Niche differentiation and neutral theory: an integrated perspective on shrub assemblages in a parkland savanna

Investigations of structure in ecological communities need to move beyond the dichotomy between niche and neutral theory to broader conceptual frameworks that accommodate both neutral stochastic and biological structuring processes in organizing species assemblages. We propose and test a framework that integrates niche and neutral-assembly perspectives and determines their relative contributions in structuring diverse shrub species assemblages in a parkland savanna. Our approach proposes that stochastic dispersal processes initially govern the assemblage of species in discrete shrub clusters developing in grassland, but that community structure subsequently develops through the progressive action of first positive, then negative interactions among species. A comparison of observed patterns of occurrence and niche models for 12 shrub cluster species against neutral predictions revealed that neutral stochastic, island biogeographic processes accounted for most patterns of species occurrence. One species showed strong evidence of successional differentiation, whereas evidence of slight recruitment biases for five others was equivocal. Our results demonstrate the usefulness of an approach that accommodates contributions of both neutral and niche assembly rather than assuming either process alone is sufficient to account for community structure. Further development and testing of robust and falsifiable neutral theory will allow ecologists to critically evaluate the relative roles of niche differentiation and neutral, stochastic processes in structuring communities.