The brilliant golden toad of Costa Rica's mountainous rain forest that seemed well-protected in the Monteverde Cloud Forest Reserve has not been known to breed since 1987 and the last two individuals were spotted two years ago. The harlequin frog has nearly vanished from the same range. Getting a handle on the overall picture is proving difficult. In many cases a local decline may have a ready explanation, such as the drought in California, pesticides from nearby farms or the introduction of fish or predatory non-native frogs. It is the sheer number of reported declines, many in well-protected wild areas, that stuns the experts and leads to fear that something more basic is going on.
Some scientists blame acid rain.
Others think the thinning ozone layer has increased ultraviolet radiation at the ground enough to devastate amphibians that are typically thin-skinned and often bask in the sun and whose eggs and larvae live in shallow, sunlit waters. Trace amounts of toxic compounds carried by the wind might conceivably deal fatal blows to animals that tend to breathe through their moist skins. But nobody claims to know for sure. Scientists are hard pressed to understand how a diverse order of animals that has been on Earth for 200 million years should be highly vulnerable to an environmental change so subtle that experts can not agree what it is. Most scientists still hope and a few believe that the wave of fear over amphibians is merely due to a weird coincidence of natural fluctuations, along with a few declines attributable to purely local causes.