El Niño Digs Shallow Graves

Toads need sunscreen, too. Andrew Blaustein monitors egg mortality at a Cascade field site.

Amphibians are dying in unprecedented number all over the world, and biologists are baffled. Now a study pegs global climate change as the ultimate culprit in the decline of Western toads. Intensified El Niño cycles can dry up streams and ponds, researchers say, which allows ultraviolet (UV) light to penetrate breeding pools and damage eggs, making them susceptible to fatal infections.

The recent declines and disappearances of many frogs, toads, and salamanders have been blamed on a number of causes, including pathogenic infection, pesticides, pollutants, increased UV radiation from ozone depletion, and local drought spurred by climate change. There's no one smoking gun, however, and the relative importance of each culprit seems to vary by region and species. Lately, researchers have begun studying interactions between such factors, and now one team has identified several links.

After a decade of studying the Western toad (Bufo boreas) in Oregon's Cascade Mountains, herpetologists Joseph Kiesecker of Pennsylvania State University, University Park, and Andrew Blaustein and Lisa Belden of Oregon State University in Corvallis found that climate change triggers a chain of ultimately fatal events. In normal years, females lay their eggs under 1 to 1.5 meters of water in remote mountain lakes and pools. But after dry winters with low snowpack, lake levels drop and toad eggs can find themselves in as little as 10 cm of water. The researchers determined that such shallow-lying eggs are exposed to more UV light, which can render the eggs susceptible to a devastating water mold, Saprolegnia ferax. El Niño patterns over the Pacific correlate well with dry winters in the Cascades, the researchers report in the 5 April issue of Nature. More frequent and stronger El Niños mean more dead eggs.

The study "demands to be taken seriously," says herpetologist and evolutionary biologist David Wake of the University of California, Berkeley. But it remains to be seen whether the same type of events threaten all amphibian populations. "There isn't any conclusive information that climate change is causing amphibian declines at the sites we've examined," says Cynthia Carey of the University of Colorado, Boulder.

Related sites

U.S. Geological Survey summary of amphibian declines
The Declining Amphibian Populations Task Force Home Pages
Joseph Kiesecker's Faculty Page
Andrew Blaustein's Faculty Page

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