For a long time, biologists literally could not believe their eyes. It appeared that frogs and other amphibians were disappearing around the world. But findings from several "hot spots" during the past year have persuaded most scientists that the declines are indeed real.
"I think we're close to consensus now," said David Wake of the University of California at Berkeley.
No one knows how many of the world's approximately 4,000 amphibian species may be affected, and some areas rich in amphibian life -- notably Mexico and Africa -- have yet to be evaluated. There are also vexing inconsistencies. Some species continue to thrive in highly polluted, densely populated areas, while other populations crash in faraway places side-by-side with species that are apparently unperturbed.
Nor do scientists yet have conclusive proof as to what may be causing any of the declines. But there are four prime suspects: increasing ultraviolet radiation resulting from ozone depletion, global climate change, pesticides and new diseases -- including a recently discovered skin infection caused by a class of aquatic fungi not previously known to affect vertebrates.
Scientists are concerned by amphibian declines for several reasons. Sometimes described as sensitive "sentinels" because of their permeable skin, variable diets and a life cycle that is partly aquatic and partly terrestrial, amphibians are without question vulnerable to environmental insult.
Scientists have long known that habitat loss through human encroachment -- road building, suburban sprawl, logging, agriculture, mining and fisheries management practices -- is the leading cause of amphibian disappearance.
But beginning in the late 1970s, field herpetologists started noticing that amphibians were becoming less abundant even in areas that seemed unaffected by human activity. The mysterious losses from supposedly pristine habitats continued through the 1980s, a decade that produced several extinctions, including the disappearance of the rare and beautiful golden toad, which vanished shortly after it was discovered in a remote nature preserve high in the mountains of Costa Rica. Other significant declines were reported from mainly mountainous regions in Australia, much of Central America, parts of South America and in the western United States and Canada.
But many scientists remained dubious. Through the mid-1990s, the scientific literature contained more sniping than hard data. Skeptics, who saw the decline reports as alarmist, argued that amphibian populations have such large natural fluctuations that no short-term study could show a long-term trend. And they pointed out that the historical prevalence of amphibians was largely unknown in most places.
But the evidence for widespread, unexplained declines mounted.
J. Alan Pounds and his colleagues in Costa Rica answered many of the skeptics late last year with a statistical study of amphibian declines in the same mountain region that was formerly home to the golden toad. Pounds's five-year survey showed that natural population fluctuations could not explain the disappearance of 20 species of frogs and toads -- about 40 percent of the local total.
This spring, researcher Karen Lips reported still more amphibian declines in highland regions of Costa Rica and Panama. Lips established a correlation between more aquatic species and the probability of decline -- a finding that she said argues for disease or chemical contamination as probable causes. Lips's work attracted attention for an additional observation: She actually found large numbers of dead and dying frogs in the jungle, in itself a startling finding.
"No dead or disabled frog can last long in the rain forest," Wake said. "Finding any at all suggests to me that there must have been many, many more that were not seen."
Necropsies performed on frogs recovered by Lips showed their skin was infected by a parasitic fungus called a chytrid, which researchers now believe may be associated with amphibian declines in Australia and Central America. Chytrid fungi, which also have infected captive amphibians in several zoos, are believed to impair respiration because many frogs and toads breathe through their skins. But it remains unclear whether the fungus is the primary cause of death or a secondary effect of some other environmental problem.
In California, Gary Fellers of the U.S. Geological Survey documented significant declines in several frog and toad species in seemingly pristine areas along the spine of the Sierra Nevada Mountains, including inside the protected confines of Yosemite National Park, where amphibian records go back 75 years.
Fellers's work, which is being done under arduous, high-altitude conditions at 4,000 remote sites, is one of the most ambitious monitoring efforts underway anywhere. What alarms scientists most about his preliminary findings is that the declines are all occurring on the western slopes of the mountains -- the side facing California's heavily agricultural central valley. Fellers believes pesticides atmospherically transported to the mountains are a likely cause of the declines.
David Green, a herpetologist at McGill University in Montreal, at first suspected at least some initial reports reflected only temporary local shifts in amphibian populations and not larger changes. Now, he said, he is surprised and distressed by the complete disappearance of the leopard frog from British Columbia.
The leopard frog is one of perhaps 10 species that Green now believes are in decline in his country, where he heads the endangered species program. "And this is happening in Canada," Green said, "which for the most part is pristine compared to the U.S."
At a world herpetology conference last summer in Prague, Wake and George Rabb, longtime head of the international Species Survival Commission, decided to seek federal assistance in creating a more centralized U.S. research effort.
Interior Secretary Bruce Babbitt last March requested a briefing by scientists studying amphibian population declines as well as the outbreak of leg deformities occurring in frogs and toads across the United States and Canada. "What these scientists told me," Babbitt said, "hit like a flash of light in the night. They illuminated a landscape of potential extinction that extends all the way around the world."
Babbitt arranged a second briefing in May for himself, Health and Human Services Secretary Donna E. Shalala and Environmental Protection Agency Administrator Carol M. Browner. The group, which also included Neal Lane, then of the National Science Foundation, and Kathleen A. McGinty, chair of the White House Council on Environmental Quality, agreed that it didn't want to create a big "Apollo Mission" type of program, but recognized that there was a pressing need for centralized coordination of the many scientists and research programs involved, Babbitt said.
Wake said the comparison some people have made between amphibians and the canaries that once warned coal miners of danger isn't quite right. "If a canary died," Wake said, "the miners got out of the mine. We don't have that option. We don't have any place to go."
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