Hyperdiverse communities have inspired a rich body of theory to explain community assembly and species coexistence. In species-rich communities, multiple processes operating at different scales may determine community assembly and patterns of biodiversity. These include ecological drift, dispersal limitation of extreme numbers of rare species, local interactions among dominant and rare species, and species sorting at landscapes scales. The relative importance of these processes has been challenging to resolve because dissimilar community assembly mechanisms can create similar spatial patterns of biodiversity. We experimentally tested the relative importance of assembly processes at local and landscape scales in the hyperdiverse longleaf-pine ecosystem of the southeastern United States. In each of two habitat types spanning a soil-resource gradient, we experimentally removed a dominant bunchgrass species to test the influence of competition from dominant species, ecological drift, and landscape-scale species sorting on spatial variation in community composition (beta-diversity).
Results/Conclusions
Using null models of community assembly, we found that beta-diversity was significantly more non-random in high-resource habitats compared to low-resource habitats. Differences in community composition were more strongly correlated with differences in local abiotic conditions within high-resource habitats compared to low-resource habitats, suggesting that the importance of species sorting increases with resource availability. In contrast, beta-diversity and composition-environment relationships were unaltered by the presence of dominant bunchgrasses in both habitats, suggesting that dominant species do not mediate the importance of species sorting. Although competition or facilitation among dominant and rare species may be relatively common in species-rich communities, our experimental results indicate that dominant species generally have a neutral influence on community dynamics. Our findings highlight the importance of dispersal limitation and species sorting at landscape scales as principal drivers of community assembly in hyperdiverse ecosystems.