Generalist Predators, Interaction Strength and Food-web Stability

Abstract

Summary The recent description of complex food webs that contain many species abundant omnivores, relatively long food chains and have a high number of links per species has cast significant doubt over our understanding of the relationships between food-web stability, interaction strength and overall web complexity. Complex food webs contradict previous theoretical observations that highly complex webs should also be unstable and hence less likely to persist. The description of a high links per species ratio in food webs suggests that generalist predators are a common feature of many systems. Despite the abundance of such predators, there have been few considerations of their role in food-web dynamics. Theoretical and empirical studies of generalist predators suggest that their role in food-web dynamics may range from being highly destabilising to having little impact on food-web dynamics. We argue that a full understanding of the role of generalist predators in food-web dynamics can only be developed when their actions are considered within a broad spatial and temporal framework. In contrast to specialist predators, generalist predators have the capacity to switch to alternative prey as preferred prey decline. Consequently, the predator's population dynamics are not necessarily closely coupled to that of their preferred prey. When, considered over short temporal scales and at small spatial scales, such a predator is clearly destabilising as local extinction of vulnerable prey may occur given that the predator's abundance can be sustained by access to alternative prey. Species loss from a community is, however, a phenomena that is dependent on the spatial and temporal scales at which it is viewed. It cannot continue indefinitely and, once predators have eliminated vulnerable prey, those that remain are likely to be little affected by that predator's actions even though significant numbers of prey individuals may still be consumed by that predator. Remaining prey may persist within the predator's habitat, through protection within refugia or alternatively by maintaining reproduction rates that exceed the rate of predation. The food web persists in a stable, albeit altered form, but is now dominated by weak, effectively donor-control, interactions. We review evidence for this perspective of generalist predation and food-web dynamics. Consideration of the role of generalist predators in food webs suggests that dynamic constraints on food-web structure may only apply in food-webs with few species, that the relative importance of top-down and bottom-up regulation of food-web structure may be strongly linked to the availability of refugia within a habitat, and that the strength and energetic importance of a link may, to some degree, be inversely related.