Effects of aquatic exposure to the insecticide carbaryl are species‐specific across life stages and mediated by heterospecific competitors in anurans

Abstract

Summary 1. Understanding the direct and indirect mechanisms by which anthropogenic stressors interact with species and communities is central to understanding ecology in a modern world. Pesticides have been implicated in amphibian declines worldwide, but no mechanism has yet been identified. 2. We tested the hypotheses that exposure to the insecticide carbaryl (2 mg/L) would mediate competitive outcomes between larvae of two anuran species (Bufo americanus, American toads and Rana pipiens, northern leopard frogs), and that post‐exposure effects would carry over to affect juveniles in the terrestrial environment. 3. These hypotheses were tested in controlled mesocosm experiments. Toad survival decreased with insecticide exposure and presence of heterospecific competitors, relative to low‐density controls. However, when exposed to the insecticide, toads experienced the greatest survival when leopard frogs were present. Larval period of toads was lengthened by carbaryl exposure and high density of conspecifics. Leopard frog survival was reduced only by high conspecific density, and larval period was shortened by carbaryl exposure. There were no significant effects of the insecticide on periphyton abundance, although phytoplankton abundance increased and zooplankton abundance decreased after exposure. 4. A subset of metamorphs of each species from each aquatic treatment was added to 2 × 2 m terrestrial enclosures to evaluate larval treatment effects on overwinter survival and growth. Despite differences in initial mass of this subset of metamorphs, mass at spring emergence was not significantly affected by any aquatic treatment for either species. However, leopard frog survival to and mass at spring emergence benefited from larger mass at metamorphosis, while toads appeared unaffected. 5. These results suggest that while tadpoles sharing a habitat may experience reduced survival from competition, the presence of multiple amphibian species may ameliorate the effects of pesticide exposure. 6. More generally, maintaining diversity may benefit ecological communities by reducing the impacts of anthropogenic threats to certain taxa.