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The Society for Integrative and Comparative Biology
How Physiological Methods and Concepts Can Be Useful in Conservation Biology1
1 Department of Integrative Physiology, University of Colorado, Boulder, Colorado 80309-0354
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SYNOPSIS |
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TOP
SYNOPSIS
INTRODUCTIONThe single and synergistic effects of man-made changes to the environment, such as habitat destruction, climate change, introduction of novel, long-lived chemicals into the environment, transport of exotic species and pathogens into new geographical areas, and other factors are predicted to cause widespread population declines and species extinctions of plants and animals in this century. From its inception, physiology has dealt with organismal capacities to deal with environmental change. This essay argues that physiologists, their methods and concepts can make more substantial contributions to Conservation Biology than they have to date. A few of the many ways in which physiologists can participate in Conservation Biology include formulating standards for proof of cause-and-effect relations and providing information about how environmental change could affect organismal energetics, host-pathogen relations, immune defenses, and others.
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INTRODUCTION |
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Environmental change has been the rule, rather than the exception, in the evolution of living organisms. The concentrations of various gases in the atmosphere have varied over time (Berner, 1991
), as have the salinity and pH of oceans. Climates (ambient temperatures and moisture patterns) have oscillated between periods of relative stability and rapid, sometimes abrupt, change (Alley et al., 1997; Severinghaus and Brook, 1999; Taylor, 1999; Overpeck and Webb, 2000).
Species extinctions have contributed importantly to the evolution of life (Myers and Knoll, 2001). Although extinctions of species have been a regular occurrence in the evolution of living organisms, many prehistoric shifts in the environment, especially dramatic and rapid climate change, have been associated with mass extinctions of many species (Donovan, 1989; Hubbard and Gilinsky, 1992; Miller and Foote, 2003). Because existing animals and plants are descendants of those that survived past environmental change, it might be expected that most living organisms possess effective physiological and behavioral mechanisms for coping with at least some degree of environmental variation.
Living organisms are confronted with a continuation of natural environmental variation but, additionally, they are now challenged by dramatic man-made environmental changes superimposed on natural change. These new challenges include habitat destruction, introduction of novel chemicals into the environment, introduction of exotic species that serve as competitors or predators, changes in the environment that foster the emergence of new pathogens or transport of pathogens into areas in which they have not previously occurred, increases in ultra-violet radiation, and synergistic effects of these and other factors. The number of relatively undisturbed ecosystems in the world is shrinking rapidly, and management of human-disturbed ecosystems will become increasingly important in the near future (Palmer et al., 2004). By the year 2050, about 15–37% of existing plant and animal species in specific geographical areas are predicted to become extinct (Thomas et al., 2004) and marine and freshwater biodiversity will be severely reduced (Jenkins, 2003). About half the extant species on earth may experience extinction by the end of this century (Myers and Knoll, 2001). Taxonomic groups that survive into the next century are likely to be composed of inordinately large proportions of "pest and weed" species (Myers and Knoll, 2001).
The field of Conservation Biology is a multi-disciplinary field dedicated to the study of the causes of species declines and extinctions and to the formulation of plans for recovery of threatened species. All too often, conservation biology as a discipline has been called upon in crises for quick management recommendations without adequate time for information gathering (Pullin and Knight, 2001). A plea has recently been made to ecologists to look to the future and incorporate the impact of human activities in ecosystems into their research agendas (Palmer et al., 2004). In a similar appeal to physiologists, this paper promotes the idea that they need to play an increasingly central role in providing specific and useful information for the conservation effort. Although many physiologists have been hesitant to become involved with "applied" science, this essay argues that physiological principles, concepts and methods that are rooted in traditional basic research in physiology, physiological ecology, and evolutionary physiology are fundamentally important in understanding the causes of population declines and in conservation planning.
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PHYSIOLOGY, ECOLOGICAL PHYSIOLOGY, AND EVOLUTIONARY PHYSIOLOGY |
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From its inception, the field of physiology has dealt with the abilities of animals to cope with environmental change. Claude Bernard was the first to recognize that the internal environment (miliéu interior) of an animal was distinct from that of the external environment (miliéu exterior). Walter B. Cannon was among the first to realize that death would result if an animal were placed in an environment in which it could not maintain a constant internal environment (homeostasis) within its range of tolerance (Piantidosi, 2003
). E. F. Adolph (1956) proposed that physiological regulation is directly responsible for the ability of an animal to adapt to new environmental conditions. Such perspectives serve as the basic foundation for current studies of complex signaling pathways and differential gene expression that foster abilities to meet new environmental challenges. Although behavior is considered by many biologists to be a field distinct from physiology, physiologists have long recognized that behavior and physiology capacities work in tandem to deal with environmental change (Bartholomew, 1966; Bennett, 1987; Huey, 2003).
