Amphibians at risk

Some 5,000 species of amphibians inhabit the world, mostly frogs, toads and salamanders, and they seem to be dying at unprecedented rates. This situation has raised alarm because amphibians are widely regarded as uniquely sensitive indicators of the planet's health. Much of the damage to amphibians comes from habitat destruction, particularly the draining of wetlands, but what has scientists particularly worried are the declines and apparent extinctions in areas far removed from obvious human intrusion. For example in the US, the areas most affected by amphibian declines are in the Rocky Mountains, and the South West as well as California.

Do reports such as these indicate a worldwide amphibian crisis? Not necessarily, according to Joseph Pechmann of Florida International University, whose work suggests that reported declines and extinctions in near-pristine environments could simply be natural year-to-year variations: a drought, for instance, that affects egg laying and larvae survival. Because of such fluctuations, it is often impossible, in the absence of more complete historical information, to judge whether a reported decline is natural or a reaction to human activity. On the other hand, many researchers, including Andrew Blaustein of Oregon State University, believe the reported rates of decline and extinction are so extraordinary that they cannot be a part of the natural cycle.

If the reported declines and extinctions are indeed highly abnormal--and this seems to be the majority viewpoint--what might have caused them? A variety of causes is probably at work here. Among the leading culprits are acid rain, synthetic chemicals, metallic contaminants and infectious diseases. Excessive ultraviolet radiation (presumably caused by the thinning of the ozone layer) working synergistically with fungal disease may explain some of the declines. Another possible cause is global warming-related droughts, a particular threat to amphibians, which generally require high humidity or an aquatic environment.

There has also been a recent surge of reported amphibian abnormalities, such as missing limbs and eyes. One of the factors may be retinoic acid, which may be naturally present in water or as a residue from pesticides; it is known to produce birth defects in vertebrates. It is not clear, however, whether the increase in reports stems from greater human activity or is simply the result of more surveys: reports on amphibian abnormalities are not new and have fluctuated in number since 1700.

One of the few areas with reasonably complete information is the U.S. The Nature Conservancy and the Natural Heritage Network have identified 242 native amphibian species that have inhabited the 50 states since the beginning of European settlement. Three of these species are presumed extinct; an additional three are classified as possibly extinct. Moreover, 38 percent are classified as imperiled or vulnerable, a level that suggests that amphibians are more affected by human activity than birds, mammals and reptiles, which are at far lower levels of risk in the U.S. Freshwater fish are at about the same level of risk as amphibians; crayfish and mussels are at substantially higher levels of risk.

The number of amphibian species in the U.S. varies considerably, with California and the South having the largest number. Amphibians tend to be at greater proportional risk in the Far West. In much of the West, habitats are discontinuous, making it more difficult for locally threatened groups to be recolonized from other areas, whereas in California's Central Valley, pesticides and herbicides are suspected of contributing to amphibian declines. Amphibians in certain parts of the West may be adversely affected by the introduction of nonnative species such as the bullfrog, which compete with native amphibians for food, or by the introduction of salmon and trout, which eat amphibian eggs, tadpoles and even adults. The Southeast--which has more areas of continuous suitable habitat (such as the southern Appalachians) and a warm, moist climate--is better suited for recolonization.