A third "inhibition" model suggests that all species resist invasions of competitors. At present there exists little evidence in support of this model. Thus they can invade and grow to maturity in the presence of those that preceded them. Later species will be those able to tolerate lower levels of resources than earlier ones. A second "tolerance" model suggests that a predictable sequence is produced by the existence of species that have evolved different strategies for exploiting resources. Evidence in support of this model applies mainly to certain primary successions and in heterotrophic succession. The first "facilitation" model suggests that the entry and growth of the later species is dependent upon the earlier species "preparing the ground" only after this can later species colonize. Three models of such mechanisms have been proposed. Several alternative mechanisms may then determine which species replace these early occupants. These species cannot invade and grow in the presence of adults of their own or other species. "Opportunist" species with broad dispersal powers and rapid growth to maturity usually arrive first and occupy empty space. The sequence of species observed after a relatively large space is opened up is a consequence of the following mechanisms.
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