A Conceptual Framework for Associational Effects: When Do Neighbors Matter and How Would We Know?

Abstract

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.

Trade‐off between early emergence and herbivore susceptibility mediates exotic success in an experimental California plant community

“…Secondly, the increased differences in emergence time between individuals in polyculture may also enhance herbivory effects. The greater size of exotic species that emerge earlier than heterospecific competitors in polyculture may mean they are relatively more apparent and accessible than when growing with conspecifics in monoculture, a form of associational susceptibility (Underwood et al., 2014). Smaller, later‐emerging native species however would be relatively less apparent and accessible when growing with larger heterospecific competitors in polyculture than with similar sized conspecifics in monoculture; therefore, growth in polyculture may provide associational resistance (Underwood et al., 2014).…”

“…The greater size of exotic species that emerge earlier than heterospecific competitors in polyculture may mean they are relatively more apparent and accessible than when growing with conspecifics in monoculture, a form of associational susceptibility (Underwood et al., 2014). Smaller, later‐emerging native species however would be relatively less apparent and accessible when growing with larger heterospecific competitors in polyculture than with similar sized conspecifics in monoculture; therefore, growth in polyculture may provide associational resistance (Underwood et al., 2014). Furthermore, native species can benefit from competitive release if early‐active exotic species are preferentially consumed (Beck, Hernández, Pasari, & Zavaleta, 2015).…”

Elevational gradients in plant defences and insect herbivory: recent advances in the field and prospects for future research

“…Nevertheless, recent evidence has emerged that 'horizontal' interactions within a trophic level -conspecific or heterospecific -may indirectly influence the amount of damage received (and defences produced) by focal plants. Competition with neighbouring plants can alter focal plant growth and quality for insect herbivores (Kim and Underwood 2015), and theory on these associational effects offers avenues to disentangle the neighbourhood effects mediated by conspecific density and focal species' frequency relative to that of heterospecific neighbours (Underwood et al 2014). Although it is not known whether these effects operate over elevational gradients, conspecific densities (and thus their frequency relative to heterospecifics) are known to respond to abiotic factors that change with elevation ( Fig.…”

Plant diversity effects on insect herbivores and their natural enemies: current thinking, recent findings, and future directions

“…At the same time, it will be important to integrate these new ideas and approaches with existing theory to develop new paradigms. In this sense, Underwood et al [37] recently proposed a framework that distinguishes between the effects of plant conspecific density from effects of heterospecific neighbor frequency.…”