Species abundance distributions: pattern or process?

Abstract

graphic properties of individuals are independent of the identity of the species to which those individuals belong can generate distributions of species abundance that replicate, to a considerable degree, those observed in real communities. The hypothesis that species are ecologically identical, at least in terms of their contribution to community and regional diversity, has been considered elsewhere (see, for example, Brown 2001; Condit et al. 2002; Clark & McLachlan 2004). One facet of the theory that has, however, received little critical attention is that of the assumptions regarding the distribution of species abundances over large sweeps of space or time. This is important because Hubbell predicts that species abundances in local communities will reflect, to an extent that depends on migration rates within the regional assemblage, the structure of the metacommunity. Hubbell argues that the diversity of a metacommunity is a function of the mode (and rate) of speciation occurring therein. He proposes two speciation mechanisms point mutation and fission (see below for details) which result in different abundance distributions. All of the analytical results presented by Hubbell and colleagues (Hubbell 2001; Volkov et al. 2003) relate to the mutation model of speciation which predicts that species abundances will be distributed according to Fisher's (Fisher et al. 1943) log-series. For example, in a recent defence of the neutral model, Volkov et al. state that: 'Under neutrality at large spatial and temporal scales, Fisher's log-series distribution is the expected steady-state distribution of relative species abundance at the speciation-extinction equilibrium in the metacommunity when the per capita birth and death rates are density independent and the same for all species, and speciation is introduced'. In this paper I therefore revisit Fisher's model and consider its potential as a descriptor of large-scale patterns of species abundance. I also highlight the need for better methods of distinguishing between rival species abundance distributions and the theories that underpin them. LOCAL COMMUNITIES AND