Concurrent niche and neutral processes in the competition-colonization model of species coexistence.

The importance of neutral dynamics is contentiously debated in the ecological literature. This debate focuses on neutral theory's assumption of fitness equivalency among individuals, which conflicts with stabilizing fitness that promotes coexistence through niche differentiation. I take advantage of competition-colonization trade-offs between species of aquatic micro-organisms (protozoans and rotifers) to show that equalizing and stabilizing mechanisms can operate simultaneously. Competition trials between species with similar colonization abilities were less likely to result in competitive exclusion than for species further apart. While the stabilizing mechanism (colonization differences) facilitates coexistence at large spatial scales, species with similar colonization abilities also exhibited local coexistence probably due to fitness similarities allowing weak stabilizing mechanisms to operate. These results suggest that neutral- and niche-based mechanisms of coexistence can simultaneously operate at differing temporal and spatial scales, and such a spatially explicit view of coexistence may be one way to reconcile niche and neutral dynamics.