Plant biodiversity and site productivity shape the complexity of multi-trophic interactions, which, in turn, can influence plant performance and community structure. Analyses of multi-trophic interactions are therefore crucial to understand the role of tree composition, diversity, and extinction for ecosystem processes in forest habitats. We will focus on the multi-trophic interaction structure within two food web systems: cavity-nesting bees and predatory and parasitic wasps (system 1), as well as interactions between leaf-sucking aphids, tending ants and their parasitoids and hyper-parasitoids (system 2). Moreover, interactions between both systems will be investigated, e.g. the effects of ants on the trophic interactions of the cavity-nesting community. We will link food web structure (system 2) to the quantity and diversity of sugars and amino acids in the aphids’ honeydew both in the main experiment and the CSPs. The importance of specific honeydew compounds to their interacting ant partners will be examined with choice experiments. In cooperation with our Chinese counterparts, we will link ant- to termite food webs to help explaining termite-mediated decomposition rates. This proposed sub-project will provide year-round data on trophic cascades and food web structure. Such data are critical to study the role of top-down versus bottom-up effects for a better understanding of how plant diversity can be used to stabilize forest ecosystems.