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Airborne Pesticides as an Unlikely Cause for Population Declines of Alpine Frogs in the Sierra Nevada, California
Citation:
BRADFORD, D. F., N. G. TALLENT-HALSELL, K. Stanley, L. McConnell, D. Sparling, M. S. NASH, S. Simonich, AND R. Knapp. Airborne Pesticides as an Unlikely Cause for Population Declines of Alpine Frogs in the Sierra Nevada, California. Presented at American Society of Ichtyologists and Herpetologists, Portland, OR, July 22 - 27, 2009.
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Presentation
Description:
Airborne pesticides from the Central Valley of California have been implicated as a cause for population declines of several amphibian species, with the strongest evidence for the mountain yellow-legged frog complex (Rana muscosa and R. sierrae) in the Sierra Nevada. We measured pesticide concentrations in multiple media at multiple times at 28 sites in the southern Sierra and evaluated the pesticide-decline hypothesis in three ways: (1) we described the temporal variation in concentrations in lake water and compared these values to established critical levels; (2) we tested the hypothesis that pesticide concentrations decrease with distance from the Valley, a pattern that could explain the east-west pattern in population declines; and (3) we tested the hypothesis that pesticide concentrations are correlated with frog population status (i.e., fraction of suitable sites occupied within 2 km of a site). Media represented were air, lake water, sediment, and tadpoles of a surrogate species (Pseudacris regilla); we also measured acetyl cholinesterase activity in P. regilla tadpoles. Results do not support the hypothesis for a pesticide effect on frog populations. Concentrations of up to nine pesticides (both currently and historically used forms) were extremely low, on the order of 1 part-per-trillion in lake water, well below critical levels. Evidence for a distance effect in concentrations or cholinesterase activity was very limited. Virtually no association was found between frog population status and any chemical metric. In contrast, two well documented causes for the dramatic and continuing population declines of these frogs are chytridiomycosis and introduced trout.