Dynamical evolution of ecosystems

The assembly of an ecosystem such as a tropical forest depends crucially on the species interaction network, and the deduction of its rules is a formidably complex problem(1). In spite of this, many recent studies(2-16) using Hubbell's neutral theory of biodiversity and biogeography(2) have demonstrated that the resulting emergent macroscopic behaviour of the ecosystem at or near a stationary state shows a surprising simplicity reminiscent of many physical systems(17). Indeed the symmetry postulate(2), that the effective birth and death rates are species-independent within a single trophic level, allows one to make analytical predictions for various static distributions such as the relative species abundance(3-12), beta-diversity(13-15) and the species - area relationship(16). In contrast, there have only been a few studies of the dynamics and stability of tropical rain forests(18-20). Here we consider the dynamical behaviour of a community, and benchmark it against the exact predictions of a neutral model near or at stationarity. In addition to providing a description of the relative species abundance, our analysis leads to a quantitative understanding of the species turnover distribution and extinction times, and a measure of the temporal scales of neutral evolution. Our model gives a very good description of the large quantity of data collected in Barro Colorado Island in Panama in the period 1990 - 2000 with just three ecologically relevant parameters and predicts the dynamics of extinction of the existing species.