Why intraguild predation should be common; a modeling approach

Background/Question/Methods   Intraguild predation (IGP) exists when a predator eats one or more foods of one or more of its own prey species. This means that the species interact via both competition and predation. The first models of 3-species systems that included IGP suggested that coexistence of all three species was difficult to achieve. In particular, enrichment of the system typically decreased the abundance of the prey species, and frequently resulted in its exclusion. In spite of the discovery of many theoretical mechanisms for persistence, workers in the field continue to describe it as puzzling because theory does not predict that intraguild predation should be common. This presentation consists of three sections. The first is a brief literature review to determine the applicability of those mechanisms are already known to permit robust coexistence of all species in IGP systems. The second is a set of models of previously unexplored mechanisms that can also allow persistence of all species at high densities, even in enriched IGP systems. The third is an evolutionary model that is based on competition within the predator level between types having more or less intraguild predation.

Results/Conclusions   All three sections suggest that there is no contradiction between the full body of theory and the observation of frequent intraguild predation in natural systems. The literature review shows that the availability of refuges and presence of self-limitation by predators (both known to promote coexistence) should be regarded as the norm in natural food webs. The new models show that life-history differences in diet, population cycles, and adaptive defense also provide mechanisms that permit IGP systems to persist over very wide ranges of enrichment. Finally, the evolutionary models show that IGP is generally superior to pure predation, and that this is particularly true of variable environments. The conclusion is that there is no contradiction between theory and data on the subject of intraguild predation. Together, they argue that the phenomenon is expected to be, and actually is, quite common.