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Stability and persistence of food webs with omnivory: Is there a general pattern?

The relationship between omnivory and stability has been the subject of a longstanding debate in ecology. Early theory predicted that omnivory would decrease the probability of food webs being stable. While early empirical data appeared to support the prediction that omnivory should be rare, detailed study of food webs later revealed that omnivory is ubiquitous across ecosystems and taxa. Recent years have seen renewed interest in the omnivory-stability debate, and advances in mechanistic non-equilibrium models demonstrated that omnivory can both increase and decrease stability. Current efforts have therefore focused on identifying biological mechanisms that promote the persistence of food webs with omnivory. We synthesize recent evidence that omnivory often stabilizes food webs when it occurs as life-history omnivory, when prey experience reduced predation rates due to refuges or adaptive antipredator defences, and when omnivores interfere with each other or feed adaptively. Empirical research has lagged behind theory and there remains a shortage of studies directly measuring the stability of diverse natural communities that vary in the number and strength of omnivorous interactions. Early microcosm experiments indicated a narrow range of conditions for the persistence of simple omnivorous modules, while studies of omnivory embedded within larger natural networks have demonstrated its stabilizing effects. These new findings alter our view of food web dynamics and show that rather than looking for a simple and general omnivory-stability relationship, we should focus on identifying conditions under which omnivory is a stabilizing feature of more complex natural systems.