Pesticide, urbanization linked to frog declines

Though the California red-legged frog recently earned sweeping federal protection from habitat destruction, researchers from UC Davis and California State University, Sacramento, have found new evidence that their decline may also be pesticide-related.

"It's the first time scientists have been able to link a known declining frog species with pesticides," said Carlos Davidson, an environmental studies professor at CSUS. "We found there is a very strong association between declines of red-legged frogs and the amount of agricultural land use upwind from the site. It strongly suggests that windborne agrochemicals may be contributing to the decline."

In a study that encompassed almost all of California, Davidson and co-author H. Bradley Shaffer, a professor at the Center for Population Biology at the University of California, Davis, mapped out the disappearance of red-legged frog populations. Using those geographic patterns they analyzed possible causes of the declines.

Mark Jennings of the U.S. Geological Survey also contributed to the study. Their findings are published in the April issue of the journal Ecological Applications.

The red-legged frog has disappeared from over 70 percent of its historic range in California. It was added to the threatened species list by the U.S. Fish and Wildlife Service in 1996.

To identify historic concentrations of red-legged frogs, Davidson and Shaffer compared museum records of their habitats, dating back to the mid-1800s, with recent survey data. "From the museum specimens we know where the frogs used to be. Recent survey data tells where they are now," Davidson said.

Of the 237 sites they looked at that once had frog populations, 48 percent no longer do.

The researchers looked at several possible causes of the declines, including global warming, ultraviolet radiation, pesticide use and habitat destruction due to urbanization and agriculture. They concluded that both urbanization and pesticides may be important factors in the declines.

At each site they calculated the predominant wind direction and the amount of agricultural land use upwind. The percentage of upwind land use in agriculture for sites where the red-legged frog has disappeared was 6 1/2 times greater than for sites where they still exist, suggesting that windborne agrochemicals may be an important factor in frog declines.

"The results were consistent," Davidson said. "We found areas with a lot of agricultural land use upwind from them are more likely to have declines than sites with less upwind agriculture."

"It's an issue that has impact far beyond California," Davidson added. "There have been amphibian declines in many locations around the world and pesticides are definitely a possibility. In both Central America and Australia, declines have been found close to major agricultural areas.

"If it turns out pesticides are the cause, we'll have to do more than set aside habitats to protect the species. We'll have to do something about the types and amounts of pesticides that are used and how they are applied."

Media contacts:
-- Carlos Davidson, CSUS Environmental Studies, (916) 278- 6063, [email protected]
-- H. Bradley Shaffer, UC Davis Evolution and Ecology, (530) 752-2939, [email protected]
-- Laurie Hall, CSUS Public Affairs, (916) 278-4282, [email protected]
-- Sylvia Wright, UC Davis News Service, (530) 752-7704, [email protected]