vol. 159, no. 1 the american naturalist january 2002 Comparing Classical Community Models: Theoretical Consequences for Patterns of Diversity Je ´ro ˆme Chave, * Helene C. Muller-Landau, † and Simon A. Levin ‡ Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey 08544-1003 Submitted October 26, 2000; Accepted July 7, 2001 abstract: Mechanisms proposed to explain the maintenance of species diversity within ecological communities of sessile organisms include niche differentiation mediated by competitive trade-offs, frequency dependence resulting from species-specific pests, re- cruitment limitation due to local dispersal, and a speciation- extinction dynamic equilibrium mediated by stochasticity (drift). While each of these processes, and more, have been shown to act in particular communities, much remains to be learned about their relative importance in shaping community-level patterns. We used a spatially-explicit, individual-based model to assess the effects of each of these processes on species richness, relative abundance, and spatial patterns such as the species-area curve. Our model com- munities had an order-of-magnitude more individuals than any previous such study, and we also developed a finite-size scaling analysis to infer the large-scale properties of these systems in order to establish the generality of our conclusions across system sizes. As expected, each mechanism can promote diversity. We found some qualitative differences in community patterns across com- munities in which different combinations of these mechanisms operate. Species-area curves follow a power law with short-range dispersal and a logarithmic law with global dispersal. Relative- abundance distributions are more even for systems with compet- itive differences and trade-offs than for those in which all species are competitively equivalent, and they are most even when fre- quency dependence (even if weak) is present. Overall, however, communities in which different processes operated showed sur- prisingly similar patterns, which suggests that the form of com- munity-level patterns cannot in general be used to distinguish among mechanisms maintaining diversity there. Nevertheless, pa- rameterization of models such as these from field data on the * Corresponding author. Present address: Laboratoire d’Ecologie Terrestre, Centre National de la Recherche Scientifique, UMR 5552, 13 avenue du Colo- nel Roche, F-31029 Toulouse cedex 4, France; e-mail: chave@cict.fr. † E-mail: helene@eno.princeton.edu. ‡ E-mail: slevin@eno.princeton.edu. Am. Nat. 2002. Vol. 159, pp. 1–23.  2002 by The University of Chicago. 0003-0147/2002/15901-0001$15.00. All rights reserved. strengths of the different mechanisms could yield insight into their relative roles in diversity maintenance in any given community. Keywords: density dependence, dispersal, ecological community, neutral model, spatial ecology, trade-off model. Many mechanisms have been proposed to explain the maintenance of species diversity within communities, one of the most fundamental questions in ecology (Hutchinson 1959; Levins 1970; May 1975; Pacala and Tilman 1993). These mechanisms shape community-level properties such as species-area curves, relative-abundance distributions, and spatial patterns of species occupancy. However, few studies have addressed the theoretical implications of di- versity-maintaining mechanisms for community patterns. These mechanisms can be broadly partitioned into “equi- librial” or “nonequilibrial.” Equilibrial mechanisms can maintain constant species composition over time. They are based on functional differences among species in life-history strategy (Grubb 1977), habitat affinity (Ashton 1969, 1998), pests or predators (Janzen 1970; Connell 1971), and/or other factors that lead species to differ in their competitive ranking in differing circumstances as influenced by spatial and/or temporal heterogeneity. This heterogeneity can be exogenous (like topography), endogenous (the presence or absence of a particular competitor or pest), or both (light availability [Canham et al. 1994], soil fertility [Newbery and Proctor 1984], or disturbance regime [Connell 1979; Salo et al. 1986]). Nonequilibrial hypotheses, in contrast, explain diversity as a balance between speciation (or immigration) and extinction, with the species composition itself constantly changing (MacArthur and Wilson 1967; Caswell 1976; Sim- berloff 1976; Hubbell 1979; Chesson and Warner 1981). All these factors have been extensively discussed theoretically and tested in the field. They have all been shown to be capable of contributing to the maintenance of species diversity. One way to test for the presence and importance of these factors in a given community is to examine com- munity-level properties such as species-area curves, rela- tive-abundance distributions, and spatial patterns in real