full text: Several species of anuran amphibians (frogs and toads) are experiencing severe population declines in even seemingly pristine areas of the Sierra Mountains of California. Among the most severely depressed species are the redlegged frog, the foothill and mountain yellow-legged frogs, the Yosemite toad, and the Cascades frog. Several factors, such as habitat fragmentation, introduced predators (especially fish), and disease, have been linked to these declines. But recent evidence from a USGS-led study shows that contaminants are a primary factor. During the past three years, researchers from the USGS Patuxent Wildlife Research Center, the Western Ecology Research Center, the USDA Beltsville Agriculture Research Center, and the Texas A&M University have teamed up to conduct an extensive study on airborne pesticides and their effects on amphibian populations in the Sierra Nevada Mountains. Previous work on environmental chemistry demonstrated that pesticides from the intensely agricultural Central Valley of California are being blown into the more pristine Sierra Nevada Mountains, especially around Sequoia and Yosemite National Parks. Several pesticides, including diazinon, chlorpyrifos, malathion and endosulfan, can be measured in snow, rainfall, and pond waters in these national parks. With the exception of endosulfan, these pesticides affect and even kill both invertebrates and vertebrate species by inhibiting cholinesterase, an enzyme essential to proper nervous system functioning. In the summer of 2001, we published a paper showing that these same pesticides are now found in adults and the tadpoles of Pacific treefrogs. The results of this landmark study showed that more than 50 percent of the tadpoles and adults sampled in Yosemite and Sequoia National Parks had detectable levels of diazinon or chlorpyrifos and that 86 percent of the Pacific treefrogs sampled in the Lake Tahoe region had detectable levels of endosulfan. In contrast, frogs that were collected along the coast, upwind of the Central Valley, contained much lower levels of contaminants. In addition, cholinesterase activity was significantly inhibited in tadpoles from Yosemite and Sequoia National Parks compared to sites along the coast and at Lassen Volcanic National Park. During the summer of 2001, we sampled water, sediments, and Pacific treefrogs throughout most of northern and central California. In an associated study, we moved tadpoles among Sequoia, Yosemite, and Lassen Volcanic National Parks. Although we are still analyzing data, it is clear that cholinesterase in tadpoles shows the same pattern as in 1999. That is, tadpoles in coastal sites (where pesticides occur in much lower levels) had higher cholinesterase production; tadpoles in the mountains (downwind of the agricultural areas in the Central Valley) had depressed levels of cholinesterase. Early analysis of tadpoles that were moved among the parks shows that up to 25 percent of those reared in Yosemite National Park had deformities in their hind limbs. By comparison, only about five percent of the tadpoles raised in the relatively contaminant-free Lassen Volcanic National Park had deformities. In another part of our study, we found that tadpoles moved from Sequoia and Yosemite National Parks had greater genetic damage than animals that were from the same egg masses and moved to Lassen Volcanic National Park.. . In 1972 Rachael Carson wrote Silent Spring, in which she foretold the decline and near extinction of top carnivore birds such as bald eagles, brown pelicans, and ospreys due to persistent pesticides such as DDT. Now pesticides seem to be implicated in an important way with the declines of another class of vertebrates, the amphibians, a class that has frequently been called the 'canary in the mine' for ecological stress. In addition, some of the environmental stressors that affect amphibians may affect humans as well, making continued research on a