Community assembly is thought to be determined by two processes — interspecific competition and environmental filtering — which are assumed to result in contrasting trait distributions. Interspecific competition is thought to result in over-dispersion of traits due to competitive exclusion of similar species, while environmental filtering is assumed to result in clustered trait distributions. Here, we examine these assumptions by modeling a community of competing species with a trait-based Lotka Volterra model. Since the carrying capacity is an external factor analogous to the environment, we assume that it is a function of the species’ trait. We examine two different carrying capacity functions (unimodal and bimodal) to mimic two different environments. We also assume that competition strength between two species depends on their traits through a competition kernel, often Gaussian in shape. We vary the width of this competition kernel relative to the width of the carrying capacity function. We focus on the long-term outcome of community assembly by numerically solving for uninvasible communities (evolutionary stable state [ESS]). We examine the ESS of these communities while varying the width of the competition kernel and the shape of carrying capacities (unimodal and bimodal).
We found that as the width of the competition kernel decreases, the number of coexisting species increases. In case of unimodal carrying capacity, when competition was very diffuse, the species with the highest carrying capacity excludes all others. As competition becomes more localized, species with lower carrying capacities can coexist. To understand the role of environment, we changed the carrying capacity to a bimodal function with equal peaks. In this case, diffuse competition results in two species with traits corresponding to the highest carrying capacities. As competition becomes more localized, we see two clusters of species corresponding to the two peaks of the carrying capacity function. However, with extremely localized competition, species with traits between the carrying capacity peaks can persist. The trait distribution becomes similar to the case with the unimodal carrying capacities, even though the underlying environment is different. These results have bearing on the attempts to infer the presence of environmental filtering and competition from trait distributions in communities. Extremely local competition can result in evenly spaced traits even in presence of environmental filtering. Thus, future empirical work needs to consider the role of localization of competition while making inferences from trait distributions in species.