Physiological ecology became an identifiable offshoot of physiology during the late 1940s (Bennett, 1987). This field blends laboratory physiology, with its emphasis on controlled experimental designs, and natural history. While traditional physiological research focused primarily on laboratory or domesticated animals, the initial focus of physiological ecology was on behavioral and physiological adaptations of novel organisms in extreme environments, such as deserts, high altitudes, cold climates, intertidal environments, etc. These studies provided excellent mechanistic examples of how environmental challenges have selected for specialized cellular and/or organ systems.
A new perspective, evolutionary physiology, emerged in the early 1980s (Feder et al., 2000). This new emphasis incorporates concepts from evolutionary biology, population genetics, systematics, and molecular biology. Among the many potential contributions of this perspective to conservation biology, my favorites are the ways in which this field seeks 1) to examine how physiological traits have evolved in the past as environments have changed and to make predictions about the phenotypic and genetic characteristics necessary for particular species to meet new challenges in changing environments, and 2) to evaluate the roles of genotype and phenotype of physiological traits in determining the performance of individuals and their ultimate fitness during environmental change. Evolutionary physiologists have already made use of a number of "natural" experiments to study natural selection on wild populations (Feder et al., 2000) and they may find equally interesting results by studying the increasing number of "unnatural" experiments that are caused by man-made alteration of the environment.
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AMPHIBIAN POPULATION DECLINES AND DEGREDATION OF CORAL REEFS |
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The following sections will highlight several areas of physiological research that can contribute importantly to conservation biology. This list is a personal selection that is by no means exhaustive, nor even comprehensive. Many of the examples cited in each section will come from two of the most widely publicized conservation problems, namely the world-wide decline of amphibian populations and the degradation of coral reefs. The focus on these examples is not intended to minimize the importance of population declines in other taxa (cf., Ceballos and Ehrlich, 2002
)
Although amphibians have existed for roughly 350 million years, drastic reductions in numbers and sizes of many populations have been documented world-wide over the last 30 years, although sporadic declines were noted before that time (Alford et al., 1999; Daszak et al., 1999; Carey, 2000; Carey et al., 2001; Stuart et al., 2004). In several cases, extinction of several species may have occurred (Pounds and Crump, 1994; Stuart et al., 2004). Initially, considerable contention existed about the possible causes of these declines, but habitat destruction, introduction of exotic species, and pathogens have gained widespread acceptance as causes (Linder et al., 2003). Habitat alteration and introduction of invasive species are clearly caused by humans. Whether human disturbances have been involved in the emergence of amphibian pathogens causing mass mortalities is still under consideration.
The history of research into the causes of coral reef degradation has many parallels with that of amphibian population declines (Carey, 2000). All existing coral reefs in the world show some degree of deterioration and up to 60% may be extinct by 2030 (Wilkinson, 2002). The potential causes of declines are still being debated, but hypotheses include overfishing of herbivores that normally maintain coral health, pathogens, elevated water temperatures, release of nutrients into the sea from adjacent coastal agricultural areas, and pollution (Hughes et al., 2003; Pandolfi et al., 2003).
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PROOF OF CAUSATION |
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It is critical to determine accurately the cause(s) of population declines and extinctions in order to provide solid recommendations to decision-makers about how, if possible, to take steps to reverse the declines before extinction results. In most cases of population declines, identification of causal mechanism or mechanisms is very difficult because a number of environmental factors vary continuously, often independently, and may have synergistic effects. Research on amphibian population declines and coral reef degradation began with correlative observations of amphibian declines or coral reef damage and changes in environmental factors. However, researchers in many instances have been satisfied with correlations without attempting to prove causality or to examine possible alternatives. This problem is particularly evident in research on climate change as a potential cause of amphibian mass mortalities (Carey and Alexander, 2003
). In fact, almost all of the existing studies concerning the theoretical effects of climate change on animals and plants are correlational (McCarty, 2001). To be sure, however, when a large number of correlative studies point to the same conclusion, the preponderance of evidence can be compelling (Root et al., 2003).
