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

Abstract

Abstract1. 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.2. 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.3. 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.4. 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.5. 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. K E Y W O R D Shabitat refuge, intraguild predation, trophic cascade, trophic level, weak interactions

Environmental constraint of intraguild predation: Inorganic turbidity modulates omnivory in fairy shrimps

“…In an IGP system, two predators share a food source (shared prey), thereby acting as competitors. The feeding of an IG predator may have important influence on community persistence (Stouffer & Bascompte, 2010) and stability (Neutel et al, 2007;Wootton, 2017). The feeding of an IG predator may have important influence on community persistence (Stouffer & Bascompte, 2010) and stability (Neutel et al, 2007;Wootton, 2017).…”

“…Besides changes in community composition (e.g. the omnivore feeds predominantly on one trophic level) will favour coexistence and increase the stability of food webs (Wootton, 2017). In general, factors that decrease the strength of omnivorous trophic interactions (i.e.…”

Flood disturbance affects macroinvertebrate food chain length in an alluvial river floodplain

“…Omnivory is ubiquitous in freshwaters, and in highly disturbed environments because consumers can shift their diet when resource availability varies (Fagan, 1997, in Wootton, 2017. Empirical studies have shown that intermediate levels of trophic omnivory increase food-web stability (Denno and Fagan, 2003;Thompson et al, 2012), and may therefore be selected for in highly disturbed environments.…”

“…We suggest that because predators feed on different resources depending on the disturbance regime, maintaining the spatial heterogeneity in disturbance within a floodplain can result in overall weaker trophic interactions between predators and consumers. Additionally, by shifting their diets, omnivorous predators may enable depleted consumer populations to better recover from environmental disturbance, resulting in the same stabilising effect in time (Abrams and Fung, 2010;Singer & Bernays, 2003;Wootton, 2017). Additionally, by shifting their diets, omnivorous predators may enable depleted consumer populations to better recover from environmental disturbance, resulting in the same stabilising effect in time (Abrams and Fung, 2010;Singer & Bernays, 2003;Wootton, 2017).…”

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

“…Food web stability, here defined as temporal constancy, is a fundamental characteristic of ecosystems (Worm & Duffy, ) that can be profoundly affected by the presence of omnivores—organisms that feed on more than one trophic level (Pimm, ; Pimm & Lawton, ). Omnivory is common in food webs across a broad range of habitats, including freshwater systems (Thompson, Dunne, & Woodward, ; Thompson, Hemberg, Starzomski, & Shurin, ; Wootton, ). Omnivores reduce the strength of consumer–resource links by shunting some of the energy up the omnivore–resource pathway and away from the consumer–resource pathway (McCann, Hastings, & Huxel, ).…”