0104791- Carroll. Ecologists have debated for more than a century in the theoretical literature over the predominant mechanisms that structure plant communities over a heterogeneous landscape. Proposed mechanisms include plant response to variable environmental conditions, differential seed dispersal, differential competitive abilities among plant species, and differential seed predation. There have been few empirical comparative studies that investigate the relative importance of these proposed hypotheses. Recently, several studies have debated the significance of differential herbivory versus plant response to environmental conditions on observed mangrove community shifts, yet no comparable investigations have been conducted in terrestrial forests. Preliminary data from a secondary coastal forest in Costa Rica suggests that herbivorous land crabs and terrestrial hermit crabs have the ability to induce the shifts in forest community structure observed in tropical coastal forests. Through the use of vegetation analyses, seed traps, crab exclosures and controls, and manipulations of environmental conditions, this dissertation research will determine the relative effect of seed and seedling predation pressure by crabs on the patterns of coastal forest regeneration. The results of this study will be of practical management use in coastal forest conservation efforts throughout the tropics. Researcher collaborations with faculty and students of the Universidad Nacional Autonoma of Costa Rica fosters an essential working relationship among native Costa Ricans and the Cabo Blanco Absolute Reserve that is viewed as the model of conservation by their Park Service.