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The Role of Pesticides in Amphibian Declines

Cascades frog in northern California creek.

Compelling evidence from many areas around the world indicate that amphibians are declining on a global basis. This decline may be one of the most serious conservation problems in the U.S. today because it is occurring in our largest park and wilderness areas where healthy, seemingly well-protected populations have disappeared for no obvious reason. These losses are important both because of the reduction in biodiversity and because amphibians are sensitive biological indicators of the health of the environment. This has led to establishment of a Declining Amphibian Task Force, sponsored by the IUCN Species Survival Commission in Lund, Sweden. This research project is designed to evaluate the status of amphibians in California, identify potential causes of decline, evaluate selected factors for their role in declining amphibians, and conduct experimental reintroductions of selected species. Only with this type of integrated approach will it be possible to solve this increasingly significant loss of biodiversity.

Project Details

This project focuses on two broad goals;

1) determine the current distribution and population status of amphibians in the non-desert parts of California, and

2) determine the cause of declines where they occur. Field work has concentrated in the Sierra Nevada and Cascades mountain ranges where the most significant declines have been observed, but extensive surveys have taken place throughout the Coast ranges as well.

Research on the causes of amphibian declines has included non-native species (e.g. fish and bullfrogs), estrogenic effects, disease, and the role of contaminants. As part of the contaminants research, we are evaluating sediment, water, air, frog tissue, and fish. 

Chlorpyrifos is the most widely used organophosphorus pesticide in California. Chlorpyrifos blocks acetylcholinesterase (AChE) at neural synapses and hence lead to repeated firing of neurons.  This can cause death through respiratory failure. In addition, chlorpyrifos can be degraded into chlorpyrifos oxon that is at least 100 times more toxic. Endosulfan is the second most commonly used pesticide in California. Endosulfan also impairs neurological function. Our experiments have shown that endosulfan is the most toxic of the commonly used pesticides in California. Whereas all three forms of endosulfan are toxic, a mixture of alpha and beta endosulfan resulted in an LC50 of 0.3 μg/kg body weight in Rana boylii (foothill yellow-legged frog) and ca. 3 μg/kg body weight in P. regilla and Bufo boreas (western toad). Approximately 86% of adult P. regilla collected from an area that had experienced declines had trace amounts of one or more of the endosulfans.  Although endosulfan use is less than that of chlorpyrifos in California, its longer half life and high toxicity may make it more dangerous. 


USGS Contact For This Project
Gary Fellers - Emeritus
(415) 464-5185
Pt. Reyes Field Station
Pt. Reyes National Seashore
Pt. Reyes, CA 94956-9799
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