A conceptual framework for associational effects: when do neighbors matter and how would we know?

Interactions between individual consumer and resource organisms can be modified by neighbors, e.g., when herbivory depends on the identity or diversity of neighboring plants. Effects of neighbors on consumer-resource interactions ("associational effects") occur in many systems, including plant-herbivore interactions, predator-prey interactions (mimicry), and plant-pollinator interactions. Unfortunately, we know little about how ecologically or evolutionarily important these effects are because we lack appropriate models and data to determine how neighbor effects on individuals contribute to net interactions at population and community levels. Here we supply a general definition of associational effects, review relevant theory, and suggest strategies for future theoretical and empirical work. We find that mathematical models from a variety of fields suggest that individual-level associational effects will influence population and community dynamics when associational effects create local frequency dependence. However, there is little data on how local frequency dependence in associational effects is generated, or on the form or spatial scale of that frequency dependence. Similarly, existing theory lacks consideration of nonlinear and spatially explicit frequency dependence. We outline an experimental approach for producing data that can be related to models to advance our understanding of how associational effects contribute to population and community processes.