources. In the case of the observed increase in body size in the Eurasian otter in Sweden, Yom-Tov found that increasing temperature reduced the length of time of ice cover on freshwater lakes in Sweden, thus increasing the access of otter to food resources (fish and invertebrates). Similarly, elevated plant growth and thus food availability under warmer climate conditions is suggested as an explanation for increase in the body size of the masked shrew (Sorex cinereus) and the American marten (Martes americana) in Alaska, and the weasel (Mustela nivalis) and stoat (Mustela ereminea) in Sweden. The story that emerges from these and many other studies is that rising global temperatures are having an impact on the performance and fitness of animal species. As we shall see later in Chapter 27, just as with body size, the variety of responses to a warming climate are as diverse and complex as the array of biological processes that respond both directly and indirectly to temperature and other features of the climate system. Summary Body Size 7.1 Size has consequences for structural and functional relationships in animals; as such, it is a fundamental constraint on adaptation. For objects of similar shape, the ratio of surface area to volume decreases with size; that is, smaller bodies have a larger surface area relative to their volume than do larger objects of the same shape. The decreasing surface area relative to volume with increasing body size limits the transfer of materials and energy between the organism’s interior and its exterior environment. An array of adaptations function to increase the surface area and enable adequate exchange of energy and materials between the interior cells and the external environment. Acquisition of Energy and Nutrients 7.2 To acquire energy and nutrients, herbivores consume plants, carnivores consume other animals, and omnivores feed on both plant and animal tissues. Detritivores feed on dead organic matter. Directly or indirectly, animals get their nutrients from plants. Low concentrations of nutrients in plants can adversely affect the growth, development, and reproduction of plant-eating animals. Herbivores convert plant tissue to animal tissue. Among plant eaters, the quality of food, especially its protein content and digestibility, is crucial. Carnivores must secure a sufficient quantity of nutrients already synthesized from plants and converted into animal flesh. Conformers and Regulators 7.3 When an animal is confronted with changes in the environment, it can respond in one of two ways: conformity or regulation. In conformers, changes in the external environmental conditions induce internal changes in the body that parallel external conditions. Conformity largely involves changes at the physiological and biochemical level that are simple and energetically inexpensive but are accompanied by reduced activity and growth. Regulators use a variety of biochemical, physiological, morphological, and behavioral mechanisms that regulate their internal environments. Regulation is energetically expensive, but the benefit is a high level of performance and a greatly extended range of environmental conditions over which an activity can be maintained. Regulation of Internal Conditions 7.4 To confront daily and seasonal environmental changes, organisms must maintain some equilibrium between their internal and external environment. Homeostasis is the maintenance of a relatively constant internal environment in a variable external environment through negative feedback responses. Through various sensory mechanisms, an organism responds physiologically or behaviorally to maintain an optimal internal environment relative to its external environment. Doing so requires an exchange between the internal and external environments. Oxygen Uptake 7.5 Animals generate energy by breaking down organic compounds primarily through aerobic respiration, which requires oxygen. Differences between terrestrial and aquatic animals in their means of acquiring oxygen reflect the availability of oxygen in the two environments. Most terrestrial animals have some form of lungs, whereas most aquatic animals use gills to transfer gases between the body and the surrounding water. Water Balance 7.6 Terrestrial animals must offset water loss from evaporation, respiration, and waste excretion by consuming or conserving water. Terrestrial animals gain water by drinking, eating, and producing metabolic water. Animals of arid regions may reduce water loss by becoming nocturnal, producing highly concentrated urine and feces, becoming hyperthermic during the day, using only metabolic water, and tolerating dehydration. Aquatic animals must prevent the uptake of, or rid themselves of, excess water. Freshwater fish maintain osmotic balance by absorbing and retaining salts in special cells in the body and by producing copious amounts of watery urine. Many marine invertebrates’ body cells maintain the same osmotic pressure as that in seawater. Marine fish secrete excess salt and other ions through kidneys or across gill membranes. Energy Exchange 7.7 Animals maintain a fairly constant internal body temperature, known as the body core temperature. They use behavioral and physiological means to maintain a heat balance in a variable environment. Layers of muscle fat and surface insulation of scales, feathers, and fur insulate the animal body core against environmental temperature changes. Terrestrial animals face a more dynamic and often more threatening thermal environment than do aquatic animals. Thermal Regulation 7.8 Animals fall into three major groups relating to temperature regulation: poikilotherms, homeotherms, and heterotherms. Poikilotherms, so named because they have variable body temperatures influenced by ambient temperatures, are ectothermic. Animals that depend