The recurrent patterns in the commonness and rarity of species in ecological communities—the relative species abundance—have puzzled ecologists for more than half a century^{1, 2}. Here we show that the framework of the current neutral theory in ecology^{3, 4, 5, 6, 7, 8, 9, 10} can easily be generalized to incorporate symmetric density dependence^{11, 12, 13, 14}. We can calculate precisely the strength of the rare-species advantage that is needed to explain a given RSA distribution. Previously, we demonstrated that a mechanism of dispersal limitation also fits RSA data well^{3, 4}. Here we compare fits of the dispersal and density-dependence mechanisms for empirical RSA data on tree species in six New and Old World tropical forests and show that both mechanisms offer sufficient and independent explanations. We suggest that RSA data cannot by themselves be used to discriminate among these explanations of RSA patterns¹⁵—empirical studies will be required to determine whether RSA patterns are due to one or the other mechanism, or to some combination of both.

1. Department of Physics, 104 Davey Laboratory, The Pennsylvania State University, University Park, Pennsylvania 16802, USA
2. Department of Renewable Resources, University of Alberta, Edmonton, AB T6G 2H1, Canada
3. Department of Plant Biology, The University of Georgia, Athens, Georgia 30602, USA
4. The Smithsonian Tropical Research Institute, Box 2072, Balboa, Panama
5. Dipartimento di Fisica 'G. Galilei', Universitá di Padova, via Marzolo 8, 35131 Padova, Italy

Correspondence to: Jayanth R. Banavar¹ Correspondence and requests for materials should be addressed to J.R.B. (Email: banavar@psu.edu) or A.M. (maritan@pd.infn.it).

Received 13 May 2005; Accepted 7 July 2005

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