Thursday, April 15, 1999 ; Page A03

An abrupt, unprecedented climate shift apparently associated with global warming appears to have caused the mysterious disappearance of 20 frog species in Costa Rica, researchers reported yesterday.

The frog declines, which included the infamous extinction of the Golden Toad, coincided with a sudden reduction in moisture levels on the continental divide atop Monteverde in Costa Rica's central highlands, according to J. Alan Pounds, of the University of Miami, and his colleagues.

The discovery is evidence that global warming is affecting wildlife in previously unrecognized ways, he said. "Biological communities are responding to climate change more quickly than we thought," he said. "We've observed a pattern here and our responsibility is to sound an alarm."

Monteverde was settled in the 1950s by Quakers from the United States who set aside a large area of "cloud forest" as a nature preserve. Moisture-laden winds off the Caribbean cool as they rise up the eastern slope, forming a cloud bank on the mountaintop and shrouding the jungle in mist.

The cloud forest is home to an enormous diversity of plants and animals dependent on its extreme moisture levels. Conversely, other species adapted to drier, warmer conditions live further down the mountainside, below the base of the cloud.

Pounds and his collaborators, Michael P.L. Fogden and John H. Campbell, discovered that amphibians and reptiles living at upper elevations had simultaneously suffered severe population reductions. At the same time, a number of bird species from lower sections of the mountain began an upward migration. Toucans, previously found only in the lowlands, now live side by side with the Resplendent Quetzal, the colorful, long-tailed bird identified with the cloud forest going back to pre-Columbian times.

All of these changes coincided with unusually warm, dry conditions produced by a combination of the El Nino weather pattern and a more general, long-term rise in sea surface temperatures, the researchers found. These effects, Pounds said, are amplified at higher altitudes and have caused the base of the cloud bank to lift. As the cloud recedes up the mountain, the misting and condensation essential to life have decreased.

When the scientists examined stream flow and ocean temperature data, plus daily records of air temperature and mist frequency near the continental divide, they discovered not only that the dry season had become warmer and drier, but that dry days now come in longer sustained runs.

The overall climate trend corresponds to a shift in bird demographics that has brought 15 new species up from lower elevations. Meanwhile, two lizards found only at higher elevations began to decline in the late 1980s and had vanished by 1996. In the same period, a third species of the small lizard that thrives in drier conditions remained stable. All of this took place against the backdrop of a massive frog population decline that began in 1987 and has since wiped out 40 percent of species present in a series of synchronous crashes that have occurred during peaks of warm and dry conditions.

Unlike birds, earthbound amphibians have limited upward mobility. The Golden Toad, which lived only in several wetlands in a small area almost at the mountaintop, had nowhere to go. It was last seen in 1989.

Global warming probably was not the immediate cause of the Golden Toad's demise, Pounds said. More likely the climate fluctuation weakened the animals and made them vulnerable to an epidemic involving a pathogen or parasite, such as the chytrid fungus implicated last year in other frog die-offs around the world. But Pounds said no one will ever know the exact cause.

"At the time of the crash we weren't aware of what was happening," Pounds said. "Nobody looked at the animals to see what killed them."

Pounds's research, published in today's issue of the journal Nature, appears to confirm the warnings of many scientists that amphibians are reacting to widespread environmental degradation in even seemingly pristine habitats.

"This is very important," said Andrew Blaustein, a biologist at Oregon State University. "It's a convincing scenario for why the Golden Toad and other species went down the tubes. It also shows how incredibly complex these environmental interactions can be."

Michael Lannoo, U.S. coordinator of the Declining Amphibian Populations Task Force, said Pounds has demonstrated the first animal extinction attributable to modern climate change. "People who say global warming won't be a problem argue that animals will simply shift to more suitable habitats as change occurs," he said. "Alan's results show there are limits to that."

Cutline: Golden Toad

Articles appear as they were originally printed in The Washington Post and may not include subsequent corrections.