Deriving a neutral model of species abundance from fundamental mechanisms of population dynamics

1. Ecologists have long sought to derive assembly rules of ecological communities from the fundamental processes of population dynamics, but this goal has remained elusive. Neutral theory has reinvigorated the search by showing that patterns of relative species abundance closely resembling those actually observed arise under the assumption that, to a first approximation, all species are demographically identical on a per capita basis. 2. Here a neutral model is proposed to incorporate all four fundamental processes of population dynamics: birth, death, immigration and emigration. This symmetric model demonstrates that patterns of relative species abundance are fully derivable from these basic processes of population dynamics. 3. The theory derived extends the concept of community by showing that a continuum exists between large-scale ('metacommunity') and small-scale ('local community') processes, eliminating the artificial distinction between the two made by the current neutral theory and by the theory of island biogeography. 4. The population-based species-abundance model describes very well the observed patterns of relative species abundance of tropical trees, breeding birds in the USA, aphids at Rothamsted, UK, and estuarine fishes in the north-east USA. 5. The study also notes that while species assemblages may be well described by the neutral processes of population dynamics, the inference of mechanisms from pattern fitting is not warranted because one-to-one relationships between generating mechanisms and community patterns usually do not exist, either in the neutral realm or in the niche world.