Multispecies Coexistence without Diffuse Competition; or, Why Phylogenetic Signal and Trait Clustering Weaken Coexistence.

Related and phenotypically similar species often compete more strongly than unrelated and dissimilar species. Much is unknown about the community-level implications of such complex interactions. Here, we study how they affect community dynamics differently from diffuse interactions (competing equally with all heterospecifics). We derive results for a model that applies to many forms of density dependence and also examine specific cases using a site-occupancy model of forest dynamics. The results indicate that nondiffuse competition produces three effects that will not occur under diffuse competition: first, the central niche effect-if a species has high niche overlap with several competitors, then its average fitness is reduced; second, the common competitor effect-if a species has high niche overlap with more common species, then its average fitness is reduced (these two effects are usually equalizing, because more favorable niches are likely to contain more species and more abundant species); and, finally, the community redistribution effect-when a species falls to low density, the relative abundance of its competitors will change, altering its ability to recover. The community redistribution effect usually undermines coexistence, because the community will usually become dominated by an invader's closest competitors. This study provides both instructions for measuring these effects in the field and a framework for analyzing how phylogenetic signal, trait-based niche axes, and other forms of nondiffuse competition affect coexistence.