In species-rich natural plant communities, such as tropical rain forests, it is sometimes argued that most species have similar habitat requirements and competitive abilities, so that community composition is the net consequence of a slow random drift in species abundances. In 'neutral models' assuming identical competitive abilities, competition occurs all the time, but community structure is strongly affected by chance and history, and changes occur only on a geological, evolutionary, timescale. The heterogeneity of the biotic and abiotic microenvironments experienced by individuals of each tree species may prevent competitive exclusion as well as any directional character displacement among a large number of competing species. Diffuse coevolution of many competing species would lead to convergence on the statistical average of biotic and abiotic neighborhood conditions. Following this radical 'symmetric neutral theory' popularized by S. P. Hubble, simple models incorporating dispersal and recruitment limitation are fully capable of maintaining arbitrary levels of species diversity of arbitrarily competent competitors.
Although this theory has proved to be a good approximation to reality in a limited number of cases, it stimulated the development of spatially explicit models of plant competition incorporating stochastic processes.
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