Environment: Why Are These Frogs Croaking?

Massive die-offs in the American tropics are an early warning of the effects of global warming

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Hardy and plentiful as they seem, frogs are actually very frail things, with a semipermeable skin that leaves them vulnerable to even the slightest hiccup in their environment. So when entire species of brightly colored harlequin frogs started dying off in the cloud forests of Central and South America about 25 years ago, scientists suspected that something in the amphibians' ecosystems--they weren't sure what--had gone awry.

Now an international team of scientists think they've solved the mystery. Comparing changes in annual temperatures with the number of frog species spotted, they've documented for the first time a direct correlation between global warming and the extinction of about two-thirds of the 110 known species of harlequin frog.

The critters in question are favorites of scientists studying climate change. Quick and polychromatic, the frogs spend their days near stream banks, where their constant motion and vibrant hues make it easy for researchers to count them. Previous studies have shown that it's not heat alone that kills harlequins but also a pathogen--the chytrid fungus--that attacks their skin. The chytrid is actually a cool-weather organism, doing best at temperatures from 63°F to 77°F. Paradoxically, an effect of global warming is to increase cloud cover in the tropical forests, lowering daytime temperatures and making the frogs more vulnerable to fungal assault.

The most persuasive piece of evidence in the new study, led by J. Alan Pounds of the Monteverde Cloud Forest Preserve in Costa Rica and published in Nature, is a graph that shows both annual changes in average temperature and the number of frog extinctions per year on the same grid: the jagged lines track each other with eerie precision. Species die-offs follow warm years 80% of the time. With tropical air temperatures from 1975 to 2000 rising three times as fast as the 20th century average, things should only get worse.

Frogs are what scientists call an indicator species: particularly sensitive animals that are the first to go when the climate starts to change. Their extinction may increase pressure on government and industry to dial back greenhouse gases. The harlequins, after all, are only the beginning.