Omnivores (defined as species feeding on more than one trophic level) should be rare (Pimm 1982, Pimm and Rice 1987). Pimm and Rice (1987) concluded that omnivory should reduce the stability of food web interactions. However, as noted earlier in the text, a number of studies have demonstrated intraguild predation (Denno et al. 2004, Erbilgin et al. 2004, Perez-Lachaud et al. 2004, Rosenheim 2005), suggesting that top-down regulation of herbivore population irruptions could be disrupted when multiple predators are present. Polis (1991b) and Reagan et al. (1996) reported that omnivory is common in food webs when arthropods are resolved to species or "kinds." In fact, they found that most species fed at more than one trophic level, often from nonadjacent trophic levels, in desert and tropical rainforest communities.
Fagen (1997) tested the effect of omnivory on stability of community structure by manipulating the degree of omnivory (excluding either a specialist predator, the nabid bug, Nabis alternatus, or an omnivorous predator, wolf spiders, Pardosa spp.) in replicated plots, then disturbing the community by applying aphicide to disrupt prey, Macrosiphum valeriani, abundance. Plots with high levels of omnivory showed significantly reduced responses to disturbance for seven of 14 species, compared to plots with low levels of omnivory; no species showed significantly increased responses to disturbance. These data indicated that omnivory increased the stability of food web interactions.
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