Dramatic population declines in red-legged frogs, foothills yellow-legged frogs, mountain yellowlegged frogs and Yosemite toads have occurred in California over the last 10-15 years, but no single cause for these declines has been positively identified. Scientists and managers have been especially concerned because many of these declines occurred in some of the state's most seemingly pristine areas. Declines have been particularly drastic in the Sierra Nevada, which lie east of the intensely agricultural San Joaquin Valley. The red-legged frog is listed as threatened under the US Endangered Species Act, and the mountain yellow-legged frog and Yosemite toad have been proposed for listing.

"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 more subtle effects on reproduction, development and behavior," said Dr. Donald Sparling, a research biologist and contaminants specialist at the USGS Patuxent Wildlife Research Center in Maryland. "Unfortunately, now there appears to be a close correlation between declining populations of amphibians in the Sierra Nevada and exposure to agricultural pesticides."

The scientists found proof pesticides are being absorbed by frogs in aquatic and terrestrial systems and are suppressing an enzyme called cholinesterase, which is essential for proper functioning of the nervous system. Modern pesticides function by binding with this enzyme in animals and disrupting nervous system activity, usually causing death by respiratory failure. Decreased cholinesterase activity can indicate exposure to certain commonly used pesticides and can be harmful to animals.

The scientists collected 170 tadpole and 117 adult Pacific treefrogs, a species fairly abundant in the Sierra Nevada, from 23 sites in six locations including coastal, foothill, Lake Tahoe Basin Yosemite and Sequoia national parks; adult frogs were also collected from Lassen National Park. They found cholinesterase activity levels in tadpoles were significantly lower in mountains east of the San Joaquin Valley compared with similar sites farther north and east of the Sacramento Valley where agricultural activity is less intense. Moreover, cholinesterase activity became decreasingly lower in tadpoles from northern and southern sample sites as the sites moved in a gradient from the coast to the higher elevations. Thus, tadpole populations in the mountains had lower cholinesterase values than those along the coast. Similar but less significant trends were seen in adult frogs.

The researchers also measured concentrations of particular pesticides in the bodies of tadpoles and adults. More than 50% of adult frogs and tadpoles at Yosemite National Park had measurable levels of chlorpyrifos or diazinon, compared to only 9% at coastal reference sites. Frogs at Yosemite National Park also had a higher frequency of detection for chlorpyrifos than those on the coast. Both diazinon and chlorpyrifos degrade very rapidly in organisms, and detection of either compound indicates recent exposure to the chemicals. of the pesticides tested in related lab studies, chlorpyrifos and diazinon, commonly used organophosphates, suppressed nervous system activity and, along with endosulfan, a frequently used organochlorine pesticide, proved the most highly toxic to frogs. Diazinon has recently been targeted for a three-year phaseout by the US Environmental Protection Agency (US EPA).

"The presence of pesticides and the decrease in cholinesterase activity in Pacific treefrogs suggest that other species, which are more closely associated with water, could be even more affected," said Dr. Gary Fellers, a research biologist and amphibian specialist at the USGS Western Ecological Research Center in California. "Mountain yellow-legged frogs, for example, spend two or three years as tadpoles before they metamorphose and then spend considerable time in the water as adults. Melting of pesticide-contaminated snow could provide a pulse of toxic chemicals at a critical time in the life history of these frogs."

As the nation's largest water, earth and biological science and civilian mapping agency, the USGS works in cooperation with more than 2,000 organizations across the country to provide reliable, impartial, scientific information to resource managers, planners, and other customers. This information is gathered in every state by USGS scientists to minimize loss of life and property from natural disasters, contribute to sound conservation, economic and physical development of the nation's natural resources, and enhance quality of life by monitoring water, biological, energy and mineral resources.

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