Realistic variation in predator species richness produces emergent biodiversity effects

Increasing predator biodiversity often strengthens herbivore suppression. Evidence for this generally stems from artificially-constructed predator communities, wherein all species co-occur at equal relative-abundances. Such experimental designs effectively isolate richness effects from other confounding factors, but necessarily sacrifice a degree of realism. Here, we adopt a somewhat different approach, quantifying species richness and composition among predatory insects naturally assembled on B.oleracea plants in the open field, and then reproducing each community in field cages to measure its impact on aphid prey. The open-field communities naturally differed in species richness and composition, allowing us to measure the contribution of each factor to prey consumption. Across this realistic diversity gradient, predator biodiversity significantly strengthened aphid suppression. This effect was correlated with predator species number but not the density of any particular species, suggesting interspecific complementarity as the underlying cause. These results closely mirrored those of earlier studies using artificially-constructed aphid-predator communities. Further examination of our open-field predator assemblages revealed that they consisted of roughly-random groupings of the available species with no systematic change in species’ relative abundances as richness increased. This suggests that artificially- and naturally-constructed predator communities produced similar diversity effects, operating through the same mechanism, because they were similarly composed.