Ecology, 88(12), 2007, pp. 3076–3087 Ó 2007 by the Ecological Society of America SEED PREDATION BY NEOTROPICAL RAIN FOREST MAMMALS INCREASES DIVERSITY IN SEEDLING RECRUITMENT C. E. TIMOTHY PAINE 1,3 AND HARALD BECK 2 1 Department of Biological Sciences, Louisiana State University, Baton Rouge, Louisiana 70803 USA 2 Department of Biological Sciences, Towson University, Towson, Maryland 21252 USA Abstract. Seed dispersal and seedling recruitment (the transition of seeds to seedlings) set the spatiotemporal distribution of new individuals in plant communities. Many terrestrial rain forest mammals consume post-dispersal seeds and seedlings, often inflicting density-dependent mortality. In part because of density-dependent mortality, diversity often increases during seedling recruitment, making it a critical stage for species coexistence. We determined how mammalian predators, adult tree abundance, and seed mass interact to affect seedling recruitment in a western Amazonian rain forest. We used exclosures that were selectively permeable to three size classes of mammals: mice and spiny rats (weighing ,1 kg), medium- sized rodents (1–12 kg), and large mammals (20–200 kg). Into each exclosure, we placed seeds of 13 tree species and one canopy liana, which varied by an order of magnitude in adult abundance and seed mass. We followed the fates of the seeds and resulting seedlings for at least 17 months. We assessed the effect of each mammalian size class on seed survival, seedling survival and growth, and the density and diversity of the seedlings that survived to the end of the experiment. Surprisingly, large mammals had no detectable effect at any stage of seedling recruitment. In contrast, small- and medium-sized mammals significantly reduced seed survival, seedling survival, and seedling density. Furthermore, predation by small mammals increased species richness on a per-stem basis. This increase in diversity resulted from their disproportionately intense predation on common species and large-seeded species. Small mammals thereby generated a rare-species advantage in seedling recruitment, the critical ingredient for frequency dependence. Predation by small (and to a lesser extent, medium-sized) mammals on seeds and seedlings significantly increases tree species diversity in tropical forests. This is the first long-term study to dissect the effects of various mammalian predators on the recruitment of a diverse set of tree species. Key words: density dependence; maintenance of biodiversity; Manu National Park, Peru; seed dispersal; seed predation; seedling recruitment; Tayassu peccari; tropical rain forest. INTRODUCTION The vast majority of seeds that fall in forest understories fail to recruit as seedlings. Mortality rates during seedling recruitment (the seed to seedling transition) are greater than at any other life stage (Muller-Landau et al. 2004). This mortality tends to be negatively frequency dependent, which increases the diversity of the seedling layer (Webb and Peart 1999, Harms et al. 2000, Hille Ris Lambers et al. 2002). The filters that limit seedling recruitment thus contribute to species coexistence. It is therefore essential to study the causes of seed and seedling mortality to understand the processes that maintain forest diversity. However, the agents of mortality are rarely identified because the death of an individual seed or seedling is unlikely to be observed through sampling. For these reasons, our understanding of the mechanisms that contribute to seed and seedling mortality in forests remains incomplete. Terrestrial mammals affect all stages of seedling recruitment. Many terrestrial mammals are predators of seeds and seedlings, and predation can dramatically affect patterns of seedling recruitment (Terborgh et al. 1993, Ostfeld et al. 1997, Notman and Gorchov 2001, Silman et al. 2003, DeMattia et al. 2004). Terrestrial mammals also disperse seeds (e.g., Brewer 2001) and trample seedlings (Clark and Clark 1989). Nevertheless, following Beckage and Clark (2005), we refer to terrestrial mammals as predators of seeds and seedlings, understanding that their effects on seedling recruitment encompass various processes. Mammals consume more seeds than do invertebrates (Holl and Lulow 1997, Notman and Gorchov 2001), and small mammals may consume more seeds than large mammals (DeMattia et al. 2004). Excluding terrestrial mammals can increase seedling recruitment and survival (Ostfeld et al. 1997, Connell et al. 2005), change seedling community composition (DeMattia et al. 2006), and increase seedling growth rates (Wahungu et al. 2002). Neverthe- less, many studies of mammalian predation on seeds and seedlings have been of short duration (6 months; Holl and Lulow 1997, Notman and Gorchov 2001, Wahungu Manuscript received 2 November 2006; revised 22 January 2007; accepted 3 April 2007. Corresponding Editor: B. J. Danielson. 3 E-mail: cpaine3@lsu.edu 3076