Background Information on Amphibians

Five mass extinction events and consequent biotic rebounds have been documented in the geologic record. The event at the end of the Permian period eliminated 96 percent of extant species (Eldredge, 1999). The causes of these historical extinctions are difficult to determine. Current theories suggest that the mass extinctions resulted from changes in global climate, increases in volcanic activity (Eldredge, 1999), or collision between the earth and some extraterrestrial object (Alvarez and others, 1980). There is strong evidence that a sixth extinction event is ongoing (Wilson, 1993). In contrast to the earlier mass extinctions, human activities, specifically direct habitat destruction, overexploitation of natural resources, and introduction of non-native species are believed to be the cause of this sixth event (Eldredge, 1999). Amphibians evolved around 350 million years ago during the Devonian period and have survived three mass extinction events. The three main groups of amphibians are frogs, salamanders, and caecilians. In North America, the greatest density of anuran (frog) species is in the south Atlantic coastal plain and the Gulf Coast, while the greatest density of salamanders is in the southern Appalachian mountains, extending south to the Gulf Coast. In the last few decades, the decline and in some cases loss of amphibian populations has been identified as a global problem (Houlahan and others, 2000). Many instances of amphibian decline have been documented, and many causes of amphibian decline have been proposed (Blaustein and Wake, 1990; Orchard, 1992; Koonz, 1992; Corn, 1994; Dodd, 1997; Alford and Richards, 1999; Rouse and others, 1999). Instances of amphibian population decline may be the result of natural fluctuations (Pechmann and others, 1991), but some of the declines are likely the result of changes in local conditions, such as the introduction of a contaminant or the alteration of a habitat. Declining amphibian populations have also been attributed to regional phenomena like changes in climate, introduction of non-native competitive species, or exposure to pathogens. Many herpetologists believe that a combination of stresses is being placed on amphibian populations, and that the combination of stresses puts some species more at risk than others. Rigorous long-term census studies of amphibians worldwide are needed to assess the magnitude and direction of changes in amphibian populations (Blaustein and others, 1994).

Almost all amphibians spend a part of their life-cycle in aquatic habitats, primarily for mating, oviposition, and larval growth (Semlitsch and others, 1996). Amphibians have many unique or unusual developmental characteristics. Some representatives from this class of animals can breath through their skin, regenerate lost body parts, change skin color, produce deadly poisonous secretions, or during cold weather, enter a state of torpor during which their heart stops and they cease to breath air only to return to normal when warmer weather returns (Cunningham and others, 1912). Because amphibians have highly permeable skins, they may be more susceptible to damage from environmental toxins than other terrestrial and aquatic invertebrates (Alford and Richards, 1999). There is a need for a better understanding of habitat suitability and the stressors that affect amphibian populations over a wide range of ecoregions. Specifically, an evaluation scheme that incorporates hydrologic variables is needed to: (1) evaluate natural habitat suitability for amphibians, (2) determine the vulnerability of the habitat to stressors such as changes in the hydrologic regime, increases in ultraviolet radiation, modification of natural water chemistry, or introduction of chemical contamination, and (3) determine if potential stressors to amphibian populations are present at biologically relevant times.

References

Alford, R.A. and Richards, S.J., 1999. Global Amphibian Declines: A problem in applied ecology. Annual Reviews of Ecological Systems 30:133-165.

Alvarez, L.W., Alvarez, W., Asaro, F., and Michel, H.V., 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208:1095-1108.

Blaustein, A.R. and Wake, D.B., 1990. Declining amphibian populations: A global phenomenon? Trends in Ecological Evolution 5(7):203-204.

Blaustein, A.R., Wake, D.B., and Sousa, W.P., 1994. Amphibian Declines: Judging stability, persistence, and susceptibility of populations to local and global extinctions. Conservation Biology, 8(1):60-71.

Corn, P.S., 1994. What we know and don't know about amphibian declines in the west. IN W. Covington and L. DeBano, eds., Sustainable ecological systems: Implementing an ecological approach to land management. USDA Forest Service General Technical Report RM-247, p. 59-67.

Cunningham, J.T., Oxon, M.A., and Boulenger, G.A., 1912. Amphibia. IN J. Cunningham, ed., Reptiles, amphibia, fishes, and lower chordata. Methuen & Co. Ltd., London, p. 157-230.

Dodd, K.C., 1997. Imperiled amphibians: A historical perspective. IN G. Benz and D. Collins, eds., Aquatic Fauna in Peril: The Southeastern Perspective. Special Publication 1, Southeast Aquatic Research Institute, Lenz Design & Communications, Decataur, GA, p. 165-200.

Eldredge, N., 1999. Cretaceous meteor showers, the human ecological niche, and the sixth extinction: IN R. MacPhee, ed., Extinctions in Near Time--Causes, Contexts, and Consequences: Kluwer Academic/Plenum Publishers, New York, p. 1-15.

Houlahan, J.E., Findlay, C.S., Schmidt, B.R., Meyer, A.H. and Kuzmin, S.L., 2000. Quantitative evidence for global amphibian population declines. Nature 404:752-755.

Koonz, W., 1992. Amphibian in Manitoba. IN C.A. Bishop and K.E. Pettit, eds., Declines in Canadian amphibian populations: Designing a national monitoring strategy. Canadian Wildlife Service Occasional Paper Number 76, p. 19-22.

Orchard, S.A., 1992. Amphibian population declines in British Columbia. IN C.A. Bishop and K.E. Pettit, eds., Declines in Canadian amphibian populations: Designing a national monitoring strategy. Canadian Wildlife Service Occasional Paper Number 76, p. 10-13.

Pechmann, J.H.K., Scott, D.E., Semlitsch, R.D., Caldwell, J.P., Vitt, L.J., and Gibbons, J.W., 1991. Declining amphibian populations: The problem of separating human impacts from natural fluctuations. Science, 253:892-895.

Rouse, J.D., Bishop, C.A., and Struger, J., 1999. Nitrogen pollution: An assessment of its threat to amphibian survival. Environmental Health Perspectives, 107(10):799-803.

Semlitsch, R.D., Scott, D.E., Pechmann, J.H., and Gibbons, J.W., 1996. Structure and dynamics of an amphibian community: Evidence from a 16-year study of a natural pond. IN M. Cody and J. Smallwood, eds., Long-Term studies of vertebrate communities. Academic Press, San Francisco, p. 217-248.

Wilson, E.O., 1993. The Diversity of Life. Harvard University Press, Cambridge, Mass.