Image: JOSEPH KIESECKER, Pennsylvania State University

Amphibian populations around the world have been declining at alarming rates for years, prompting a confounding variety of explanations from researchers. Now new research, reported today in the journal Nature, indicates that for at least one species, the cause is climate change.

Biologist Joseph Kiesecker of Pennsylvania State University and his colleagues reached this conclusion after investigating how differing water levels impacted the development of toad eggs in lakes and ponds in western North America. "For over 10 years, we have been collecting data at a number of sites in the Cascades, 4,000 to 7,000 feet above sea level, where there are large breeding populations of western toads," Kiesecker says. The team used boxes designed to anchor the toad eggs at various depths, to assess whether a thicker blanket of water protects the eggs from the harm of ultraviolet light.

"We have found that water levels are shallower during years when there is less snow, which exposes the eggs to more ultraviolet light and makes them susceptible to disease outbreaks," Kiesecker explains. They found that the developing embryos were killed by a water-mold pathogen, Saprolegnia ferax, which usually only attacks organisms that are injured or under some sort of stress. "Around the early 1990s, we started to see 80 to 100 percent mortality," says Kiesecker.

Although Kiesecker sees global warming as the culprit in this specific case, he blames various causes of stress for amphibian declines around the world. "Our results indicate that interpreting the impacts of global climate change will require a detailed mechanistic understanding of local natural history coupled with long-term data that will permit us to gauge responses associated with climate change," he remarks. Team member Andrew Blaustein adds: "This study shows that if we want to understand the complex ecology of the world around us, we must start looking at the big picture, and there may not be simple or easy answers."