Interactions of an insecticide with larval density and predation in experimental amphibian communities

This study examines the effects of the short-lived insecticide carbaryl, a neurotoxin, on amphibian communities experiencing natural stresses of compe tition and predation. Tadpoles of three species (Wood-house's toad /Bufo wo odhousii/, gray treefrog [Hyla versicolor], and green frog [Rana clamitans] ), representing a commonly encountered assemblage in Missouri, were reared in outdoor polyethylene pond mesocosms. We determined the effects of initia l tadpole density (low or high), Predation (newts [Notophthalmus viridescen s] absent or present), chemical exposure (0, 3.5, or 7.0 mg/L carbaryl) and their interactions on body mass, larval period, and survival to metamorpho sis. Green frogs in high-density ponds did not reach metamorphosis, but met amorphs in but-density ponds and tadpoles in high-density ponds were not si gnificantly affected by treatments or their interaction. Carbaryl reduced s urvival to metamorphosis in toads and treefrogs and increased mass at metam orphosis in treefrogs. Effects of carbaryl varied with predator environment and initial larval density. Interactions of carbaryl with predator and wit h density may result in an indirect effect of carbaryl causing increased fo od resources through the elimination of zooplankton populations that may co mpete for similar resources. Our results indicate that differences in bioti c conditions influenced the potency of carbaryl and that even but concentra tions induce changes that may alter community dynamics in ways not predicte d from single-factor, laboratory-based studies.