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.