A framework exists within which causality may be inferred from multiple possible causative factors (Lilienfeld and Stolley, 1994). Space limitations do not allow a detailed examination of these recommendations, but investigators seeking to find more compelling evidence of causality than simple correlation might take additional steps to find evidence of a) strength of association, b) consistency of the observed association, c) specificity of the association, d) temporal sequence of events, e) dose–response relationships, f) biological plausibility of the observed association, and g) experimental evidence (Lilienfeld and Stolley, 1994).
The long tradition in physiology of examining cause-and-effect relationships through rigorous experimental design and control studies can make an important contribution to conservation biology. This tradition might prove especially useful in the design of field experiments, where a number of factors need to be controlled as carefully as possible. The need for well-designed experiments in the field is exemplified by the controversy over whether or not ultra-violet B (U-V B) radiation has caused amphibian declines (Blaustein et al., 1998; Corn, 1998; Palen et al., 2002; Licht, 2003).
Physiologists also have a long tradition of defining cause-and-effect sequences, such as signaling pathways or multi-step processes, like that beginning with neural output from the brain and culminating with muscle contraction. Cause-and-effect must be demonstrated at each step of the sequence. As biologists attempt to put multiple factor, cause-and-effect sequences together to explain mass mortalities, the temptation exists to use correlations at some of the steps, rather than proof of causality (cf., Kiesecker et al., 2001). Physiologists can be particularly helpful in setting standards for the quality of evidence that would constitute compelling proof of cause-and-effect relationships.
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ENERGETICS |
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One of the most important contributions of physiological ecology has been the examination of energy exchange between organisms and their environment. The relationship between energy availability in the environment and competing requirements for energy expenditure within an organism for maintenance, growth, reproduction and other activities has been the subject of countless studies (Bennett, 1987
; Brown, 2004). Because of the importance of the energy exchange of an organism to every aspect of its life and its fitness, the potential contributions of physiologists to understanding how man-made changes in the environment can impact animal and plant survival are too numerous to list comprehensively. However, here are a few examples.
Global temperatures are predicted to increase more rapidly in the next century than in any other equivalent time interval in the last 10,000 years. Additionally, temperature increases, averaging between 1.7 and 4.9°C, will be accompanied by more frequent extreme temperature and precipitation events than in the past (Houghton et al., 2001; Karl and Trenberth, 2003). These types of thermal and moisture changes will inevitably influence microclimatological parameters that will have physiological consequences, such as changes in metabolic costs of maintenance, food requirements, etc. (Huey, 1991). Therefore, organisms must either physiologically and/or behaviorally adjust to new thermal and hydric conditions in their current habitats, or move to new geographic areas that foster survival within lethal limits. Physiological ecologists have extensively evaluated how physiological rate processes vary with temperature and how acclimatization can adjust lethal limits and physiological responses to thermal change within those limits. In recent years, these whole-organism studies have been supplemented by research on the molecular mechanisms that underlie these adjustments (cf., Feder and Hofmann, 1999). As a result, physiologists can assist with making predictions about the physiological and biochemical mechanisms that would foster survival in new thermal and hydric environments.
The symbiotic relationship between corals and their algal zooanthellae provides an example of the consequences of sudden variation in ambient conditions. An unusually high amount of coral "bleaching," caused by algal death, was associated with an average 1°C increase in sea surface temperatures during an unusually strong El Niño in 1998 (see review by Knowlton, 2001). Bleaching leads to low growth rates, reduced fecundity, and ultimately death (Hughes et al., 2003). Whether other factors, such as over-fishing of herbivorous fish that perform vital services for coral health, serve to make coral reefs more vulnerable to temperature changes is still being debated (Pandolfi et al., 2003). The observations that non-bleached corals frequently occur adjacent to non-bleached corals of the same species suggest that genetic variation in the ability of corals to survive sudden environmental change may prove to be important in future survival (Hughes et al., 2003). Physiological capacities of various species for acclimatization to higher temperatures will undoubtedly be an important determinant of survival of coral reefs at progressively warmer ocean temperatures (Gates and Edmunds, 1999; Knowlton, 2001).
