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. Neighborhood-level associational effects are hypothesized 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, particularly in systems with mainly generalist herbivores. 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 neighbors predict the direction and strength of associational effects. We provide evidence that this may be due to...