Hydrologically defined niches reveal a basis for species richness in plant communities

Species-rich plant communities are prized repositories of biodiversity and a dwindling resource, but how the large numbers of species that characterize such communities are able to coexist is poorly understood. Resource-based competition theory predicts that stable coexistence between species depends on each being a superior competitor in its own niche(1). The theoretical problem is that plants all require the same resources and acquire them in a very limited variety of ways, so observed niche overlaps are high(2,3) and exclusion of all but the best competitor is the predicted result. This problem, central to community ecology, has elicited a variety of theoretical solutions(4-7), several of which invoke some degree of niche separation in time or space(8,9). The signature of niche separation in the field is to be found in community structure, which should indicate (i) smaller than expected niche overlaps on relevant niche axes and (ii) a trade-off between species' resource use on orthogonal axes. Here we provide evidence for the existence of both these conditions in a species-rich plant community