Declining populations of mountain yellow-legged frogs: a reassessment of the evidence implicating pesticides

Abstract

The decline in populations of the Mountain Yellow-legged frogs, Rana sierrae and Rana muscosa, in the Sierra Nevada in California is consistent with a worldwide trend in frog decline that has resulted in nine species extinctions, four of these in Australia. Spray drift from pesticides applied to agricultural crops in California’s Central Valley was widely reported as causing the decline, and the claim central to a successful campaign to ban the use of cholinesterase-inhibiting pesticides in California. We reassess the scientific and historical evidence implicating pesticide use in frog decline in California using Hill’s Criteria of Causation and show the extent to which the claim is based on correlation, analogy and extrapolation from computer modelling. Critically, concentrations of pesticides in National Parks that have experienced severe decline in frog numbers are orders of magnitude below those shown to produce sub-lethal effects in the laboratory. Of particular concern, output from influential computer modelling work was never validated; no water samples were ever taken to ground truth the calculated pesticide levels.