April 3, 2001
Research Supports Pesticide Link
In Disappearance of Threatened Frogs
Though the California red-legged frog recently earned sweeping
federal protection from habitat destruction, researchers from
California State University, Sacramento and UC Davis have
found new evidence that their decline may also be pesticide-related.
"It's the first time scientists have been able to link
a known declining frog species with pesticides," says
Carlos Davidson, an environmental studies professor at CSUS.
"We found there is a very strong association between
declines of red-legged frogs and the amount of agricultural
land use upwind from the site. It strongly suggests that windborne
agrochemicals may be contributing to the decline."
In a study that encompassed almost all of California, Davidson
and co-author H. Bradley Shaffer, a professor at the Center
for Population Biology at the University of California, Davis,
mapped out the disappearance of red-legged frog populations.
Using those geographic patterns they analyzed possible causes
for the declines. Mark Jennings of the U.S. Geological Survey
also contributed to the study. Their findings were published
in the April issue of the journal Ecological Applications.
The red-legged frog has disappeared from over 70 percent of
its historic range in California. It was added to the threatened
species list by the U.S. Fish and Wildlife Service in 1996.
To identify historic concentrations of red-legged frogs, Davidson
and Shaffer compared museum records of their habitats, dating
back to the mid-1800s, with recent survey data.
"From the museum specimens we know where the frogs used
to be. Recent survey data tells where they are now,"
Davidson says. Of the 237 sites they looked at that once had
frog populations, 48 percent no longer do.
The researchers looked at several possible causes of the declines
- including global warming, ultraviolet radiation, pesticide
use and habitat destruction due to urbanization and agriculture
- and concluded that both urbanization and pesticides may
be important factors in the declines.
At each site they calculated the predominant wind direction
and the amount of agricultural land use upwind. The percentage
of upwind land use in agriculture for sites where the red-legged
frog has disappeared was six and a half times greater than
for sites where they still exist, suggesting that windborne
agrochemicals may be an important factor in frog declines.
"The results were consistent," Davidson says. "We
found areas with a lot of agricultural land use upwind from
them are more likely to have declines than sites with less
"It's an issue that has impact far beyond California,"
Davidson adds. "There have been amphibian declines in
many locations around the world and pesticides are definitely
a possibility," Davidson says. "In both Central
America and Australia, declines have been found close to major
"If it turns out pesticides are the cause, we'll have
to do more than set aside habitats to protect the species.
We'll have to do something about the types and amounts of
pesticides that are used and how they are applied."
Media assistance is available from the CSUS public affairs
office at (916) 278-6156 or the UC Davis news service at (530)
752-7704. Davidson can be reached at (916) 278-6063 and Shaffer
at (530) 752-7266.
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