Agricultural pesticides are disrupting the nervous systems of frogs in Yosemite National Park and elsewhere in the Sierra Nevada, which could explain why many amphibians have undergone dramatic declines in the mountains, federal scientists announced Thursday.
The scientists with the U.S. Geological Survey and U.S. Department of Agriculture discovered that commonly used pesticides suppress an enzyme in frogs that controls the nervous system. Such a disruption could cause frogs to die from respiratory failure.
Because of California's prevailing winds, farm chemicals sprayed in the San Joaquin Valley blow eastward, directly toward the mountains.
The scientists say they clearly proved that the pesticides--diazinon and chlorpyrifos--are getting into the bodies of frogs and tadpoles in the Sierra Nevada and are having a toxic effect.
But they don't know how much the contamination may be responsible for the disappearance of various species, including red-legged frogs, yellow-legged frogs and Yosemite toads. Dramatic drops in populations of those species have occurred over the last 10 to 15 years.
"Unfortunately, now there appears to be a close correlation between declining populations of amphibians in the Sierra Nevada and exposure to agricultural pesticides," said Donald Sparling, a contaminants specialist at the U.S. Patuxent Wildlife Research Center in Maryland who was one of the scientists conducting the study.
For several decades, biologists have chronicled severe declines in amphibians throughout the world, from mountains to rain forests--even in areas considered remote and pristine.
"The problem is even worse than we imagined, and it's getting worse," said James Hanken, a herpatologist with Harvard University and chairman of an international task force on the issue.
Adding to the mystery, deformed frogs with missing or extra legs have shown up increasingly in some areas of the United States.
Scientific debate over the causes has intensified in recent years. In fact, the only thing that seems clear is that reasons for the decline vary from place to place.
"What everyone agrees on is that there is no single cause of amphibian decline," Hanken said.
Pesticides "certainly cannot be the universal cause for all amphibian declines," said Andrew Blaustein, an Oregon State University zoology professor who has studied frogs in the Cascade Mountains.
Also, he said, if the disruptions of nervous systems are triggering respiratory failure, then "where are the carcasses?" Frogs are not turning up dead. Instead, their numbers are smaller, suggesting a problem with reproduction as well.
Blaustein suspects pesticides are a problem for frogs but that ultraviolet radiation from a thinning ozone layer is a major culprit, too.
Amphibians, he said, "face multiple insults. All of these things are hitting them at once."
Other possible causes include parasites, disease, destruction of habitat and predators. Amphibians are vulnerable to environmental factors because they have permeable skin and have multi-step life cycles that can be disrupted at key developmental times.
The federal researchers agreed that other causes also are likely to be contributing to the frog decline in the Sierras. For example, trout introduced into lakes prey upon some species.
Bob Krauter of the California Farm Bureau said "there has been a lot of scientific focus" on the frog decline, but it is still debatable how pesticides are affecting them.
"We need to get a greater understanding of it all," he said.
In the study, 170 tadpoles and 117 adult Pacific tree frogs from 23 sites from the coast to the Sierra Nevada were tested for pesticides and levels of cholinesterase, an enzyme that controls the nervous system.
The scientists found that suppression of the nervous system increased in frogs from west to east, the direction of the prevailing winds. Also, more suppression was found in animals in the mountains east of the San Joaquin Valley than in mountains farther north and east of the Sacramento Valley.
The tree frog used in the study is fairly plentiful. Others, such as red-legged frogs, are more vulnerable because they spend more of their lives in water, said Gary Fellers, an amphibian specialist at the USGS Western Ecological Research Center, one of the study's authors.
More than half of the frogs tested in Yosemite National Park had chlorpyrifos or diazinon in their bodies, compared with 9% of frogs in coastal mountains.
Also, "the contaminant levels are higher in the Sierra Nevada than in the Coast Range," Fellers said.
The U.S. Environmental Protection Agency recently announced that it is phasing out uses of the two pesticides because of potential neurological effects on children.
The pesticides are used to keep insects off a number of crops in the San Joaquin Valley, Krauter said. He said farmers have been developing new spraying techniques to reduce the drifting of pesticides.