Scaling biodiversity under neutrality

Introduction To better understand biodiversity scaling, it may be useful to characterize the biodiversity scaling relationships that arise from the neutral perspective. This exercise will help in formulating quantitative null hypotheses for observed biodiversity scaling relationships. Here we use the term biodiversity to refer not only to species richness, but also to relative species abundance, and biodiversity scaling to refer to how patterns of biodiversity change on increasing sampling (spatial) scales. The study of biodiversity includes questions about the spatial dispersion and geographic range of species, relative species abundance, endemism, and beta diversity, the turnover of species across landscapes. A full discussion of all-taxa biodiversity scaling in the context of neutral theory is beyond the scope of the present theory because the theory currently applies only to communities of trophically similar species (a tree community, for example). It is also individual based rather than biomass based. Despite these limitations neutral theory is nevertheless a mechanistic, dynamical theory of community assembly based on fundamental demographic processes (birth, death, dispersal, speciation) – even though its postulated assembly rules are extremely simple. Under current neutral theory, speciation, ecological drift (demographic stochasticity in birth and deaths), and dispersal govern the presence or absence and the abundance of species in communities over local to global scales. Extinction is also critically important, but under neutrality, the extinction rate can be predicted once the other three processes are known. Neutral theory asserts that, to a first approximation, trophically similar species are identical (symmetric) in these vital rates on a per-capita basis. According to the theory, species develop differences in abundance due to demographic stochasticity and accidents of dispersal. Once species become differentiated in abundance, their expected fates diverge. Common species are expected to have longer life spans and to be more persistent members of local communities than rare species.