The global decline in amphibians has been attributed to everything from UV radiation to global warming. The first concrete evidence that agriculture may play a role in amphibian decline was presented by Carlos Davidson of the University of California, Davis, at the June 1999 Society for Conservation Biology meeting.
Davidson and H. Bradley Shaffer, also of UC Davis, studied the decline of the California red-legged frog, which was once found all the way from the coast to the Sierra Nevada mountains but today is restricted primarily to the central coast.
The researchers determined the frogs' historical distribution based in part on museum records and their current distribution based on recent surveys. They then looked for links between the frogs' decline at a given site and factors including UV radiation, global warming, urbanization and agriculture.
Agriculture had the strongest link to the red-legged frog decline: frogs were more likely to have died out at sites that were upwind of greater amounts of agriculture. This suggests that wind-borne agrochemicals may have caused the frogs' decline. Davidson and Shaffer determined the prevailing wind direction for each site and then drew a 90-mile long triangle facing into the wind for each site. The researchers found that in sites where red-legged frogs have died out, about 19% of this triangle was agricultural land.
In contrast, in sites where red-legged frogs still survive today, only about 3% of this triangle was agricultural land.
This finding makes sense in light of the fact that red-legged frogs have declined dramatically in the Sierra Nevada foothills, which lie above the agricultural land of the Central Valley. Moreover, previous studies have shown that the wind carries pesticides from the Central Valley up into the Sierra Nevada.
Davidson cautions that while these findings are significant, they do not prove that agricultural chemicals are wiping out red-legged frogs. "It's a huge leap to say that the amounts of pesticides that get up to the Sierra Nevada kill frogs," he says. "But it's a pattern that suggests that we should look into the role of pesticides in frog declines further," he says.
Materials provided by Society For Conservation Biology. Note: Content may be edited for style and length.
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