vol. 172, no. 1 the american naturalist july 2008  Beta Diversity in Spatially Implicit Neutral Models: A New Way to Assess Species Migration Franc ¸ois Munoz, 1,2,* Pierre Couteron, 1,2,† and B. R. Ramesh 1,‡ 1. French Institute of Pondicherry, 11 Saint Louis Street, Pondicherry 605 001, India; 2. Institut de Recherche pour le De ´veloppement (IRD), Unite ´ Mixte de Recherche AMAP (Botanique et Bioinformatique de l’Architecture des Plantes), Boulevard de la Lironde, TA A-51/PS2, F-34398 Montpellier Cedex 5, France Submitted February 6, 2007; Accepted January 22, 2008; Electronically published June 5, 2008 Online enhancements: appendixes. abstract: The spatially implicit neutral model (SINM) of S. P. Hubbell predicts species’ abundance distributions at two levels: local communities where extinction balances immigration, characterized by the immigration number I, and the metacommunity, a source pool of migrants where speciation balances extinction. Previously, a plot’s I was estimated from its species abundance distribution. Here, we relate neutral theory to the additive partitioning of species di- versity and calculate the immigration rate into different plots from the variation in species composition among them. We revisit the G ST statistic of population genetics to introduce a new version, G ST (k), conditional on each community sample k. We derive an analytical expectation of G ST (k) as a function of the local immigration number, I(k), under a generalized version of the SINM, which allows the pool of migrants to deviate from the large-scale speciation-extinction bal- ance. Simulations and field data suggest that G ST (k) provides rea- sonable estimates of immigration numbers, which were compared with the results from alternative likelihood-based estimations. Keywords: diversity partitioning, G ST , immigration rate, sampling, spatially implicit neutral model, similarity estimators. Whittaker (1972) analyzed biodiversity in terms of species diversity within local sampling areas (alpha diversity), var- * Corresponding author; e-mail: francois.munoz@cirad.fr. † E-mail: pierre.couteron@mpl.ird.fr. ‡ E-mail: ramesh.br@ifpindia.org. Am. Nat. 2008. Vol. 172, pp. 116–127.  2008 by The University of Chicago. 0003-0147/2008/17201-42395$15.00. All rights reserved. DOI: 10.1086/587842 iation in species composition between locations (beta di- versity), and global diversity (gamma diversity). This dis- tinction has been central to community ecology ever since. The past decade has seen a renewed interest in the topic thanks to additive schemes of diversity partitioning (Lande 1996), which allow the expression of beta diversity as a function of environmental variables and/or distance be- tween sampling locations (Couteron and Pe ´lissier 2004; Pe ´lissier and Couteron 2007). Most of these efforts, how- ever, have focused on methods of analyzing field data rather than modeling community composition and diver- sity dynamics. In fact, few dynamical theories have been developed to model species composition and relative abundances in species-rich communities: this may be one reason why models of neutral communities have aroused such great interest among ecologists (Bell 2000; Hubbell 2001; Chave and Leigh 2002; Chave 2004; Hu et al. 2006). These neutral models can predict diversity-related pa- rameters, assuming that all trees have equal prospects of dying or reproducing, whatever their species or habitat (Hubbell 1979, 2001). Whether this assumption holds or not (Hubbell 2006; Zillio and Condit 2007), neutral mod- els can help ecologists grasp how the interaction between migration and local “ecological drift” (Hubbell 2001) may shape taxonomic composition. Conversely, one may be interested in using the neutral theory to estimate immi- gration rates in local communities from the variation in species composition among a set of community samples (beta diversity), as Wright (1931, 1943) and Slatkin (1985) inferred migration rates within a species from variation in genetic composition in a subdivided population. Basically, published neutral models are either implicit or explicit regarding the role of local dispersal limitation in broad-scale migration patterns. The spatially explicit modeling approach is appealing in explorations of the in- fluence of fine-scale processes of dispersal limitation on community structure (Condit et al. 2002). Expectations for a similarity statistic closely related to beta diversity have been derived by Chave et al. (2002) and Chave and Leigh (2002) from the spatially explicit framework of Ma- le ´cot (1969), which assumes space homogeneity and spatial