Explaining variation in plant-herbivore associational effects in a tree biodiversity experiment

Within biodiversity-ecosystem function research, a major outstanding question is how herbivory, a critical ecosystem function at the base of the food web, changes along gradients of plant biodiversity. Neighbourhood-level associational effects are hypothesised to be a strong driver of biodiversity-herbivory relationships, but we lack a successful framework that explains the wide variation observed in the sign and magnitude of plant-herbivore associational effects.In this study, we combine measurements from a tree biodiversity field experiment with simulation to provide a framework for explaining variation in plant-herbivore associational effects, particularly when herbivores that feed on many different species (e.g. generalists) cause most damage. We show that monoculture herbivory levels of focal species and their neighbours predict the direction and strength of associational effects. We provide evidence that this may be due to a "spillover effect", in which some insect herbivores attracted to focal individuals ultimately end up feeding on neighbouring individuals.With an empirically parameterised simulation, we explain how spatial organisation modifies biodiversity-ecosystem function relationships when associational effects operate. We suggest a set of experiments to test the generality of our conceptual framework, to elucidate the underlying mechanisms that produce the patterns we find, and to ultimately increase the predictability of plant-herbivore associational effects. We conclude by discussing how our results might inform pest management in diversified agroecosystems and reforestation sites.Synthesis. Our results provide a potential framework for explaining why positive and negative plant-herbivore associational effects are often balanced in systems with primarily generalist herbivores and point to a path forward for predicting when increased plant biodiversity will be associated with increased, decreased or unchanged levels of insect herbivory on individual plant species in such systems.