on internally produced heat to maintain body temperatures are endothermic. They are called homeotherms because they maintain a rather constant body temperature independent of the environment. Many animals are heterotherms that function either as endotherms or ectotherms, depending on external circumstances. Poikilotherms 7.9 Poikilotherms gain heat from and lose heat to the environment. Poikilotherms have low metabolic rates and high thermal conductance. Environmental temperatures control their rates of metabolism. Poikilotherms are active only when environmental temperatures are moderate; they are sluggish when temperatures are cool. They have, however, upper and lower limits of tolerable temperatures. Most aquatic poikilotherms maintain no appreciable difference between body temperature and water temperature. Poikilotherms use behavioral means of regulating body temperature. They exploit variable microclimates by moving into warm, sunny places to heat up and by seeking shaded places to cool off. Many amphibians move in and out of water. Insects and desert reptiles raise and lower their bodies to reduce or increase conductance from the ground or for convective cooling. Desert animals enter shade or spend the heat of day in underground burrows. Homeotherms 7.10 Homeotherms maintain high internal body temperature through the oxidiziation of glucose and other energy-rich molecules. They have high metabolic rates and low thermal conductance. Body insulation of fat, fur, feathers, scales, and furlike covering on many insects reduces heat loss from the body. A few desert mammals employ heavy fur to keep out desert heat and cold. When insulation fails during the cold, many homeotherms resort to shivering and burning fat reserves. For homeotherms, evaporative cooling by sweating, panting, and wallowing in mud and water is an important way of dissipating body heat. Trade-offs in Thermal Regulation 7.11 The two approaches to maintaining body temperature, ectothermy and endothermy, involve trade-offs. Unlike poikilotherms, homeotherms are able to remain active regardless of environmental temperatures. For homeotherms, a high rate of aerobic metabolism comes at a high energy cost. This cost places a lower limit on body size. Because of the low metabolic cost of ectothermy, poikilotherms can curtail metabolic activity in times of food and water shortage and temperature extremes. Their low energy demands enable some terrestrial poikilotherms to colonize areas of limited food and water. Heterotherms 7.12 Based on environmental and physiological conditions, heterotherms take on the characteristics of endotherms or ectotherms. Some normally homeothermic animals become ectothermic and drop their body temperature under certain environmental conditions. Many poikilotherms, notably insects, must increase their metabolic rate to generate heat before they can take flight. Most accomplish this feat by vibrating their wings or wing muscles or by basking in the sun. After flight, their body temperatures drop to ambient temperatures. During environmental extremes, some animals enter a state of torpor to reduce the high energy costs of staying warm or cool. Their metabolism, heartbeat, and respiration slows, and their body temperature decreases. Hibernation (seasonal torpor during winter) involves a complete rearrangement of metabolic activity so that it runs at a very low level. Heartbeat, breathing, and body temperature are all greatly reduced. Unique Physiological Means to Maintain Thermal Balance 7.13 Many homeotherms and heterotherms employ countercurrent circulation, the exchange of body heat between arterial and venous blood reaching the extremities. This exchange reduces heat loss through body parts or cools blood flowing to such vital organs as the brain. Some desert mammals use hyperthermia to reduce the difference between body and environmental temperatures. They store up body heat by day, then release it to the cool desert air by night. Hyperthermia reduces the need for evaporative cooling and thus conserves water. Some cold-tolerant poikilotherms use supercooling, the synthesis of glycerol in body fluids, to resist freezing in winter. Supercooling takes place when the body temperature falls below freezing without freezing body fluids. Some intertidal invertebrates survive the cold by freezing, then thawing with warmer temperatures. Habitat 7.14 The place where an animal is found is called its habitat. A species’ habitat must provide the wide array of resources necessary to sustain growth and reproduction. The environment must provide essential food resources, cover from potential predators, areas for successful reproduction (courtship, mating, nesting, etc.), and a substrate for a wide array of life activities. In mobile animals, the behavioral process of selecting a location to occupy is called habitat selection. Climate Change and Body Size Ecological Issues & Applications Bergmann’s rule states that for endotherms, body size for a species tends to increase with decreasing mean annual temperature. Studies have documented recent changes in animal body size for local populations over the timescale of decades to a century that are correlated to changes in local temperature. Recent global warming has resulted in both increases and decreases in the average size of different animal species. Decreases in body size have been related to the benefit of smaller body size in thermal balance, whereas increases in body size have been associated with increases in food availability under warmer climates. (Smith 148-149) Smith, Thomas M. Smith and Robert L. Elements of Ecology, 9th Edition. Pearson Learning Solutions, 06/2016. VitalBook file. The citation provided is a guideline. Please check each citation for accuracy before use.
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