A tale of two pesticides: how common insecticides affect aquatic communities

1. Recent ecotoxicology studies show that pesticide exposure can alter community composition, structure and function. Generally, community responses to pesticides are driven by trait‐ and density‐mediated indirect effects resulting from sublethal and lethal effects of pesticide exposure on vulnerable taxa. These effects depend upon the concentration of the pesticide and the frequency of exposure. 2. While more research is needed to understand community‐level responses to pesticide exposure, testing the effects of multitudes of registered chemicals on ecologically relevant communities is overwhelming. Recent reviews suggest that contaminants with similar modes of action should produce comparable community‐level responses because they have similar direct effects and, as a result, similar indirect effects; this hypothesis remains largely untested. 3. We subjected pond communities containing zooplankton, phytoplankton, periphyton and leopard frog tadpoles (Rana pipiens)] to several applications (single applications of medium or high concentrations or weekly applications of a lower concentration) of two acetylcholine esterase inhibiting insecticides, malathion and carbaryl that have comparable toxicity for aquatic organisms. 4. We found that both insecticides cause comparable trophic cascades that affect zooplankton and phytoplankton abundances; however, their effects on amphibians diverged, especially when exposed to higher concentrations of insecticides. Malathion caused a trophic cascade beginning with a decline in cladocerans followed by increases in phytoplankton. At a medium concentration, this cascade also caused a subsequent decrease in periphyton. Carbaryl caused a similar trophic cascade with the highest application, a weak trophic cascade with the medium application and no cascade with smallest application. Malathion directly reduced tadpole survival at all concentrations. Survivors in the two higher treatments were larger at metamorphosis while survivors in the lowest treatments were smaller and developed slowly. In contrast, carbaryl was not directly toxic to tadpoles, but indirectly reduced survival because slow growth and development prevented some tadpoles from metamorphosing before the mesocosms dried at medium and low applications. 5. These results suggest that these common pesticides, which share the same mode of action, have similar effects on zooplankton and algae, but differences in the strength and timing of their effects on tadpoles reduce the generality of responses at higher trophic levels. Overall, general predictive models of contaminant effects could be improved by incorporating the relative timing of direct and indirect effects of exposure.