Pesticides likely to cause Californian amphibian decline

Scientists from the United States Geological Survey (USGS) have confirmed that agricultural contaminants may be an important factor in amphibian declines in the state of California.

Research by the team of scientists indicates that organophosphorus pesticides from agricultural areas, which are transported to the Sierra Nevada on summer winds, may be affecting populations of amphibians that breed in mountain ponds and streams. Dramatic declines in red-legged frogs, foothills yellow-legged frogs, mountain yellow-legged frogs and Yosemite toads have been occurring for the last 10 to 15 years, in some of the states most pristine areas (see related story).

“While crucial to the agriculture industry, pesticides by their very nature can result in serious harm to wildlife both by directly killing animals and through more subtle effects on reproduction, development and behaviour,” said Dr Donald Sparling, a research biologist and contaminants specialist at the USGS Patuxent Wildlife Research Centre in Maryland. “Unfortunately, now there appears to be a close correlation between declining populations of amphibians in the Sierra Nevada and exposure to agricultural pesticides.”

The scientists found proof that pesticides are being absorbed by frogs in both aquatic and terrestrial systems and are suppressing an enzyme called cholinesterase, which is essential for the proper functioning of the nervous system, usually causing death by respiratory failure. The USGS scientists used both suppressed cholinesterase levels and direct findings of pesticides for evidence of the chemicals.

“The presence of pesticides and the decrease in cholinesterase activity in Pacific treefrogs suggest that other species, which are more closely associated with water, could be even more affected,” said Dr Gary Fellers, a research biologist and amphibian specialist at the USGS Western Ecological Research Centre in California. “Mountain yellow-legged frogs, for example, spend two or three years as tadpoles before they metamorphose and then considerable time in the water as adults. Melting of pesticide-contaminated snow could provide a pulse of toxic chemicals at a critical time in the life history of these frogs.”

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