Community Assembly Rules, Morphological Dispersion, and the Coexistence of Plant Species

In order to find and define any assembly rules for communities, we must first investigate the patterns among species assemblages. We used a series of null models to test for patterns in wetland plant composition at the level of species, functional guilds, and traits. At the species level, we found significant checkerboard and nestedness patterns. Three functional guilds had some tendency to contribute a constant percentage to species richness, but after Bonferroni correction there was no significant pattern. Coexisting plant species showed no consistent overall pattern of morphological dispersion. However, when we considered each of 11 traits in turn, we found that 4 traits were overdispersed and 3 were underdispersed. Thus there are morphological assembly rules that constrain wetland plant community composition. These results reconcile contrasting views of community assembly. Communities can be simultaneously structured by a tension between two forces: abiotic external forces that constrain certain traits within limits and biotic internal forces that tend to keep coexisting species from being too similar. Because our sites vary along a fertility/disturbance gradient, we also investigated how trait dispersion varies in space. Trait dispersion increases with soil fertility; soil phosphorus explains about 36% of the variance in mean nearest neighbor distance. Species richness tends to decline with mean nearest neighbor distance, which contrasts with the general pattern for animal assemblages.