Amphibian declines: chemicals, stress and disease

Tyrone Hayes

University of California, Berkeley, USA

Amphibian populations are declining at an alarming rate, and many causes for these declines have been hypothesized. While some populations may be susceptible and decline due to single causes, I propose that in many (if not most) cases, multiple causes interact to cause population declines and that individual populations within a species may decline for different sets of interacting causes. Using an approach that integrates laboratory experiments and extensive field work, my laboratory has generated data to support this hypothesis. In laboratory studies, we showed that pesticide mixtures alter growth, development and immune function in amphibian larvae (leopard frogs: Rana pipiens) and render exposed animals more susceptible to environmental stressors (high temperatures, desiccation, crowding). These interacting stressors result in animals that are more susceptible to predation and disease. Further, in field studies, we examined three different causes for the decline of the endangered northern red-legged frogs (Rana aurora) along the Salinas River. At the river mouth, competition and predation by the invasive American bullfrog (Rana catesbeiana) limit its numbers; further downstream, desiccation and resulting overcrowding and extreme temperatures limit survivorship of all anurans including the invasive species; while at the end of the river, agricultural run-off and pesticide exposure retard growth and development and impair reproduction and immune function. Laboratory simulations further show that these factors interact, enhancing each other’s negative effects on survivorship.