Image: courtesy of the U.S.G.S.

During the past 10 to 15 years, populations of certain amphibian species in Californianamely, red-legged frogs, foothills yellow-legged frogs, mountain yellowlegged frogs and Yosemite toadshave declined at an alarming rate. The exact cause for these declines, however, has managed to evade researchers until now. At a press conference held last Thursday in Washington, D.C., scientists from the U.S. Geological Survey (USGS) and the U.S. Department of Agriculture (USDA) confirmed that agricultural contaminants may be the culprits. The new results could explain why pristine areas like the Sierra Nevada, which lies near the agriculturally important San Joaquin Valley, have been particularly hard-hit.

Investigators collected tadpoles and adult frogs from 23 sites in California (right) and found that the animals were suppressing an enzyme called cholinesterase, which can indicate exposure to commonly used pesticides that can be harmful to animals. According to the results of their study, recently accepted for publication by the journal Environmental Toxicology and Chemistry, organophosphorous pesticides from nearby farming regions may reach the Sierra Nevada amphibians by way of the prevailing summer winds. "While crucial to the agriculture industry, pesticides by their very nature can result in serious harm to wildlife both by directly killing animals and through subtle effects on reproduction, development and behavior," said U.S.G.S. biologist Donald Sparling. "Unfortunately, now there appears to be a close correlation between declining populations of amphibians in the Sierra Nevada and exposure to agricultural pesticides."