Location: Hydrology and Remote Sensing Laboratory
Title: Pesticide distributions and population declines of California alpine frogs, Rana muscosa and Rana sierrae Authors
Submitted to: Environmental Toxicology and Chemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 28, 2010
Publication Date: December 1, 2010
Citation: Bradford, D.F., Knapp, R.A., Sparling, D.W., Nash, M.S., Stanley, K., Tallent-Halsell, N.G., Mcconnell, L.L., Simonich, S.M. 2011. Pesticide distributions and population declines of California alpine frogs, Rana muscosa and Rana sierrae. Environmental Toxicology and Chemistry. 30:682-691. Interpretive Summary: Some populations of amphibians in the Sierra Nevada Mountains of the United States (US) have declined drastically over the last 30 years. A number of factors have been examined in this decline including the introduction of game fish into amphibian habitats, the disease chytridiomycosis, and the atmospheric transport of pesticides from the San Joaquin Valley of California. This project examined pesticide concentrations in air, tadpoles, and sediment from 28 sites in high elevation sites of the Sierra Nevada Mountains along with a biological indicator of a specific type of pesticide exposure in the tadpoles. Each site was also classified as to the abundance of amphibians in the area. These results were compared with their distance from the Valley using statistical tools. The chemical measurement data did not show strong relationships to the abundance of amphibians or distance from the Valley, indicating that pesticides are not an important predictor at these high elevation sites. However, the relationship between distance from the Valley and amphibian population were strongly correlated. This may reflect a west to east spread of chytridiomycosis disease rather than an effect of emissions from the Valley.
Technical Abstract: 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 the 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.