Frogs

Judging by recent observations, the prospects for many species of frogs is grim. During the 1990s, numerous species of frogs apparently vanished from nature without any obvious cause of their demise. For example, a newly described, extremely unusual Australian frog, (Rheobatrachus silus), could not be found in its only known habitat the following year. Numerous other Australian frogs have also disappeared. Similarly, the golden toad (Atelopus zeteki) of Costa Rica, which once occurred in large numbers, has apparently disappeared. The populations of the Yosemite toad (Bufo canorus) in the Sierra Nevada of California have plummeted. Similar reports have come from other parts of the world, and there is now an international group of biologists investigating the causes of the apparently simultaneous declines of many species of frogs.

Some biologists believe that the cause of the loss of these species may be somehow related to pollution caused by human activities. Emissions of chemicals known as chlorofluourocarbons, for example, may be causing the stratospheric ozone layer to become thinner, allowing greater amounts of ultraviolet energy to reach Earth's surface. There is some evidence that this environmental change may be a cause of the decline of the boreal toad (Bufo boreas) of the northwestern United States. This species breeds in open ponds at high altitude, and the intensified exposure to ultraviolet light may be killing its eggs.

Chemical pollutants may also be spread widely through the atmosphere, or be transported by surface water to places far from their original source of emission. Consequently, trace amounts of pesticides have been found in frogs living far from human populations. Although these poisons do not seem to kill the adults, they may be interfering with reproductive processes, and may be causing unusual deformities in rapidly developing tadpoles and juvenile frogs. Because frogs have such delicate, water-absorbing skin, they may be serving as environmental "canaries." Like the actual canaries that coal miners used to take into the mines as an early warning of the presence of toxic gas, frogs seem to be among the most sensitive indicators of ecological damage caused by toxic chemicals.