Though more work must be done to plug the gaps in our knowledge of amphibian declines, these studies allow us to draw an important conclusion: amphibian populations, in far-flung locations, are indeed disappearing even in seemingly virgin environments. The challenge, therefore, is no longer merely to preserve habitat, though that is still a vital task. We must also discover and address the less obvious reasons for the demise of these creatures as well as determine what fate they might portend for other species, including ourselves.
Prominent among the suspects thought responsible for declining amphibian populations, at least in specific locales, include agricultural chemicals and pesticides. In many parts of the world, certain amphibian species have thrived in agricultural areas, taking advantage of artificial water bodies used for irrigation and watering livestock. But the chemicals found in farmland breeding sites interfere with normal amphibian development. Michael Tyler, a biologist at the University of Adelaide in Australia and a board member of the Declining Amphibian Populations Task Force, explains that the problem with some herbicides is not the active ingredient itself, for example glyphosate, but rather a detergent additive that acts as a dispersant or wetting agent. The detergent breaks down the surface tension at the leaf surface to enable spray droplets to completely cover the leaf. However, the agent also interferes with respiration in frogs through the skin and even more so with respiration of tadpoles through gills. Michael Lannoo, a biologist at Ball State University, also points out that some pesticides such as methoprene (used for mosquito control) break down into a compound resembling retonic acid, which has been shown in the laboratory to produce severe amphibian limb deformities that would render individuals incapable of escaping predators.
Other pollutants under investigation are being blamed for more regional amphibian declines. Among the leading culprits for these losses may be acid rain. In fact, researchers have found that almost all amphibian eggs or larvae tested so far cannot survive in water with a pH of less than 4.5. Yet acid rains, commonly in the 3.5 range, can lower the pH of ponds and streams from a normal average of about 7.0 to lethal levels. In fact, acid rain has been identified as a cause of amphibian declines in lakes and ponds in Canada, Scandinavia, and Eastern Europe.
Chief among the candidates likely to be responsible for amphibian declines on an even wider, perhaps global, basis is ozone depletion. Recent studies in Oregon have shown that rising levels of ultraviolet-B (UV-B) radiation resulting from the depletion of the earth’s ozone layer have undermined the hatching success of eggs in some native amphibian species. The researchers suggest that other amphibians most likely to be affected by increased UV-B radiation-which, at elevated levels, breaks down the DNA molecule-are those living at cooler, higher elevations and extreme latitudes, where the ozone layer is thinnest but where amphibians must bask in the sunlight to regulate body temperature.
Environmental estrogens may also be responsible for global declines. Researchers believe that these pollutants, which result from the chemical breakdown of pesticides such as DDT, are likely to severely affect the reproductive biology of amphibians, as they have been shown to do in other aquatic organisms, such as fish and alligators. In fact, in laboratory studies, Tyrone Hayes, an endocrinologist at the University of California, Berkeley, found that such environmental estrogens masculinized female Japanese tree frogs, Buergeria buergeri, and feminized male pine woods tree frogs, Hyla femoralis, causing both populations to become sterile. These estrogens, whose molecules do not break down easily in the environment, stockpile in silt on the bottoms of ponds and lakes, where they are ingested by bottom-feeding amphibian larvae. Some of these agents are effective in very small concentrations and are easily wind-borne, making them a global threat regardless of their point of origin.