A spatially implicit model of associational effects: Neighborhood structure affects resource competition

Background/Question/Methods

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 and plant-pollinator interactions. Unfortunately, there are few appropriate models and data to determine how neighbor effects on individuals contribute to net interactions at population and community levels. We develop and analyze a model of two plant species and an herbivore, where the effects of the herbivore depend on the local density and frequency of each plant species.  Distributions of neighborhood types are defined probabilistically, rather than by spatially explicit locations, in order to achieve exact numerical solutions.  Plant densities are a function of previous population sizes and herbivory; we consider cases with either constant or variable herbivore pressure.

Results/Conclusions

Our results demonstrate that associational effects can alter the outcome of competition, when compared to a nonspatial model of plant competition that is based on mean species abundances.  Local neighborhood structure creates heterogeneity in the susceptibility of individuals to herbivory, creating a type of ‘refuge effect’.  This can be either stabilizing or destabilizing, depending on the symmetry of associational effects, the functional form of local frequency dependence, and strength of intra- and interspecific aggregation.  Symmetric associational effects (greater resistance for one species implies greater susceptibility for the other) contribute to competitive exclusion, where asymmetric associational effects can ameliorate competitive interactions and promote coexistence.