Tree diversity has contrasting effects on predation rates by birds and arthropods on three broadleaved, subtropical tree species
Plant diversity is hypothesized to strengthen biological control by promoting top-down pressure of predators on herbivores. However, studies on the effects of plant diversity on actual predation rates are still scarce, particularly in forest ecosystems. We analyzed the effect of tree species richness, and the potential influence of neighbor tree density, on predation rates of arthropods and birds on artificial clay caterpillars in a large-scale forest biodiversity experiment in south-east China. Our study was focused on three broadleaved tree species that are frequently damaged by lepidopteran caterpillars. Predation rates were influenced by tree species richness on only one of the three tree species, on which arthropod predation increased and bird predation decreased with increasing tree species richness. Importantly, these relationships were mediated by neighbor tree density, being most pronounced when focal trees had fewer surrounding neighbor trees. Our findings indicate that low tree density reduced arthropod predator abundances and predation rates, but that negative effects of this reduction were compensated for in more diverse tree mixtures by a functionally more diverse predator community. In contrast, lower tree densities might have benefited insectivorous birds by making trees more accessible particularly in monocultures, which are often structurally more uniform and denser than tree mixtures. Overall, our results point to an important role of species-specific and density-dependent mechanisms in modifying the consequences of biodiversity loss on top-down effects in forest ecosystems. Future work should aim at separating the effects of different predator guilds and those of host diversity from host density.