Omnivory and stability in freshwater habitats: Does theory match reality?

Abstract

Abstract Omnivory, feeding at more than one trophic level, is a prevalent feature of freshwater ecosystems. Understanding where and when omnivory is important, its relevance for sustaining diversity, and the effect it may have on ecosystem responses to disturbances, are necessary for effective management of freshwater ecosystems. The many theoretical predictions of the effects of omnivory are often contradictory, and empirical studies aimed at understanding omnivory have been difficult and contingent on a number of factors. Here, I synthesise theoretical evidence to generate five predictions of where omnivory will be most important in freshwater ecosystems, how it is maintained and the effect it will have on communities. First, theory indicates that, while strong omnivorous interactions are destabilising, weak omnivorous interactions usually enhance stability. Therefore, mechanisms which decrease the strength of omnivorous interactions should favour their occurrence and increase stability of food webs. Secondly, omnivorous interactions which are theoretically unstable may be found in stable food webs due to stabilising features of the web as a whole. Thirdly, omnivory is likely to persist primarily at intermediate productivity levels and be more common in disturbed environments. Fourthly, omnivory is likely to decrease the strength of trophic cascades. Finally, omnivores should generally make more successful invaders. These predictions are important for effective freshwater management because actions which decrease the strength of omnivorous interactions, such as maintaining habitat refuges for consumers (e.g. woody debris and aquatic plants), may be essential for sustaining biodiversity. In addition, if omnivores make better invaders, effective invasion management may benefit from focussing resources on omnivorous invaders to limit their spread and impact. Overall, this synthesis of theoretical and empirical studies indicates that, while their predictions may be frequently at odds, with deeper investigation they are largely reconcilable and can be used to make practical suggestions for the careful management of omnivory in freshwater food webs.