Scientists home in on what’s killing frogs, and raise new questions about how far the damage could spread.

or years, scientists have puzzled over the worldwide die-off of frogs and other amphibians. Because they are extremely sensitive to pollution and environmental change, frogs have been seen as a sort of early warning sign — the proverbial canary in the coal mine — indicating that ecosystems are in peril. Pinpointing the cause of the die-off has proven tricky, however.

In California’s Sierra Nevada mountain range, the red-legged frog has disappeared from 70 percent of its historic range over the past three decades. The range of the mountain yellow-legged frog has shrunk by 90 percent. Carlos Davidson, an environmental sciences professor at California State University in Sacramento, began studying the die-offs in 1993. He looked at a number of possible culprits, including habitat destruction, global warming, and ultraviolet radiation associated with the earth’s thinning ozone layer. Now, he’s convinced that the real killer is none of the above.

When he started his research, Davidson knew that pesticides were found at very high elevations in the Sierra Nevada, apparently blown in on the wind. Only minute amounts showed up there, however, and a direct connection to frog declines had never emerged.

But when Davidson looked at wind patterns and maps of agricultural land, he saw a stark correlation: The greatest declines in frogs were downwind of major agricultural areas such as the Central Valley, the farming capital of the world. This was true in even in the most pristine mountain forests, such as Lassen Volcanic National Park, which has lost nearly all of its Cascade frogs. The prevailing winds in Lassen come directly from the Central Valley, where 165 million pounds of pesticides are used every year. In contrast, in the nearby Trinity Alps, where the wind blows in from the Pacific Ocean, Cascade frogs are abundant. "I actually did not think it had anything to do with pesticides," says Davidson. "The results I got were really a surprise."

Unanswered questions

The link to pesticides has propelled the case forward, but it hasn’t fully solved the mystery of the Sierra’s disappearing frogs. The amounts of pesticides found in mountain frogs are at least ten times lower than the doses that are lethal to frogs in the laboratory, says Davidson.

"We really don’t know yet how pesticides affect the frogs. But there are lots of other ways to kill frogs other than killing them outright," he says. His hunch is that the chemicals disrupt the frogs’ endocrine system, either interfering with their reproductive systems or weakening their immune systems to the point where they are killed by disease.

Davidson and his colleague, Roland Knapp, a research biologist at the Sierra Nevada Aquatic Research Laboratory in Mammoth Lakes, are exploring the possible interplay between predatory fish and pesticide drift. Frog decline is greater in the presence of non-native trout, which prey on frog eggs and tadpoles, and likely kill more frogs indirectly, they say.

Frogs that are stressed by predators may be more susceptible to the effects of pesticides. A study on frogs in the Eastern United States suggests that even small doses of pesticide become considerably more lethal when a predator is nearby.

"All of these synergistic effects are not figured out yet," says Davidson. Nonetheless, the mounting evidence of the pesticide link has prompted one environmental group, Californians for Alternatives to Toxics (CAT), to sue the state Department of Pesticide Regulation, demanding that the agency re-evaluate eight different agricultural chemicals that have been found in the Sierras.

Department spokesman Glenn Brank says the agency is watching the frog studies closely, but so far has seen no evidence of a cause-and-effect relationship between pesticide drift and frog declines. Farmers, too, caution against jumping to conclusions. California Farm Bureau spokesman Bob Krauter says California farmers are already under the strictest pesticide regulation in the country. "Pesticides are a convenient target," says Krauter. "It really begs for more information than we have right now."

But CAT Executive Director Patty Clary says the studies should trigger closer investigation by the state. "They say there’s no proof that pesticides are killing frogs. But they aren’t looking for it, either. All we’re asking is that they take the good hard look that the law requires them to."

The ripple effect

Meanwhile, scientists are beginning to look at how pesticide drift is affecting other wildlife in the Sierra. Roland Knapp will complete a study this year that should determine whether the pesticides — formulated to kill agricultural insect pests — are having an effect on native insect populations.

It’s also a safe bet that the disappearance of Sierra Nevada frog populations has a ripple effect on other species. The terrestrial garter snake, for example, feeds primarily on mountain yellow-legged frogs. Not surprisingly, Knapp and his colleagues have found that in lakes and streams where the yellow-legged frog has disappeared, the garter snake has vanished as well.

It’s not hard to imagine even more far-reaching scenarios, says Knapp. Take the Clark’s nutcracker, a bird in the jay family that eats frogs. If the bird were affected by amphibian declines, he says, it could change the way the Sierra Nevada landscape looks today. That’s because the white pines that are prevalent in the high elevations depend on the nutcracker to distribute their seeds (HCN, 12/4/00: Last chance for the whitebark pine). "You’re pulling threads out of a tapestry," says Knapp. "At some point, that tapestry is going to unravel."

The author covers the environment for the Sacramento News and Review.

Carlos Davidson, California State University, Sacramento 916-278-6063

Californians for Alternatives to Toxics 707-445-5100,

California Department of Pesticide Regulation 916-445-4300,