DECLINING DOWNWIND: AMPHIBIAN POPULATION DECLINES IN CALIFORNIA AND HISTORICAL PESTICIDE USE

Abstract

Pesticides have long been proposed as a possible cause of amphibian population declines, but due to a number of challenges there has been relatively little ecotoxicological research on pesticides and declines in natural populations. My study examines the association between the spatial patterns of declines for five California amphibian species and historical patterns of pesticide use in California from 1974 to 1991 based on Department of Pesticide Regulation records. Information on declines was derived from maps of historical sites and current population status for the Yosemite toad (Bufo canorus), California red-legged frog (Rana aurora draytonii), foothill yellow-legged frog (R. boylii), Cascades frog (R. cascadae), and the mountain yellow-legged frog (R. muscosa). Multiple logistic-regression and generalized additive models were used to analyze the relationship between site status (present or absent) and total upwind pesticide use, 64 pesticide classes and groups, and covariates including precipitation, elevation, surrounding urban and agricultural land use, and spatial location. Total pesticide use was a strong, significant variable in logistic-regression models for all species, except B. canorus. Total pesticide use was a significant variable even when spatial autocorrelation was accounted for by inclusion of a spatial location covariate. Cholinesterase-inhibiting pesticides (most organophosphates and carbamates) stood out as more strongly associated with population declines than any other class of pesticides. This is the first study in which population declines of multiple declining species have been associated with historical pesticide applications.