Towards a really unified theory for metacommunities

Abstract

Traditionally, community ecologists assumed that spe-cies differ in some aspects of their traits or responsesto the environment (i.e. their niches), which allow themto coexist in the same habitat (Hutchinson 1957, 1959).Recently, Hubbell (2001) and others (e.g. Bell 2001,2003) have suggested that this view is inadequate toexplain the diversity often observed in speciose systems.For example, hundreds to thousands of tree species livein tropical forests, which only have a handful of limitingresources such as water, light, and a variety of macro- andmicronutrients. Such high diversity, with so few resources,they argue, cannot be explained by niche theory.As an alternative to the traditional niche theory,Hubbell (2001) developed a neutral theory of commu-nity structure (see also Caswell 1976; Hubbell 1979;Hubbell & Foster 1986; Bell 2000, 2001, 2003; Chave L Chave 2004). In the neutral theory, pat-terns of species diversity, relative abundance, andcomposition are primarily a function of the size of themetacommunity, the dispersal rate of organisms withinthe metacommunity, and the rates of generation (spe-ciation) of new species (Bell 2001; Hubbell 2001; Chave2004). Because species are assumed to be identicalecologically, Hubbell termed his a ‘neutral theory’, bydirect analogy to neutral genes in population genetics.Hubbell further termed his theory ‘unified’ in that it issimultaneously able to predict diversity and relativeabundance of organisms in a locality, as well as bioge-ographic patterns of species composition.In this essay, I overview the key assumptions (inputs)required and the insights (outputs) that can be gainedfrom each framework. While a complete discussion ofall of the assumptions and predictions of these modelsis beyond the scope of this essay (but see Chase