Direct evidence for the role of pesticides in amphibian population declines in the Sierra Nevada Mountains, California, USA

Authors: COWMAN, D - Us Geological Survey (USGS); SPARLING, D - Southern Illinois University; FELLERS, G - Us Geological Survey (USGS); McConnell, Laura; LACHER, T - Texas A&M University

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/30/2013
Publication Date: N/A
Citation: N/A

Interpretive Summary: For 20 years, conservationists have agreed that amphibian populations around the world are declining. Proposed causes of these declines include habitat loss, environmental contaminants, disease, introduced predators, global climate change, and others. The Sierra Nevada mountain region has seen significant declines, and all five native ranid species are in need of protection. In the past, scientists have utilized artificial laboratory experiments to examine effects of pesticides on amphibians. This study was designed to observe the development of caged frogs through metamorphosis as they lived in ponds located in areas of the Sierra Nevada Mountains where amphibian population declines are occurring. Simultaneous experiments were carried out in Lassen Volcanoes, Yosemite and Sequoia National Parks, California, USA. Sequoia, which historically has experienced the greatest declines among the three parks, had the highest overall pesticide concentrations found in water, sediment and frog tissues. Frog death was significantly greater in Sequoia than in Lassen or Yosemite. Other more subtle effects were related to higher contaminant concentrations. Yosemite had a significantly higher rate of malformations than the other two parks, and the types of malformations were consistent with contaminant exposure. However, no specific contaminant could be identified as a cause. Results suggest that while some contaminants are more toxic than others, many of the effects observed were most likely due to a combination of chemicals rather than any specific contaminant.

Technical Abstract: For 20 years, conservationists have agreed that amphibian populations around the world are declining. Proposed causes of these declines include habitat loss, environmental contaminants, disease, introduced predators, global climate change, and others. Substantial but indirect evidence through laboratory or mesocosm studies and through measurement of contaminant concentrations in affected areas has supported the role of contaminants in these declines. This study, however, is the first to our knowledge that directly links pesticide contamination to amphibian mortality and sublethal damage in situ, in areas actually experiencing declines. Larval Pseudacris regilla were translocated among Lassen Volcanoes, Yosemite and Sequoia National Parks, California, USA and caged from early development through metamorphosis. Sequoia, which historically has experienced the greatest declines among the three parks, had the highest overall pesticide residues in water, sediment and larval tissues. Tadpoles had an average of 1.32 to 5.9 (maximum= 10) contaminants in their tissues. Mortality was significantly greater in Sequoia than in Lassen or Yosemite. Sublethal effects including reduced growth and development rates, increased genotoxicity, and reduced cholinesterase activities were related to higher contaminant concentrations. Yosemite had a significantly higher rate of malformations than the other two parks, and while the etiology of these malformations was consistent with contaminant exposure, no specific contaminant could be identified as a cause. Data suggest that while some contaminants are more toxic than others, many of the effects observed were most likely due to a constellation of chemicals that the tadpoles are exposed to than any specific contaminant.