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Pesticide Distributions and Population Declines of California Alpine Frogs, Rana Muscosa and Rana Sierrae
Citation:
BRADFORD, D. F., R. A. Knapp, D. W. Sparling, M. S. NASH, K. A. Stanley, N. G. TALLENT-HALSELL, L. L. McConnell, AND S. M. Simonich. Pesticide Distributions and Population Declines of California Alpine Frogs, Rana Muscosa and Rana Sierrae. ENVIRONMENTAL TOXICOLOGY AND CHEMISTRY. Society of Environmental Toxicology and Chemistry, Pensacola, FL, 30(3):682-691, (2011).
Impact/Purpose:
The Sierra Nevada and adjacent Cascade Mountains of California and Nevada have been a global hotspot for amphibian population declines in recent decades, with at least seven species strongly affected (Anaxyrus [=Bufo] canorus, Lithobates [=Rana] pipiens, Rana boylii, Rana draytonii, Rana muscosa, Rana sierrae, Rana cascadae (e.g., [1-3]). Rana muscosa and R. sierrae (southern mountain yellow-legged frog and Sierra Nevada yellow-legged frog, respectively; formerly both R. muscosa [4]) in the Sierra Nevada provide particularly dramatic examples of population declines. These two closely-related species were once nearly ubiquitous among water bodies at high elevation (mostly >2200 m) in the mountains, but remain extant at only 5-7% of their historic localities at most ([4,5]; R. Knapp, unpublished data; V. Vredenburg, San Francisco State University, San Francisco, CA, USA, unpublished data). A number of causes for the declines of these species have been evaluated, with substantial support provided for introduced game fishes (e.g., [6,7]) and the emerging amphibian disease, chytridiomycosis (e.g., [8-10]). Atmospherically deposited pesticides originating from the adjacent intensively cultivated Central Valley of California have also been implicated, acting either separately or in conjunction with chytridiomycosis. Many pesticides whose likely primary origin is the Central Valley have been found in multiple media throughout all elevations in the Sierra Nevada (e.g., [11-18]).
Description:
Atmospherically deposited pesticides from the intensively cultivated Central Valley of California have been implicated as a cause for population declines of several amphibian species, with the strongest evidence for the frogs Rana muscosa and Rana sierrae at high elevation in the Sierra Nevada mountains. Previous studies on these species have relied on correlations between frog population status and either a metric for amount of upwind pesticide use or limited measurements of pesticide concentrations in the field. The present study tested the hypothesis that pesticide concentrations are negatively correlated with frog population status (i.e., fraction of suitable water bodies occupied within 2 km of a site) by measuring pesticide concentrations in multiple media twice at 28 sites in the southern Sierra Nevada. Media represented were air, sediment, and Pseudacris regilla tadpoles. Total cholinesterase (ChE), which has been used as an indicator for organophosphorus and carbamate pesticide exposure, was also measured in P. regilla tadpoles. Results do not support the pesticide-site occupancy hypothesis. Nine pesticide compounds were detected with ≥ 30% frequency, representing both historically and currently used pesticides. In stepwise regressions with a chemical metric and linear distance from the Central Valley as predictor variables, no negative association was found between frog population status and the concentration of any pesticide or tadpole ChE activity level. In contrast, frog population status showed a strong positive relationship with linear distance from the Valley, a pattern that is consistent with a postulated west-to-east spread of the amphibian disease, chytridiomycosis.