Pesticides linked to amphibian declines

November 21, 2002

California is a hotspot of amphibian decline - half of the state's frogs and toads are in trouble -- and new research suggests that agricultural pesticides may be one of the biggest reasons.

"Declines of four...species were strongly associated with the amount of upwind agricultural land use, suggesting that wind-borne agrochemicals may be a factor," say Carlos Davidson of California State University, Sacramento; Brad Shaffer of the University of California, Davis; and Mark Jennings of Rana Resources in Davis, California, in the December issue of Conservation Biology.

Davidson and his colleagues studied the effects of four factors thought to play a role in amphibian declines (pesticide use, habitat destruction, UV-B radiation and climate change) on eight species that are declining in California (four frogs, two toads, one spadefoot toad and one salamander). To identify areas where each species used to live but no longer exists, the researchers compared historic distribution records with recent surveys.

Davidson and his colleagues found that of the factors considered, only habitat destruction and pesticides were linked to declines of the amphibians studied. Habitat destruction was based on the amount of urban and agricultural land use in a 3-mile radius around the site, and likely pesticide exposure was based on the amount of upwind agricultural land use.

Habitat destruction was linked to the declines of two species: sites that have lost the California tiger salamander and the Western spadefoot toad had 3 and 6 times more local urbanization, respectively, than sites where these species still live. The spadefoot toad depends on vernal pools, many of which have been destroyed by urbanization or conversion to agriculture.

Pesticides were linked to declines of all four frog species (the mountain yellow-legged frog, foothill yellow-legged frog, California red-legged frog and Cascades frog). Sites that have lost these frogs had about 2-4 times more upwind agricultural land use than sites where these species still live.

There is plenty of other evidence that pesticides may contribute to amphibian declines. Pesticides applied on Central Valley farmlands can end up in the Sierra Nevada mountains that lie to the east, and low levels of pesticides can cause fatal immune suppression in amphibians. In addition, major amphibian declines have occurred downwind of large agricultural areas in both Australia and Central America.

Davidson is currently studying whether patterns of historic pesticide use correlate with amphibian declines in California, and he and Shaffer are studying the effects of very low pesticide doses on amphibians' susceptibility to disease.

Carlos Davidson (916-278-6063, Brad Shaffer (530-752-2939,


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