Changes in biodiversity within a single trophic level can have cascading effects to the associated multitrophic community and related ecological function. In this context, plant diversity is predicted to mediate the abundance and diversity of predator communities, with consequences for top-down herbivore regulation, the so-called “Enemies Hypothesis”. While the Enemies Hypothesis has been well tested within agriculture systems, relatively less is known about such dynamics in natural systems. In addition, it is unknown to what extent plant diversity mediates top-down control through effects on predator abundance vs. predator diversity. Accordingly, we tested the Enemies Hypothesis in a large-scale tree diversity manipulation in the Yucatan peninsula of Mexico. Specifically, we assessed the effect of tree diversity on insectivorous bird predators and herbivore predation by comparing tree monoculture and polyculture plots. We used plasticine caterpillar models to measure relative predation rates from birds among experimental plots paired with observations of foraging birds. In so doing, we determined the relationship between tree diversity, predator abundance, predator diversity (phylogenetic and functional) and predatory effects in the context of a non-agricultural, forested ecosystem.
We observed an overall positive effect of tree diversity on top-down control, corresponding to a 64% increase in predation rates between monoculture and polyculture plots. This net diversity effect was largely driven by positive complementarity among tree species, where individual tree species had consistently higher predation in polyculture plots. Producer diversity also mediated the diversity of the foraging bird community, reflecting greater variation in ecologically-relevant traits of predators with higher tree diversity. Both functional and phylogenetic diversity of insectivorous birds perform better than traditional metrics of taxonomic diversity in predicting plot-level predation rates.