The effects of pesticides, pH, and predatory stress on amphibians under mesocosm conditions.

Pesticides are applied throughout the world often with unintended consequences on ecological communities. In some regions, pesticides are associated with declining amphibians, but we have a poor understanding of the underlying mechanisms. Pesticides break down more slowly under low pH conditions and become more lethal to amphibians when combined with predatory stress, but these phenomena have not been tested outside of the laboratory. I examined how pH, predatory stress, and a single application of an insecticide (carbaryl) affected the survival and growth of larval bullfrogs (Rana catesbeiana) and green frogs (R. clamitans) in outdoor mesocosms. Decreased pH had no effect on survival, but caused greater tadpole growth. Low concentrations of carbaryl had no effect on either species, but high concentrations caused lower survival and greater growth in bullfrogs. Predatory stress and reduced pH did not make carbaryl more lethal likely due to the rapid breakdown rate of carbaryl in outdoor mesocosms. Thus, whereas the stress of pH and predators can make carbaryl (and other pesticides) more lethal under laboratory conditions using repeated applications of carbaryl, these stressors did not interact under mesocosm conditions using a single application of carbaryl.