Seed dispersal of woody plants in tropical forests: concepts, examples and future directions

Introduction Understanding seed dispersal is critical to understanding plant population and community dynamics (Nathan & Muller-Landau 2000), especially in tropical forests where seed rain of virtually all plant species is sparse and patchy (Hubbell et al. 1999; Muller-Landau et al. 2002). Seed rain determines potential population growth rates and spatial patterns, as well as the relative influences of postdispersal processes such as seed prédation (e.g. Wright et al. 2000), microhabitat requirements for establishment (e.g. Svenning 1999) and density-dependent survival (e.g. Harms et al. 2000). Despite its importance, we know very little about seed dispersal of tropical trees, because it has been studied in only a tiny proportion of the many tropical tree species and seed dispersers, and because the patterns that have been observed have largely eluded easy generalization. Just as there is a greater diversity of plant species and animal species in the tropics than in other regions, there is also a greater diversity of seed-dispersal strategies and patterns. Seed dispersal by animals predominates it is the main strategy of 70%-90% of tropical forest plant species (Willson et al. 1989) and involves a tremendous diversity of animal species and behaviours. Birds, bats, arboreal and terrestrial mammals (everything from mice to elephants), ants, dung beetles, even ñsh can disperse seeds (Levey et al. 1994). Animals may consume fruit and drop, spit or defecate the seeds, carry seeds in their coats or scatter-hoard seeds for later consumption. Abiotic strategies such as vwnd, water and ballistic dispersal form the main mode of seed movement for the remaining 10%-30% of tropical tree species (Willson et al. 1989). The seeds of many species may experience both primary dispersal (movement before the seed reaches the ground) and secondary dispersal (subsequent movement) usually by different modes or different animal species (e.g. movement by ballistic dispersal and then