Interaction between stress induced by competition, predation, and an insecticide on the response of aquatic invertebrates
This study investigated the effects of competition and predation on the sensitivity of aquatic organisms to the insecticide chlorpyrifos. Two laboratory experiments were performed using food as a limiting resource. In the first experiment, the combined effects of chlorpyrifos and different levels of intraspecific and interspecific competition were assessed on Gammarus pulex survival using Asellus aquaticus as a competing species. In the second experiment, the combined effects of chlorpyrifos and intraspecific and interspecific competition, and predation were assessed on Daphnia pulex abundance using Brachionus calyciflorus as a competing species and Chaoborus sp. larvae as a predator.
In the first experiment, intraspecific and interspecific competition increased the survival of G. pulex, probably due to the cannibalistic nature of G. pulex and its predation response on A. aquaticus under stress conditions. In the second experiment, intraspecific and interspecific competition significantly affected the D. pulex population structure, but did not influence the total population size. Predation was found to significantly increase the sensitivity of D. pulex at low exposure concentrations, but interacting effects were less noticeable at high exposure concentrations. This study shows that species interactions do not always increase the vulnerability of aquatic populations to chemical stress and that a priori unexpected species interactions (e.g. cannibalism and competitor predation) or reduced predator grazing pressure can alleviate competition and predation stress on population-level insecticide effects under food limiting conditions.