Frog Extinctions Linked to Global Warming

Brian Handwerk
for National Geographic News
January 12, 2006
Global warming may cause widespread amphibian extinctions by triggering
lethal epidemics, a new study reports.

J. Alan Pounds and colleagues suggest that many harlequin frog species (Atelopus) across Central and South America have disappeared due to deadly infectious diseases spurred by changing water and air temperatures.

"Disease is the bullet killing frogs, but climate change is pulling the trigger," said Pounds, lead study author and resident scientist at Costa Rica's Monteverde Cloud Forest Preserve.

"Global warming is wreaking havoc on amphibians and will cause staggering losses of biodiversity if we don't do something fast."

Biodiversity refers to the number of species in a given area. It is often used to gauge the health of an ecosystem.

The study appears in today's issue of the journal Nature.

Vanishing Frogs

About two-thirds of the 110 known harlequin frog species are believed to have vanished during the 1980s and 1990s. The primary culprit, Pounds suggests, is the disease-causing chytrid fungus Batrachochytrium dendrobatidis.

Amphibian skin is extremely thin, which makes frogs acutely sensitive to even minor changes in temperature, humidity, and air or water quality. It also makes frogs more susceptible to chytrid fungus.

The new study suggests that temperature extremes may have previously helped keep the deadly disease in check. But new climate cycles are now moderating those annual temperature swings.

Global warming has increased evaporation in the tropical mountains of the Americas, which in turn has promoted cloud formation, the study reports. That cloud cover may have actually decreased daytime temperatures by blocking sunlight. At the same time, it may have served as an insulating blanket to raise nighttime highs.

Pounds believes the combination has created ideal conditions for the spread of the frog-killing fungus, which grows and reproduces best at temperatures between 63° and 77°F (17° and 25°C).

Study Skeptic

But not all experts are ready to pin amphibian deaths on climate change just yet.

Cynthia Carey, an amphibian-disease expert at the University of Colorado, warns that the study may have shown a circumstantial correlation between temperature data and amphibian deaths. But the report has not shown cause and effect.

"[The study authors] haven't met the rigorous criteria for proving that the frogs in Monteverde even died from chytrid," she said.

"And while the globe is certainly warming, [researchers] haven't proven that the fungal outbreaks are driven by increases in temperature."

Carey cited several research studies that document the ability of chytrid fungus to kill at temperatures near the freezing point. She noted that many documented amphibian deaths from such outbreaks occur in winter.

Frogs in the Coal Mine?

Climate scientists have long warned that global warming could spur deadly disease epidemics. The study suggests that such a scenario may already be unfolding in the amphibian world.

If so, humans and other species should consider themselves duly warned.

Because amphibians are particularly sensitive to environmental change, they may serve as proverbial "canaries in a coal mine" that warn of such climate change dangers.

A two-year-old study by a scientist at Britain's University of Leeds suggests that some 15 to 35 percent of land-dwelling plants and animals, or about a million species, would be extinct or committed to extinction by 2050.

Other climate scientists have calculated that half of the planet's species are already affected by global climate change.

The news for amphibians is particularly bad.

In 2004 a global amphibian assessment by the World Conservation Union, Conservation International, and NatureServe reported that about one-third of all amphibian species were in decline.

New data will soon supplement that study, and those data will tell a familiar and sad story, according to lead author Simon Stuart, of Washington, D.C.-based Conservation International.

"They'll show a continuing deterioration, I'm afraid," he said. "Things have gotten worse."

Important Clue?

Though his 2004 report did not specify a cause for amphibian extinctions, Stuart believes Pounds's new research is an important clue.

The new study of harlequin frog species has "certainly taken us much further down the road towards understanding what's going on, towards getting the mechanism of this decline clarified," Stuart said.

If the data can be verified, that may hold a ray of hope for beleaguered frogs.

Bruce Young, a NatureServe zoologist and study co-author, said: "The good news, such as it is, is that the new findings will open up avenues of research that could provide conservationists with the means to save the amphibians that still survive. If this cloud has any silver lining, that's it."

Meanwhile, extinctions in protected and otherwise pristine environments such as Monteverde could illustrate global risks associated with a warming climate.

"Most of the studies that have been done on the impacts of climate change on species have focused on shifting habitats on a habitat moving higher up a mountainside or closer to the poles," " Stuart said.

"What we're seeing now is a much more rapid impact of climate change, not just climate change per se but climate change rendering species vulnerable to another threat."

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