Equilibrium theory and alternative stable equilibria

Most ecologists now accept the existence of alternative stable communities. Theoretical models and laboratory experiments have shown that a community's final composition, or the relative abundances of its constituent species, can depend sensitively on the sequence of arrival of the various species and the lengths of the intervals between arrivals. Alternative stable communities may also occur in nature. Several authors have argued that this phenomenon complicates community theory and precludes the development of any general model for community development. The importance of such alternative equilibria for biogeography, however, is unclear. For the equilibrium theory of island biogeography (ETIB), what matters is not the composition or structure of any stable communities but their species richnesses. For alternative stable states to damage the ETIB seriously as a general theory, the different communities must contain substantially different number of species. Curiously, both experimental and comparative biogeographical data suggest that although colonization sequence may influence the final composition of a biota it has little effect on the final species richness. In this paper we examine three other factors that might result in alternative equilibria in species number.