Neighborhood effects and competition alter plant dispersion and coexistence

Background/Question/Methods

Local plant community composition can play a major role in how individual plants interact with their environment. This may occur via direct competitive interactions between plants or indirectly through neighbor-mediated effects on herbivory. Associational resistance occurs when neighbor presence reduces damage caused by herbivores and associational susceptibility occurs when neighbor presence increases damage. Such “neighborhood effects,” have the potential to modify plant competitive interactions. Work on herbivore-mediated neighborhood effects in plant communities often focuses on individual plants and/or short-term population dynamics, however, neighborhood effects may play a role in shaping patterns at larger temporal and spatial scales. Here, we address three questions: (1) how does competition interact with herbivore-mediated neighborhood effects to change patterns of species’ coexistence, (2) do neighborhood effects have the potential to modify plant spatial distributions in predictable ways, and (3) do neighborhood effects lead to covariance between density and spatial aggregation? We address these questions using an individual-based, spatially explicit modeling framework that calculates the effect of herbivory and competition on plants as functions of weighted distances between individuals. We examine a range of competitive and neighborhood effect intensities, focusing on both transient population dynamics and population states at equilibrium.

Results/Conclusions

Using locations of conspecific and heterospecific plants, we determined the overall densities and the degree of aggregation or segregation of each species. Changing the strength of neighborhood effects moves the boundaries between competitive exclusion and coexistence such that as the herbivore effect strength increases, competitive exclusion occurs even with weaker competition. There is also an interaction between neighborhood effects and spatial aggregation, where stronger levels of segregation occur as neighborhood effect strength decreases. For species that receive associational resistance, as associational effect strength decreases, aggregation increases. The species that receives associational susceptibility, however, has stronger aggregation with greater associational effect strengths. Beyond this, the model suggests that there is a strong negative covariance between density and aggregation, with greater levels of aggregation at low densities. Overall, this suggests that the spatial distributions and densities of plant populations may be jointly determined by their neighbors, both through direct effects from competition and indirect effects from herbivory.