The Influence of Intraguild Predation on the Suppression of a Shared Prey Population: An Empirical Reassessment

Abstract

The experimental literature on the effects of intraguild predation on population growth rates of herbivorous arthropod prey has expanded substantially in the last decade, creating a body of results that can be used to test hypotheses relevant to biological control. Here we present a formal meta-analysis of the published experimental literature to assess two hypotheses: (1) intraguild predation causes an increase in the density of the shared herbivore prey, and (2) ‘coincidental intraguild predation’, in which a predatory arthropod (the ‘intraguild predator’) consumes a herbivore that harbors a developing parasitoid (the ‘intermediate predator’), is less likely to disrupt biological control than is ‘omnivorous intraguild predation’, in which the intermediate predator is consumed directly. The meta-analysis reveals that intraguild predation does not universally cause an increase in the density of the shared prey; instead, the mean effect size viewed across all studies is not significantly different from zero, and there is strong variability in effects across studies. The meta-analysis also reveals a marginally significant difference between the effects of coincidental and omnivorous intraguild predation: inclusion of a coincidental intraguild predator significantly enhances biological control, at least in the short-term trials included in our database, whereas inclusion of an omnivorous intraguild predator has little overall effect. Thus, our analysis highlights the diversity of effects generated by intraguild predators within arthropod communities. The discrepancy between theory and empirics appears likely to stem from their different time-frames, with theory often emphasizing equilibria and experimentation examining instead short-term transients, and also with the artificial simplification of arthropod communities depicted in theoretical treatments. More work, both theoretical and empirical, is needed to bridge the gap between theory and observation and to develop a deeper understanding of factors generating the observed diversity of intraguild predator effects.