The shadow of niche partitioning on resource consumption

Much ecological and biological control theory suggests that an increase in natural enemy diversity can lead to increased pest suppression. Indeed, there is growing empirical evidence that pest densities are lowest when predator diversity is highest. However, it is exceedingly difficult to distinguish between heightened predator facilitation, reduced scramble competition (niche partitioning), or reduced interference competition as the mechanism underlying the effectiveness of diverse predator assemblages. We present a novel means to distinguish among these mechanisms, combining field experimentation and modeling to evaluate the effects of multiple aphid predators on aphid suppression. In the field, we manipulated predator diversity and density, and recorded impacts on aphid densities. We found that predator diversity’s benefits for aphid suppression increased with predator density, suggesting a link between predator competition and biodiversity effects. We then used a multiplicative risk model to correct for prey removal due to scramble competition, to look for any non-additive predator effects operating through other mechanisms. Aphid suppression was sub-additive in single species treatments, suggesting that, in addition to scramble competition, intraspecific interference reduced predator efficiency at high densities. However, after correcting for prey removal, aphid suppression was no different than additive in high diversity treatments, suggesting that only scramble competition impacted predation at high predator densities. Therefore, we provide compelling evidence that it was a dilution of intraspecific interference among predator species that led to the very effective pest control typical of diverse communities.