Food Webs

Historically, food web research has focused on how local deterministic interactions will influence web structure. Considering food webs in a spatial context, however, provides alternative mechanisms for observed phenomena. When species do not interact, we expect that subdivision will only decrease population sizes and make species more vulnerable to stochasticity. Interacting species, however, can use space as a mechanism ofpersistence and coexistence thereby promoting a more complex food web structure.

Ultimately, the degree to which metacommunity dynamics influence food web properties over local community dynamics will depend on dispersal, patchiness, and the interaction between the two. Spatial subsidies of immigrants or energy can stabilize or destabilize local interactions that would otherwise be determined by environmental factors. Patchiness in the form of environmental heterogeneity will allow for site-specific demography and alternative food web states within patches, though the resultant food web structure will also depend on the rate of dispersal of and colonization by both competitors and their consumers. Lastly, patchi-ness in the form of the size and distribution of habitat patches will influence the extent to which species of varying life histories and dispersal abilities experience subdivision. Together, these factors will influence interactions within and between trophic levels. Where they contribute to the stabilization of local interactions, we expect to see longer food chains or more complex food webs.

Though there is a long history of seeking to explain food chain length with productivity, recent work has shown that ecosystem size may be a more consistent predictor of chain length. This could be a result of metacommunity dynamics. If a food web is under donor-control, the criteria for the existence of higher trophic levels will become stricter for each succeeding level. If basal species have low colonization and high extinction rates, and their habitat is rare, we might expect short food chains because, by virtue of being dependent on lower trophic levels for resources, each succeeding trophic level must have extinction rates equal to or higher than the previous trophic level. Increasing colonization rates or the number of habitat patches, or decreasing extinction should enhance regional mechanisms of persistence and coexistence, stabilize local interactions, and allow for greater trophic diversity.

Spatial structure can also influence the impact of consumers on their resources. A consumer's ability to optimize energy intake by switching prey and feeding location can dampen fluctuations in prey populations, stabilizing local food webs. Even when predators are not actively selecting prey and habitat, space can create refugia for prey populations and, in some systems, decrease top predator density and allow other predators to coexist. Dispersal by consumers can also link resources and habitats in ways that are detrimental for some prey species. For example, shared predation by a mobile consumer may cause two species that never co-occur locally to exhibit apparent competition, or, oppositely, may facilitate the coexistence of two species that otherwise could not coexist locally. All of these factors affect the stability of local interactions and thus the complexity of the food web.

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