Omnivory and food web dynamics

Intraguild predation is a trophic interaction in which two consumers compete for one resource and where one of the consumer species may also feed on its competitor. The intraguild predator's diet follows from the relative strength of its interactions with its potential prey. Current view holds that weak interactions between species promote the stability of food webs. To the contrary, nutrient enrichment is predicted to destabilize ecosystems. We present a theoretical analysis of the interplay between intraguild predation and nutrient enrichment in a Marr-Pirt chemostat model of a microbial food web. We perform a two-dimensional bifurcation analysis, along a gradient of allochtonous nutrient levels and a gradient of one out of two biologically plausible strategies to explore the spectrum of the intraguild predator's foraging interactions. Both strategies show that intraguild predation may stabilize food chains; eliminate chaos, predicted by food chain models; give rise to multiple stable states; be favored in systems with low turn-over rates, where the intraguild predator has a low interaction strength and a low yield on the basal resource. (C) 2002 Elsevier Science B.V. All rights reserved.