Ecologists have debated for more than a century in the theoretical literature over the predominant mechanisms that structure plant communities over a gradient. However, there have been few empirical comparative studies that investigate the relative importance of these proposed hypotheses. In this dissertation, I described the present composition and structure of a 40-year old secondary coastal tropical forest in Costa Rica. I assessed and tested the importance of seed rain distribution, environmental microhabitat along a coastal gradient, and seed and seedling predation by land crabs on tree recruitment. Forest composition shifted with distance from shore: density and species richness of tree seedlings, saplings, and adults increased with distance. From a non-metric multidimensional scaling gradient analysis, compositional differences among the coastal and inland forest zones were apparent. Soil texture and fertility, topography, and land crab densities all changed with the coastal gradient. By overlaying the environmental variables on the forest compositional differences, I found that soil characters, topography, and canopy cover did not correlate with the spatial variation in forest composition, but distance from shore and crab burrow density did. Over a two-year seed rain study, I found that seed rain density and species richness varied in space (related to adult distributions with long-distance dispersal surpassing adult distributions) and time (related to rainfall seasonality but no inter-annual variation). Seed dispersal does not appear to limit the recruitment of tree species in the coastal area. To determine the importance of seed and seedling predation by the land crabs relative to canopy cover on seedling establishment, I conducted seed removal and seedling establishment experiments over a two-year period. I found: (1) crab predation affected seed and seedling survival rates, (2) crabs differentially preferred seeds and younger seedlings over older seedlings but showed no species preferences; and (4) canopy cover affected seedling survival. My results show that crab predation pressure has an important role as a limiting factor in tree recruitment in coastal forest ecosystems.
