Intraguild interactions among generalist predator functional groups drive impact on herbivore and decomposer prey

We found no positive effect of increasing predator functional group richness on prey control. However there was evidence for strong composition effects between the functional groups. The presence of ants in predator assemblages reduced the prey suppression through mostly trait-mediated intraguild interactions, while hunting and web-building spiders contributed additively to prey suppression and reduced the density of herbivore and decomposer prey by 50-60%. A trophic cascade on plant biomass triggered by web-builders and hunting spiders was diminished at levels of higher predator group diversity. In conclusion, our experiments showed that intraguild interactions strongly influence the strength of top-down control by generalist predators. Among spiders there was evidence for a positive relation between functional group richness and prey suppression but the overall outcome strongly depended on the occurrence of interference, driven by trait-mediated indirect interactions.