Pesticides and amphibian population declines in California, USA

Several species of anuran amphibians have undergone drastic population decl ines in the western United States over the last 10 to 15 years. In Californ ia, the most severe declines are in the Sierra Mountains east of the: Centr al Valley and downwind of the intensely agricultural San Joaquin Valley. In contrast, coastal and more northern populations across from the less agrar ian Sacramento Valley are stable or declining Less precipitously. In this a rticle, we provide evidence that pesticides are instrumental in declines of these species. Using Hyla regilla as a sentinel species, we found that cho linesterase (ChE) activity in tadpoles was depressed in mountainous areas e ast of the Central Valley compared with sites along the coast or north of t he Valley. Cholinesterase was also lower in areas where ranid population st atus was poor or moderate compared with areas with good ranid status. Up to 50% of the sampled population in areas with reduced ChE had detectable org anophosphorus residues, with concentrations as high as 190 ppb wet weight. In addition, up to 86% of some populations had measurable endosulfan concen trations and 40% had detectable 4,4 ' -dichlorodiphrnyldichloroethylenr, 4, 4 ' -DDT, and 2,4 ' -DDT residues.