Pesticide use, already considered a factor in the decline of the threatened California red-legged frog, may also be affecting three additional frog species, according to a new study being released this week.
California State University, Sacramento environmental studies professor Carlos Davidson, who had previously linked pesticide use to reductions in red-legged frog populations, says upwind agricultural use is also associated with declines in the mountain yellow-legged frog, the foothill yellow-legged frog and the Cascades frog. "It strongly suggests wind-borne agrochemicals may be a factor," he says.
The findings are featured in the current issue of the journal Conservation Biology.
Davidson, along with colleagues Brad Schaffer of the Center for Population Biology at the University of California, Davis, and Mark Jennings of Rana Resources, studied the effects of four environmental factors - windborne pesticide use, habitat destruction, UV-B radiation and climate change on eight declining amphibian species.
To control for regional factors, the study combined multiple species with distinct ranges throughout California, including the California red-legged frog, California tiger salamander, western spadefoot toad, arroyo toad and Yosemite toad in addition to the mountain yellow-legged frog, foothill yellow-legged frog and the Cascades frog.
The researchers compared historic populations with recent survey data and then analyzed the geographic patterns for possible causes of the declines. The percentage of formerly occupied sites that are now absent was high for all species, from 33 percent absent in the tiger salamander to 83 percent absent in the mountain yellow-legged frog.
Of the four factors, upwind agricultural land use - with the potential for windborne pesticides - could be linked to the declines in all four frog species. Sites that had seen declines had up to four times more agricultural land use than sites where the species still exist.
Habitat destruction was found to be a factor in declines among tiger salamanders and the spadefoot toads.
Concerns over sharp declines led the U.S. Fish and Wildlife Service to add red-legged frogs to the threatened species list in 1996. The Center for Biological Diversity is now suing the Environmental Protection Agency for allegedly failing to consult with Fish and Wildlife about the possible affect on the frogs when pesticides are reregistered.
Davidson is now looking at the association between historic pesticide use and amphibian declines in California. He and Schaffer are also studying the effects of low-doses of pesticides on amphibian susceptibility to disease.
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