Since the 1970s, three frog species have disappeared from Puerto Rico's mountain forests, all after unusually warm years. Between 1987 and 1988, the hottest year on record until then, the golden toad and Monteverde harlequin frog disappeared from the Monteverde Cloud Forest Preserve in Costa Rica. That same year, the jambato toad vanished from the highlands of Ecuador, which was also unusually hot and dry. "We have a tendency for species to disappear after warm years," says Alan Pounds, scientist-in-residence at Monteverde.
Scientists consider these extinctions part of an ongoing worldwide amphibian die-off. The World Conservation Union (IUCN) estimates that 32 percent of all amphibians are in danger of going extinct. Of 5,918 known species – and new species are constantly being discovered – 165 may have already disappeared. Two-thirds of all harlequin frogs, which range throughout New World tropics, are already gone, Dr. Pounds says. Scientists have blamed habitat loss, increased ultraviolet radiation, pollution, and chytrid, a fungus that humans may have inadvertently transported around the globe.
To that list of suspects, Dr. Pounds and many other scientists now add climate change. Scientists rarely expect animals to drop dead from heat exhaustion. Instead, they anticipate warming will do away with food sources and habitats or trigger outbreaks of disease – and not always in obvious ways. Pounds thinks that this is what happened at Monteverde. Warming helped the chytrid fungus kill off the golden toad.
The fungus, which grows on the skin of amphibians and is lethal to many of them, thrives in the relatively cool conditions found in tropical highlands. So how could global warming enable it? What if warming pushed average temperatures at Monteverde up into the fungus's optimal range, between 63 and 77 degrees F.? Rather than suppressing the cool-weather fungus, rising temperatures may have led to ideal growing conditions. "It's the opposite of what you would expect," Pounds says.
Disease isn't the only threat from warming. Steven Whitfield, a PhD candidate at Florida International University in Miami, analyzed 35 years of amphibian and lizard population data at La Selva Biological Field Station on Costa Rica's Caribbean plain. To his surprise, he found that researchers today catch one-quarter of the number of frogs and lizards they caught then.
"These declines were not subtle," he says. And Pounds's chytrid hypothesis doesn't explain them: The fungus doesn't grow well in the hot lowlands, and lizards don't get chytrid. That suggested something else. Mr. Whitfield guesses it is the leaves rotting on the jungle floor.
In those same 35 years, average temperatures at La Selva have risen. Higher temperatures, especially at night, perhaps make trees put out fewer leaves. But they also increase the decomposition rate of fallen leaves on the jungle floor, where frogs and lizards feed, lay eggs, and take shelter.
"If there's less leaf litter on the ground, then frogs and lizards have [fewer] hiding places," Whitfield says.
Both these hypotheses underscore global warming's indirect effect on wildlife. "I don't think that a single factor is going to explain all of the trends that we're seeing worldwide," Whitfield says. But "interaction between these factors is going to be hugely important."