Combined effects of acidification and competition on the feeding preference of a freshwater macroinvertebrate, Asellus aquaticus (Crustacea: Isopoda): a laboratory experiment.

To determine the combined effects of stream acidification and competition on the feeding preferences of benthic detritivores we compared, before and after sublethal acid exposure, lab-cultured populations of Asellus aquaticus reared either singly or with the closely related species Proasellus coxalis sensu lato in artificial channels. Both abiotic and biotic stressors reduced A. aquaticus density and affected its food intake. Whereas the presence of P. coxalis sensu lato increased the mass-specific ingestion rate and niche breadth of A. aquaticus according to optimal foraging theory, ingestion rate was reduced following acid exposure. Despite the increased variability in the consumption rate, variability of diet composition among individuals of A. aquaticus declined after acidification above all in the presence of the other species. Resource preferences changed, as a possible result of physiological accommodation to stress and/or selection of individuals that preferentially consumed the most processed plant detritus. The symmetry of niche overlap between the two species increased, strengthening the risk of competitive exclusion of A. aquaticus. The presence of the detritivores increased the buffering capacity of the artificial channels, reducing the rate at which the temporary hardness declined.