A great mystery is unfolding amidst the world's forests, ponds, streams and mountains. But unlike the tales that delight millions of readers and moviegoers on every continent, this mystery is not bound to make many people happy, nor is it likely to have the tidy denouement that is expected in the genre. Amphibians - creatures of the water and land that have roamed the Earth for hundreds of millions of years, surviving eco- catastrophes that wiped out the dinosaurs among other former residents - are in serious decline, disappearing 10 times faster than birds and mammals in many habitats around the globe. No master sleuth has yet emerged to explain the sudden disappearance of such prized species as the golden toad of Costa Rica and Australia's gastric brooding frog, nor can anyone account for all the deformed amphibians that are turning up in increasing numbers - frogs and salamanders with extra legs, missing legs or missing eyes. Scientists attribute these unsettling findings to a combination of factors rather than to a single cause.
Increased exposure to ultraviolet-B radiation from the sun due to ozone thinning is a major suspect, having been shown to prevent amphibious eggs from hatching and to cause visual impairment in frogs that would make the animals easy targets for predators. Acid rain poses another threat; the acidity of the most severely affected streams and ponds in North America and Europe is lethal for most amphibious larvae. Nitrogen-based compounds, introduced to the environment through the application of crop fertilizers, can kill some amphibians when mixed in water at concentrations that are considered safe for humans. Unfortunately, water contaminated to that extent is commonly found near agricultural areas. Other culprits include habitat destruction, naturally occurring parasites, and the introduction of new predators. Researchers believe these factors can work synergistically: With an immune system ravaged by environmental stresses, an animal becomes more susceptible to pathogens it might otherwise fend off.
Amphibians are particularly vulnerable because their eggs lack a protective covering and their skin readily absorbs air and water that might be tainted with pollutants. Some researchers compare these animals to the proverbial "canaries in coal mines," warning humans of environmental changes we are not yet sensitive enough to detect. The problem is not confined to amphibians, as recent findings suggest that reptiles are on the wane as well. What species will be next to succumb, and how far behind are humans?
More than half of the wetlands in the United States have been lost to human activities and as much as 90 percent of the wetlands in states such as California and Ohio have been wiped out.