Amphibian population declines: evolutionary considerations.(Teaching Biology)

Numerous factors, such as global environmental changes, habitat destruction, introduced species, diseases, and chemical pollution, appear to be contributing to amphibian population declines. Moreover, the life history characteristics and behavior of many amphibian species appear to be placing them in jeopardy. Such behaviors and ecological attributes were molded over evolutionary time under selection pressures that acted on amphibians in a variety of ways. Many biologists who study amphibian population declines, however, have failed to consider some of these evolutionary aspects. Better understanding of amphibian population declines requires that scientists and policymakers consider the ecological processes associated with the declines in light of evolutionary principles such as these: Evolution is limited by historical constraints; not all evolution is adaptive; adaptations are often compromises; evolution can only alter existing variations; and evolution takes time.

Keywords: amphibians, population declines, evolution, UVB radiation, amphibian pathogens

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Large losses in biodiversity are being documented around the world in almost all classes of plants and animals (Lawton and May 1995). Although the exact number of species being lost is unknown, some researchers estimate that the rate of extinction is greater than any known in the last 100,000 years (Wilson 1992). This biodiversity crisis is exemplified by the population declines and extinctions of amphibian species around the world (Blaustein et al. 1994a, 1994b, Houlahan et al. 2000, Stuart et al. 2004). In at least some cases, amphibian losses appear to be more severe than losses in other taxa (Pounds et al. 1997, 1999, Stuart et al. 2004). Moreover, declines in amphibian populations are prominent because many of them are occurring in areas that remain relatively undisturbed by humans, such as national parks, conservation areas, and rural areas some distance from urban centers.

There is concern about amphibian population declines in part because many biologists consider amphibians excellent indicators of environmental change and contamination (Blaustein 1994, Blanstein and Wake 1995). Their skin is permeable and exposed (not covered by scales, hair, or feathers), and their eggs, which have no shells, may readily absorb substances from the environment. The complex life cycle of many amphibian species potentially exposes them to both aquatic and terrestrial environmental changes. These attributes, and the fact that amphibians are ectotherms, make them especially sensitive to changes in temperature and precipitation, and to other environmental changes such as greater ultraviolet (UV) radiation.

There appears to be no single cause for amphibian population declines. Like other animals, amphibians are assaulted by a barrage of environmental insults that often act in a complex way (Blaustein and Kiesecker 2002). The causes for declines may vary from region to region and even among different populations of the same species. Synergistic interactions between two or more agents may be involved. There may be interspecific differences, as well as differences between life stages, in how amphibians react to a potentially damaging agent.

Numerous factors, such as global environmental changes, habitat destruction, introduced species, diseases, and chemical pollution, appear to be contributing to amphibian population declines (Lannoo 2005). Adding to the complexity associated with amphibian population declines are observations that the behavior and life history characteristics of many amphibian species appear to place these animals in jeopardy. Such behaviors and ecological attributes were molded over evolutionary time under selection pressures that acted on amphibians in a variety of ways. Many biologists who study amphibian population declines, however, have failed to consider some of these evolutionary aspects. Better understanding of amphibian population declines requires that scientists and policymakers consider the ecological processes associated with the declines in light of evolutionary principles such as these: Evolution is limited by historical constraints; not all evolution is adaptive; adaptations are often compromises; evolution can only alter existing variations; and evolution takes time.

It is important to realize that much evolution occurs through genetic changes that affect rates of development and mechanisms that control specific biological processes. Furthermore, evolutionary changes occur at different rates and on different timescales, depending on the organism. For example, the evolutionary timescale may be in hours for bacteria, in weeks for certain fly species, and in decades for some vertebrate groups.

A historical perspective

Amphibians have evolved behaviors, morphologies, and lifestyles that have allowed them to persist for millions of years, since before dinosaurs existed on Earth and after dinosaurs' extinction. But under today's environmental conditions, these same behaviors and life history characteristics appear to be placing amphibians in harm's way. To illustrate these points, we explore amphibians' responses to two basic problems that many animals confront every day: (1) potential overexposure to sunlight with damaging UVB radiation and (2) exposure to diseases. These two problems are especially significant because they appear to be contributing factors in the declines of a number of amphibian populations around the world (Lannoo 2005).

The examples and generalizations described below may apply differently in different regions of the world, and may vary between species and even between life stages and populations of the same species. However, we believe our points about evolution are relevant to the phenomenon of amphibian population declines in general (figure 1), not only to the examples we provide.

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Amphibian evolution and exposure to sunlight

Over evolutionary time, selection pressures have shaped the life history characteristics and behaviors of amphibians in ways that relate to their exposure to sunlight. Amphibians seek sunlight for thermoregulation and to maximize their growth and development. Especially in temperate regions, larvae often seek shallow, warm water, which ultimately results in an increase in their growth rate (Wollmuth et al. 1987). For example, Cascades frog (Rana cascadae) tadpoles are frequently observed in sunlit, warmer areas in the afternoon (Wollmuth et al. 1987). Mountain yellow-legged frog (Rana muscosa) tadpoles concentrate where water temperatures are highest (near the shore during the day, deeper in the late afternoon and evening; Bradford 1984). In one study, all but the latest-stage bullfrog (Rana catesbeiana) tadpoles selected the warmest microhabitats (Wollmuth and Crawshaw 1988).

Many frog species bask in sunlight for prolonged periods (Hutchinson and Dupre 1992). For example, Lillywhite (1970) found that more than 70 percent of the bullfrogs present in…