The impact of future temperature changes on each species will depend on a number of factors, including the rate of temperature change, the ability to acclimatize to new temperatures, and the capacity for new acclimatory capacities to develope quickly. Several new studies point to evidence that some organisms with limited capacities to acclimatize to thermal or hydric change may be limited in their ability to survive future climate changes (Stillman, 2003; Hoffmann et al., 2003). A thermal change in a given habitat will not only affect the energetics of each species, but will affect the interactions, such as predator-prey or competition, among species. As a result, the structure of entire ecosystems will change. For example, a 3.5°C increase in water temperature over a 10-yr period directly caused unpredictable, but striking, changes in the abundance of various taxonomic groups of a marine benthic community. These changes in the population sizes of various taxons then had dramatic indirect effects on interactions among species (Schiel et al., 2004).
Several studies have noted a correlation between animal energy requirements, ambient temperature, and latitudinal distributions of animals (Root, 1988; Allen et al., 2002). One of most frequently observed responses of animals and plants to climate change has been alteration in geographical distributions (Root et al., 2003; Crozier, 2004). Relatively little information is available to date about the consequences of these distributional shifts. Movement into new habitats is likely to expose organisms to a new suite of predators and/or competitors. Additionally, because the distributions of animals and their food supplies are unlikely to change simultaneously, mismatches between energy demands and food supplies, like those already been noted in a few populations of birds (Thomas et al., 2001), are likely to become increasingly frequent. This phenomenon may prove especially acute for herbivores, since phenology of many plant species may prove to be more vulnerable to environmental change than that of animals (Sparks and Carey, 1995).
Man-made environmental changes can affect fitness of some species by changing the availability of critical nutrients for reproduction. For instance, acid rain in Europe has caused both a decline in the availability of snails used by Great Tits (Parus major) as a source of calcium for eggshell synthesis and a corresponding decline in reproductive success of these birds (Graveland et al., 1994).
Animals in areas polluted with sub-lethal levels of novel chemicals that have been introduced into the environment by human activities are likely to have significant energetic effects that could affect fitness. For example, some species may face food limitations because their prey, such as insect populations, could be decimated by toxic chemicals. Alternatively, the metabolic costs associated with combating the toxic biochemical and cellular effects of contaminants could prove debilitating. For instance, metabolic rates of larval bullfrogs (Rana catesbeiana) exposed to the residues from coal combustion were significantly (30– 100%) higher than metabolic rates of similarly-sized tadpoles in control sites (Rowe et al., 1998). When tadpoles from an uncontaminated site were transplanted to the contaminated site, metabolic rates increased within a few days to levels of tadpoles that had been continuously held at those sites. The elevated metabolic costs associated with exposure to chemical contamination can result in retarded growth rates, delayed metamorphosis, and other factors that will diminish fitness.
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EMERGENCE OF NOVEL PATHOGENS AND PATHOGEN POLLUTION |
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Infectious pathogens have plagued animals and plants throughout the evolution of life and have periodically caused population declines or even extinctions (Wilson et al., 1994
). These pathogens presumably evolved over time through mutation and other natural mechanisms. However, a surprising number of novel pathogens for both humans and wildlife appear to have "emerged" in the last few decades (Colwell and Patz, 1998; Daszak et al., 2000). Emerging diseases are those that have recently increased in incidence, switched hosts or geographical distribution, or increased their virulence (Daszak and Cunningham, 2003). Increased surveillance by world-wide medical communities may contribute to the perception that the number of newly emerged pathogens is rising. However, the unusually large number of new observations of recently "emerged" pathogens of humans and wildlife suggests that man-made environmental change may be fostering a greater rate of emergence than has occurred previously.
Pathogens have been implicated in the destruction of some coral reefs and in the decline of many amphibian populations (Carey, 2000). Identification of the pathogen(s) attacking coral reefs is still under examination, but a novel, newly emerged chytrid fungus, Batrachochytrium dendrobatidis, appears to be responsible for widespread population extinctions and even perhaps several species extinctions of amphibians on five continents (Carey et al., 2003). Most fungi of the Phylum Chytridiomycota break down vegetable matter in aquatic systems, but some are parasitic on diatoms or insects. Batrachochytrium is first known to be pathogenic to a vertebrate (Longcore et al., 1999). This fungus attacks keratinocytes in the epithelium of metamorphosed individuals and the keratin in tadpole mouthparts, but the mechanism by which it kills amphibians is still unknown (Carey et al., 2003). The high mortality (50–100%) (Carey et al., 2003) of infected amphibian populations suggests that Batrachochytrium is a novel pathogen to which they have not been exposed before. Although it remains unclear whether anthropogenic environmental disruption may have played a role its emergence from non-pathogenic forms, humans have probably facilitated movement of this pathogen around the world by transporting infected amphibians, such as the African clawed frog (Xenopus laevis) or bullfrogs (Rana catesbeiana), for medical research or for commercial purposes (Mazzoni et al., 2003; Daszak et al., 2004).
