Washington

Drifting agricultural pesticides may be eroding once-healthy frog populations in the pristine mountain areas of California, US government scientists say.

Researchers from the US Geological Survey (USGS) and the US Department of Agriculture have found that pesticides used by Californian farmers can disrupt an enzyme that regulates the nervous system of frogs in the Sierra Nevada mountains, downwind of farming regions. These same areas are those hardest hit by amphibian losses.

But this is not the whole story of global amphibian decline, according to researchers who met last week in Washington to discuss the issue. The meeting was organized by the biological division of the USGS.

“I think that one thing everyone can agree on is that there is no single cause. There are many interactions,” says Harvard University biologist James Hanken.

Amphibians, with their moist, sensitive skins, unprotected eggs and semi-aquatic lifestyle, have long been viewed as biological indicators of environmental health.

It's no croak: a combination of environmental factors is behind frog deformities and deaths. Credit: AP

Biologists became aware of a problem in the 1980s, after reports began to accumulate of dwindling or lost frog populations and unusual deformities in amphibians. Evidence came to light that ultraviolet-B radiation, an iridiovirus, a chytrid fungus and fluke parasites could each damage amphibian populations. And a study released last April showed that one frog species disappeared when lakes in the California Sierras were stocked with non-native trout.

Another study showed that tadpoles carry much higher loads of parasites in the presence of predatory fish. This indicates that the effects of separate factors can be compounded when they are combined.

Large numbers of frogs with skeletal abnormalities such as missing or extra limbs have also been seen in some populations over the past ten years. Researchers first blamed the deformities on a pollutant — retinoic acid — but later found that extra limbs could be caused by naturally occurring parasites.

The increase in abnormalities may indicate an environmental imbalance. But Carol Meteyer, a speaker at the meeting and a USGS veterinary pathologist, said that such malformations did not themselves contribute to the decline of the species they affect.

Despite the uncertainty, USGS biologist Gary Fellers is confident that pesticides are damaging the Californian frogs. He predicts that firm evidence confirming his theory will emerge within two or three years.

→ http://www.usgs.gov/amphibian_images.html

→ http://www.frogweb.gov/tadd/publications.html