The Changing View of Pesticides as a Driver of Frog Declines

Pesticides have long been hypothesized to be drivers of amphibian declines in the Sierra Nevada. In the early 1970s, Lawrence Cory and colleagues published a study showing that DDT residues could be detected in mountain yellow-legged frogs throughout the Sierra Nevada, and they suggested that this was the result of DDT applications in the Central Valley. Since then, a myriad studies have detected many additional pesticides in numerous media in the Sierra Nevada, including water, lake sediments, air, snow, rain, fish, and frogs. 

The hypothesis that pesticides could be the cause of frog declines is rooted in the observation that declines observed during the 1980s and 1990s were most severe close to the Central Valley but populations in the High Sierra seemed to be relatively healthy. In an effort to test the pesticide hypothesis, in the mid-1990s, Gary Fellers and colleagues moved southern mountain yellow-legged frogs (Rana muscosa) from Sixty Lake Basin near the Sierra Nevada crest (where frogs were still abundant) west to the Tablelands area of Sequoia National Park where frogs had disappeared a decade earlier due to unknown causes. During this study, pesticide concentrations made in Sixty Lakes Basin and Tablelands indicated that levels were generally higher in the Tablelands area. The frog populations translocated to the Tablelands declined rapidly to extinction, a result at least consistent with the pesticide hypothesis. 

In 2007, Carlos Davidson and I published a paper showing that the distribution of mountain yellow-legged frogs across the central and southern Sierra Nevada was correlated with a distance-weighted metric describing the amount of pesticides applied upwind. This provided additional suggestion that pesticides might be involved in the decline of mountain yellow-legged frogs, but the evidence was correlative and based on the untested assumption that the variable describing the amount of pesticides applied upwind was in fact a good predictor of contaminant levels across the high country. As such, the role of pesticides in causing the mountain yellow-legged frog's decline remained largely a possibility based on circumstantial evidence.

During the past year, David Bradford and colleagues have published three papers that provide for the first time detailed field measurements of concentrations of dozens of pesticides across Sequoia-Kings Canyon National Park. Collectively, these papers cast doubt on the idea that pesticides are an important driver of amphibian declines in the high elevations of the Sierra Nevada. First, concentrations of all detected chemicals were extremely low, averaging in the low parts-per-billion in lake sediments and tadpoles, and 10 picograms/m3 in the air. For reference, one part-per-billion is roughly equivalent to a single drop in an Olympic-sized swimming pool, and a picogram is one-trillionth of a gram! Second, there was no relationship between measured pesticide concentrations and distance to the Central Valley, indicating that the pesticide metric that Carlos and I used in our paper was likely not a good predictor of pesticide exposure. Third, there was no correlation between the occurrence of mountain yellow-legged frogs and measured pesticide concentrations. 

So, these latest studies suggest that pesticides are unlikely to be playing an important role in amphibian declines in the high elevation portions of the Sierra Nevada. Whether such effects could be occurring in lower elevation habitats that are closer to the Central Valley and that therefore receive higher levels of airborne contaminants remains possible, although still largely untested. 

Bradford, D. F., E. M. Heithmar, N. G. Tallent-Halsell, G.-M. Momplaisir, C. G. Rosal, K. E. Varner, M. S. Nash, and L. A. Riddick. 2010. Temporal patterns and sources of atmospherically deposited pesticides in alpine lakes of the Sierra Nevada, California, U.S.A. Environmental Science and Technology 44:4609-4614.

Bradford, D. F., K. Stanley, L. L. McConnell, N. G. Tallent-Halsell, M. S. Nash, and S. M. Simonich. 2010. Spatial patterns of atmospherically deposited organic contaminants at high elevation in the southern Sierra Nevada mountains, California, USA. Environmental Toxicology and Chemistry 29:1056-1066.

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. 2011. Pesticide distributions and population declines of California, USA, alpine frogs, Rana muscosa and Rana sierrae. Environmental Toxicology and Chemistry 30:682-691 (PDF).

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