"Pathogen pollution" is a new term applied to the transmission of pathogens by human activities across geographical and/or evolutionary boundaries to host species into which they have not previously occurred (Daszak and Cunningham, 2003; Cunningham et al., 2003). When humans knowingly or unknowingly foster the movement of pathogens into new animal hosts, the results can be devastating for the hosts (Cunningham et al., 2003). In some cases, human activities contribute in multiple ways to the transmission of pathogens. For instance, approximately 225 million wild house finches (Carpodacus mexicanus) have died in the last few years from Mycoplasma gallicepticum, a common disease of poultry (Nolan et al., 1998). The transmission of the pathogen to and among house finches was facilitated both by raising poultry in crowded conditions in which their food, feces, and other materials attract foraging house finches, and also by providing backyard seed feeders that foster pathogen transmission within abnormally high densities of house finches. As humans continue to encroach into wildlife habitat and spread pathogens through agricultural practices and trade in wildlife, plants, and soil, pathogen pollution is predicted to result in increasing numbers population and species extinctions of wildlife and plants (Daszak and Cunningham, 2003).
Basic research on host-pathogen interactions and the effects of reduced host genetic diversity and the effects of human activities on these interactions is greatly needed. While this research will require a multi-disciplinary approach, physiologists can contribute specific information concerning how homeostatic processes become exaggerated or attenuated during disease and how such responses may be detrimental to the organisms (Piantidosi, 2003). For instance, hyperplasia of the ventral pelvic skin of amphibians is an apparent pathophysiological response to infection by Batrachochytrium dendrobatidis (Carey et al., 2003). While this response may be a short-term attempt to protect the animal from the pathogen, it is hypothesized that the interference of the thickened skin layers with ion and water transport may cause the amphibian's death.
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COMPARATIVE IMMUNOLOGY |
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As human activities increase the exposure of plants and animals to novel pathogens, improved understanding of immune systems of living organisms is greatly needed. Comparative and environmental physiology textbooks have traditionally ignored immunology. Comparative Immunology exists as a field of study, with its own society, The International Society of Developmental and Comparative Immunology, and journal, Developmental and Comparative Immunology. The field of Evolutionary Immunology has recently been defined (Warr et al., 2003
). Its goals are to understand how both plants and animals define self and defend self against non-self (parasites and microbial pathogens) and how immune mechanisms have evolved in response to new pathogens and environmental challenges.
The ability to recognize and defend against pathogens arose early in evolution, with the result that all multicellular animals posses at least some components of the "innate" immune system. Plants share with animals a few "innate-like" immune strategies (Staskawicz et al., 2001). Innate immune systems are effective against the majority of pathogens. The adaptive immune system (T- and B-cell mediated responses to specific pathogens) evolved in fishes and is present in the immune systems of all advanced vertebrate taxa. Although many aspects of animal immune systems, such as signal transduction pathways and transcription factors, are remarkably conserved, sufficient information exists about various immune systems to conclude that the characteristics of the immune system of one group cannot accurately be extrapolated even to those of close taxonomic relatives (Warr et al., 2003).
Immune systems of multicellular invertebrates, such as corals (Hayes and Goreau, 1998), have received relatively less study than those of mammals and other vertebrates. In some taxa, such as amphibians, the immune system of only one species (Xenopus) out of about 4,500 extant groups has been relatively well characterized. When mass mortalities due to disease are noted in the field, the lack of information about immune systems of the hosts becomes critical. For instance, our knowledge about immune system of Xenopus has proven relatively unhelpful in determining why so many species of amphibians are so susceptible to Batrachochytrium because Xenopus appears to be resistant to this pathogen (Carey et al., 1999, 2003).
