Dominance and Diversity in Land Plant Communities: Numerical relations of species express the importance of competition in community function and evolution.

Most plant communities consist of several or many species which compete for light, water, and nutrients. Species in a given community may be ranked by their relative success in competition; productivity seems to be the best measure of their success or importance in the community. Curves of decreasing productivity connect the few most important species (the dominants) with a larger number of species of intermediate importance (whose number primarily determines the community's diversity or richness in species) and a smaller number of rare species. These curves are of varied forms and are believed to express different patterns of competition and niche differentiation in communities. It is probably true of plants, as of animals, that no two species in a stable community occupy the same niche. Evolution of niche differentiation makes possible the occurrence together of many plant species which are partial, rather than direct, competitors. Species tend to evolve also toward habitat differentiation, toward scattering of their centers of maximum population density in relation to environmental gradients, so that few species are competing with one another in their population centers. Evolution of both niche and habitat differentiation permits many species to exist together in communities as partial competitors, with distributions broadly and continuously overlapping, forming the landscape's many intergrading communities.