vol. 170, no. 1 the american naturalist july 2007  Are Plant Populations Seed Limited? A Critique and Meta-Analysis of Seed Addition Experiments C. J. Clark, 1,2,* J. R. Poulsen, 2,† D. J. Levey, 2,‡ and C. W. Osenberg 2,§ 1. School of Natural Resources and the Environment, University of Florida, Gainesville, Florida 32611; 2. Department of Zoology, P.O. Box 118525, University of Florida, Gainesville, Florida 32611 Submitted April 22, 2006; Accepted February 8, 2007; Electronically published May 21, 2007 Online enhancements: appendix, data files. abstract: We examine the relative importance of processes that underlie plant population abundance and distribution. Two opposing views dominate the field. One posits that the ability to establish at a site is determined by the availability of suitable microsites (estab- lishment limitation), while the second asserts that recruitment is limited by the availability of seeds (seed limitation). An underlying problem is that establishment and seed limitation are typically viewed as mutually exclusive. We conducted a meta-analysis of seed addition experiments to assess the relative strength of establishment and seed limitation to seedling recruitment. We asked (1) To what degreeare populations seed and establishment limited? (2) Under what con- ditions (e.g., habitats and life-history traits) are species more or less limited by each? (3) How can seed addition studies be better designed to enhance our understanding of plant recruitment? We found that, in keeping with previous studies, most species are seed limited. How- ever, the effects of seed addition are typically small, and most added seeds fail to recruit to the seedling stage. As a result, establishment limitation is stronger than seed limitation. Seed limitation was greater for large-seeded species, species in disturbed microsites, and species with relatively short-lived seed banks. Most seed addition experi- ments cannot assess the relationship between number of seeds added and number of subsequent recruits. This shortcoming can be over- come by increasing the number and range of seed addition treatments. Keywords: seed limitation, establishment limitation, meta-analysis, seed addition, seed mass. * Corresponding author; e-mail: cclark@zoo.ufl.edu. † E-mail: jpoulsen@zoo.ufl.edu. ‡ E-mail: dlevey@zoo.ufl.edu. § E-mail: osenberg@zoo.ufl.edu. Am. Nat. 2007. Vol. 170, pp. 128–142.  2007 by The University of Chicago. 0003-0147/2007/17001-41789$15.00. All rights reserved. Identifying mechanisms that determine the abundance and distribution of plant and animal populations is a central challenge of ecology (Tilman 1997; Levine and Rees 2002; Osenberg et al. 2002; Coomes and Grubb 2003; Turnbull et al. 2005). The failure of a species to recruit at a given site can result from processes that occur at practically any life-history stage and include propagule production and transportation, competition, predation, and herbivory. De- spite this range of disparate processes and stages, several of the best-known models of species coexistence are fo- cused on propagule availability in space or time (Sale 1982; Tilman 1994; Hurt and Pacala 1995; Pacala and Levin 1997; Coomes and Grubb 2003). These models are bol- stered by empirical studies across diverse systems, dem- onstrating that early life-history events (e.g., during the transition from seed to seedling or from larva to juvenile fish) can be bottlenecks for recruitment (Persson and Greenberg 1990; Chambers and MacMahon 1994; Fenner 2000; Doherty 2002). Indeed, there is growing consensus that processes underlying mortality at early stages in the life cycle may disproportionately influence the structure, dynamics, and species composition of communities. This consensus is particularly evident in studies of plant com- munities. Two processes thought to limit plant recruitment at early stages in the plant life cycle are seed limitation and establishment limitation. Seed-limited populations have fewer individuals than possible because seeds fail to arrive at saturating densities at all potential recruitment sites (Eriksson and Ehrle ´n 1992; Nathan and Muller-Landau 2000; Turnbull et al. 2000; Svenning and Wright 2005). Seed limitation can be partitioned into two processes that restrict the ability of seeds to reach recruitment sites: (1) “source limitation,” that is, not enough seeds are produced to saturate potential recruitment sites even if the seeds could reach all sites, and (2) “dispersal limitation,” that is, not enough seeds reach all recruitment sites, even though enough are pro- duced to saturate sites (Clark et al. 1998; Schupp et al. 2002).