Consequences of consumer origin and omnivory on stability in experimental food web modules

Abstract Food web stability, a fundamental characteristic of ecosystems, is influenced by the nature and strength of species interactions. Theory posits that food webs are stabilised by omnivory and disrupted by novel consumers. To test the effects of secondary consumer origin and trophic level on basal resource stability, we constructed crayfish–snail–algae modules using four congeneric species of crayfish ( Faxonius spp.), two from native populations ( Faxonius propinquus and Faxonius  virilis ) and two from non‐native populations ( Faxonius limosus and Faxonius rusticus ). We performed surgical manipulations of crayfish feeding structures to create omnivore food web and predator food chain modules. We compared the temporal stability of these modules using measures of the coefficient of variation of the basal resource (benthic algae). Consistent with theoretical and empirical predictions, food web modules with omnivory had the lowest coefficient of variation. However, contrary to prediction, we did not find consistently higher coefficients of variation in modules with non‐native species. Rather, across species, we found the lowest coefficient of variation in modules with one of the non‐native species ( F. rusticus ) and one native species ( F. virilis ), owing to stronger interactions between these crayfish species and their snail and algal food resources. The results suggest that omnivory is indeed stabilising and that very weak interactions or very low attack rates of the consumer on the basal resource can be unstable. Thus, we demonstrate that omnivores may have different impacts than predators when introduced into a novel ecosystem, differences that can supersede the effect of species identity.