Tree diversity enhances predation by birds but not by arthropods across climate gradients

Tree diversity can promote both predator abundance and diversity. However, whether this translates into increased predation and top-down control of herbivores across predator taxonomic groups and contrasting environmental conditions remains unresolved. We used a global network of tree diversity experiments (www.treedivnet.ugent.be; TreeDivNet) spread across three continents and three biomes to test the effects of tree species richness on predation across varying climatic conditions of temperature and precipitation. We recorded bird and arthropod predation attempts on plasticine caterpillars in monocultures and tree species mixtures. Both tree species richness and temperature increased predation by birds, but not by arthropods. Furthermore, the effects of tree species richness on predation were consistent across the studied climatic gradient. Our findings provide evidence that tree diversity strengthens top-down control of insect herbivores by birds, underscoring the need of implementing conservation strategies that safeguard tree diversity to sustain ecosystem services provided by natural enemies in forests.

Methods
The data and R (R markdown) code used in this study are included here.  The data file shows plot-level data for tree species richness, arthropod, and bird predation, alongside site-specific variables in 14 experimental sites belonging to the TreeDivNet global network of tree diversity experiments.