Seed dispersal in a tropical forest: Empirical patterns, their origins, and their consequences for community dynamics

Seed dispersal is a very important component of plant community dynamics, and yet one that is poorly understood---especially in diverse ecosystems such as tropical forests. In this dissertation, I analyze patterns of interspecific and interannual variation in seed dispersal among trees in a tropical forest, explore ultimate evolutionary and proximate ecological causes of this variation, and examine the consequences of limited seed dispersal for forest dynamics.; In a theoretical study, I consider how species-specific natural enemies will influence the evolution of dispersal, especially long-distance dispersal. Such natural enemies play a major role in tropical forests, and their impact on dispersal evolution had not previously been considered. Results from a variety of theoretical approaches show that specialized pests exert strong selection pressure for longer distance dispersal, especially in highly diverse communities (Chapter 2).; Then I turn to the site of the empirical studies of the dissertation, Barro Colorado Island, Panama. The studies here employ numerically intensive inverse modelling methods to estimate seed production and seed dispersal for individual species from 13 years of data on seedfall within a mapped 50 ha plot (Chapter 3).; I analyze interannual variation in seed dispersal relationships and link it to causal factors. Dispersal distances were longer in years of higher windspeed for 10 of 10 wind-dispersed species, and in years of lower crop size in 28 of 42 animal-dispersed species (Chapter 4).; I compare seed dispersal and recruitment limitation among 81 and 176 tree species, respectively (Chapter 6), in part using novel methods developed here (Chapter 5). Recruitment limitation is almost universally high, thereby reducing interspecific competition. Differences in seed production per tree explain most interspecific variation in seed arrival and limitation, with seed dispersal playing a lesser role. Seed size correlates positively with seed and seedling survival, and is strongly inversely related to seed production, and in wind-dispersed species, to seed dispersal distance. Thus, there is a tradeoff ability to colonize sites with seeds and ability to compete for sites after seed arrival. This competition-colonization tradeoff may contribute strongly to the maintenance of species diversity in this and other tropical forests.