SACRAMENTO (AP) — Scientists have known for years that pesticides drift from California's agricultural heartland and accumulate in frogs at the highest elevations of the Sierra Nevada mountains.
Now they have a better understanding of exactly which ones collect in tissue, potentially a first step toward stopping the sharp decline of threatened amphibians across the environmentally sensitive mountain range.
"The ultimate goal here is to identify whatever it is that is contributing to the decline of amphibians. If it turns out to be one or several pesticides then people would have to figure out whether there are alternative compounds with less of an impact," said Gary Fellers, a research biologist with the U.S. Geological Survey.
Both the Sierra Nevada yellow-legged frog and the Yosemite toad, which once thrived in the range, are proposed for federal Endangered Species Act protection. Populations of many other amphibians have declined in recent years.
Back in the 1990s, Fellers was part of a team of researchers who discovered that pesticides used in California's Central Valley could be detected in mountain amphibians, underscoring the danger posed by toxic compounds that wind and rain can carry hundreds of miles.
The new study conducted in national parks and monuments tested for nearly 100 of California's most commonly used pesticides, herbicides and fungicides. It found heaviest concentrations of the mold-preventing fungicides tebucanoazole and pyraclostrobin and the herbicide simazine in the tissue of Pacific chorus frogs, chosen because it is still abundant.
The fungicides are commonly used to stop blight in grapes, tomatoes, corn and rice — crops grown in abundance across the Central Valley.
"Documenting the presence of environmental contaminants in amphibians found in our protected federal lands is an important first step in finding out whether the frogs are experiencing health consequences from such exposure," says Patrick Kleeman, a USGS amphibian ecologist who collected the frog samples.
A trade group for the pesticide industry said it will continue to monitor the study.
"Much remains to be learned about how pesticides impact amphibians and other animals. It is equally factual that wildlife is often exposed to a myriad of contaminants in the environment that makes it extremely difficult to isolate contaminates responsible for a specific set of problems," said Richard Cornett, spokesman for the Western Plant Health Association. "Industry is very aware of its responsibilities in this regard..."
The California Farm Bureau Federation was cautious in its response, reiterating that a clear link between exposure and population declines hasn't been established.
"...Exposure does not necessarily equate to harm," spokesman Dave Kranz said. "Farmers will continue to monitor the latest science about crop protection and work to produce food in the safest, most efficient way possible."
This study found that frogs across the range have far greater concentrations of the three pesticides in their tissue than exist in their ponds and meadows, an indication that levels of the toxins bio-accumulate. It also suggests that frogs might be a better way to measure toxin levels than water or sediment.
"I don't want to say it's a warning, but the point is to be able to let the public and scientists know what we're finding in frogs so that we can understand what the effects are," said Kelly Smalling, a research hydrologist with the USGS, who also worked on the study. "The science needs to come first then we can report to regulatory agencies and let them make the decisions."
After Feller's 1990s study, the use of the organophosphates he detected in frogs declined across the state. The new study found DDE, a breakdown product of the long-banned pesticide DDT, but failed to detect those old organophosphate pesticides in the newly tested frogs.
"Now we're finding a different class of compounds," Smalling said.
Scientists say that crop chemicals might be one of the reasons the frog populations are declining across the Sierra, but they also say that livestock grazing and climate change likely play roles. Scientists also would like to understand whether pesticide exposure is contributing to amphibian decline from the chytrid fungus, an infectious disease killing frogs around the world.
"Any species involved in the food web is important," Smalling said. "What we are finding is frogs are exposed to a cocktail of different contaminates over their life cycles, plus disease, habitat loss and climate change. We haven't been able to pinpoint one thing, but we're trying to look into all options."
The study took 15 frogs from seven sites two years in a row. The sites included Lassen Volcanic National Park in the north to Giant Sequoia National Monument in the south. The research was conducted by the USGS's California Water Science Center and its Western Ecological Research Center.