Major gaps exist in our knowledge of how immune systems function in field situations, especially under conditions of changing temperature. Even less information is available concerning how immune systems function against a pathogen outside the laboratory. The tremendous diversity of organisms, pathogens, and immune effectors poses an obvious challenge: how do we select the appropriate pathogens and organisms as model systems with which to predict how a multitude of species will survive at onslaught of novel pathogens, particularly if they have to cope with additional stressors at the same time?
If infectious disease is going to play an increasingly important role in the survival or extinctions of organisms, additional research is needed to understand the energetic consequences of maintaining the immune system and mounting an immune defense. Maintenance of immunological competence may be the "most important determinant of life-time reproductive success and fitness for many species" (Lochmiller and Deerenberg, 2000, p. 96). Although combating the damaging effects of toxins and cellular destruction produced by a pathogen are energetically costly themselves, the activation of an immune defense itself can be sufficiently expensive that energetic trade-offs are required. For example, the breeding success, food intake, and feather growth of female pied flycatchers (Ficedula hypoleuca) were significantly reduced below comparable levels of control birds after injection of an inactivated pathogen (Ilmonen et al., 2000). Although the costs associated with mounting an immune defense appear to largely represent up-regulation of the acquired immune system found only in vertebrates, a few experiments on invertebrates indicate that the costs of meeting the challenge of an infectious pathogen can also have marked effects on energy expenditure and fitness. For example, bumblebees (Bombus terrestris) may have to increase food intake to meet the costs of activating innate immune pathways (Moret and Schmid-Hempel, 2000). Additionally, in laboratory exposures to the parasite Asobara tabida, the costs of mounting an immune response caused decreased population growth rates in adult Drosophila melaogaster (Hoang, 2002)
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SPECIES AND INDIVIDUAL VARIABILITY |
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Individual and species variation in physiological capacities have been recognized from the beginning of the study of physiological adaptation to the environment. Significant progress has been made in understanding the genetic underpinnings of some aspects of physiological variation among individuals or species. Study of such variation will become even more critical than it has been in the past in a world in which environmental stressors will test the limits of survival. The animals that will survive in the future may not be those that might have been the most fit in past environments. Those that could run the farthest or the fastest are not necessarily those that possess slight advantages in tolerance of single or multiple interactions of increased temperatures, man-made toxins, and other novel environmental conditions.
Understanding the physiological role of specific genes, their distribution, dominance, and interaction with other genes, coupled with the ability to map genes having fitness-related traits, will become increasingly important as population sizes of some species decline. Small, isolated populations of declining species are threatened by effects of genetic drift and inbreeding on fitness. Additionally, small population sizes may interact synergistically in specialized species to elevate the risk of extinction (Davies et al., 2004). Several natural experiments on small populations of endangered species have found greater inbreeding depression in more stressful environments than in others (Hedrick and Kalinowski, 2000) and such inbreeding has demonstrable negative effects on survival of catastrophes, such as extreme weather events. For instance, about 89% of adults in a population of song sparrows (Melospiza melodia) died during a winter storm. Since individuals in the population had been marked and inbreeding coefficients were known for each, it was concluded that those that survived were significantly less inbred than those that died (Keller et al., 1994).
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CONCLUSIONS |
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Living organisms are currently confronted with an array of natural and man-made environmental challenges. Although climate change, pathogen pollution, toxic chemicals, habitat alteration, and introduction of invasive species are only a few of these challenges, living organisms will increasingly have to deal with combinations of factors and synergisms among factors, rather than single threats.
Corals and amphibians are ancient lineages that play vital roles in their respective ecosystems. Loss of many, if not most, of these organisms to their ecosystems will result in potentially irreversible disruption of these ecosystems. And yet, world-wide declines in coral and amphibian populations are only two examples of many similar phenomena that are rapidly becoming evident. Difficult decisions will have to be made about which few species can be saved with the limited funds available for conservation. Physiologists can contribute a wealth of knowledge, concepts and perspectives that should make them invaluable participants in making such decisions.
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ACKNOWLEDGMENTS |
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Preparation of this review was assisted by an NSF IRCEB grant DEB 0213851.
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FOOTNOTES |
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1 From the Symposium EcoPhysiology and Conservation: The Contribution of Endocrinology and Immunology presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 5–9 January 2004, at New Orleans, Louisiana.
2 E-mail: careyc{at}colorado.edu
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