Pesticides May Be Killing Off Sierra Frogs, Other Amphibians

Scientists Say Chemicals Are Wafting from Central Valley

by Carl T. Hall
San Francisco Chronicle - December 8, 2000

Pesticides borne aloft by summer winds may be contributing to the decline of frogs and other beleaguered amphibians in the Sierra Nevada, researchers reported yesterday.

New findings by scientists with the U.S. Geological Survey and Department of Agriculture show that organophosphate chemicals, commonly used by Central Valley farmers, are finding their way to seemingly pristine Sierra habitats of the red-legged frog, mountain and foothill yellow-legged frogs and the Yosemite toad.

The research is part of a worldwide scramble to uncover the many roots of amphibian die-offs and deformities being observed in a wide array of habitats and species. James Hanken of Harvard University, chairman of an international research consortium that has been examining amphibian declines, said the latest figures seem to be confirming early anecdotal reports that something was terribly wrong.

"We now have much more reliable estimates," he said, concerning the status of about 275 species of amphibians in the United States and 5,000 worldwide. "If anything, the problem is even worse than we had imagined."

Most scientists agree that many different factors are playing a role, often acting in combination, and that the causes vary from one region to the next. Some animals are hit much harder than others by a particular problem, depending on diet, lifestyle and fundamental biological distinctions.

Suspected causes include habitat destruction, predation by nonnative fish, infectious diseases and parasite invasions. Certain developmental defects, including deformed and multiple limbs, have been traced to disturbances in hormones as well as to parasites.

Chemical poisons also have long been suspects, but there has been scant evidence of any clear links. The new studies presented yesterday are among the first to offer at least strong circumstantial evidence that chemicals play a role -- even in remote high-mountain watersheds. Studies show relatively low levels of contaminants are reaching the Sierra, one-tenth or less the exposure considered lethal.

Still, researchers suspect that the contaminants may be potent enough to have a biological effect in some vulnerable amphibians by suppressing an enzyme necessary for healthy nervous- system activity. Details from the Sierra study are scheduled for publication in the journal Environmental Toxicology and Chemistry.

The findings were summarized yesterday, along with results from other amphibian hot spots around the country, during a national conference on amphibian declines held in the Washington, D.C., area. Research biologists Donald Sparling of the Geological Survey and Gary Fellers of the Point Reyes National Seashore said they found "very definite and distinct patterns" pointing to pesticide contamination when they looked at declines of California frog and toad species.

In California's Coast Range, which is not subjected to chemical-laden western crosswinds blowing up from the Central Valley, "frogs seem to be doing rather well," Fellers said. That was not the case in the Sierra, where the most significant decreases in amphibian populations were found in west-facing habitats -- particularly those directly in the path of seasonal winds that rake the heavily farmed San Joaquin Valley.

Biological findings are indirect, however, derived by studying adult Pacific tree frogs, a species still found in large enough numbers in the Sierra to allow scientists to sacrifice some animals for the sake of the research.

A total of 170 tadpoles and 117 adult tree frogs were collected from 23 sites, including Yosemite, Lassen and Sequoia national parks, coastal and foothill areas, and the Lake Tahoe Basin.

Researchers looked at the enzyme cholinesterase, which helps clear nerve junctions for repeated firing. The enzyme was found at significantly lower levels in the animals taken from sites east of the San Joaquin Valley compared with those taken from similar sites to the north or east of less heavily farmed regions.

Just how a depressed level of that enzyme might be enough to kill large populations of amphibians is unclear. One possibility is that sublethal doses of chemical poisons may be harming nerve activity in a way that makes frogs more vulnerable to other hazards -- such as by making them too slow-moving to escape predators.

Water-dwelling species such as the mountain yellow-legged frog may be more vulnerable than other species that spend much of their time on land, the scientists said.

They emphasized that the findings are early, and warned that the results showing potential toxic effects in amphibians cannot be easily applied to other species. Still, some experts raised the possibility that maimed or dying amphibians may be early signals of much wider trouble in the environment.

"We cannot ignore the potential . . . risk to other vertebrates and invertebrate species," said Carol Meteyer, a Geological Survey clinical pathologist studying amphibian developmental problems in New England.

One clear implication of the latest research is that even some of the most remote, beautiful places on the planet are not immune to subtle environmental degradation.

About half the frogs studied in Yosemite had measurable traces of organophosphates called chlorpyrifos and diazinon, for example, compared with 9 percent of the coastal frogs. Diazinon has been a widely used commercial pesticide, now being phased out for some kinds of applications by the U.S. Environmental Protection Agency. Chlorpyrifos, sold under the trade name Dursban, is also being phased out.

Tess Dunham, director of environmental protection at the California Farm Bureau Federation, said it is too soon to point fingers at the state's huge farm industry. She said organophosphates are also widely used in suburban areas for landscaping and the like.

"To automatically say it's all coming from agriculture is a big presumption to be making at this point," she said.

For more information about the latest research on amphibian declines, visit the U.S. Geological Survey Web site at www.usgs.gov/amphibians.html.