| In order to investigate the effects of habitat fragmentation on the floral and faunal communities of a tropical dry forest, I conducted a series of observational studies on recently isolated land-bridge islands in Lago Guri, Venezuela. These studies focused on understanding the impacts of altered herbivore abundances on ecosystem processes and how these effects can ramify through the food web to affect the diversity and persistence of other taxonomic groups. Specifically, I show that 'hyper-abundant' populations of herbivorous red howler monkeys (Alouatta seniculus ) cause a significant decrease in the availability of soil nutrients (N and P) for plant uptake at the whole island scale, yet also cause a six-fold increase in tree growth rates (Chapter 2). I hypothesize that this seemingly paradoxical result is attributable to the latrining behavior of the monkeys which causes the formation of localized patches of increased nutrient availability (Chapter 3).; Expanding on these results, I shift the focus of my study to address the potential for changes in plant productivity to affect the communities of forest-interior birds nesting on the islands. I show that the diversity of nesting birds is positively correlated with tree growth and that there is a positive indirect effect of howler monkey density on bird diversity (Chapter 4). In addition, utilizing a ten-year (1993--2003) record of avian censuses, I investigate the relationship between rates of local extinction and several variables related to island biogeographic and higher-level (i.e. trophic) effects. I show that the best model describing rates of bird extinction does not include island area or nest predation as is generally predicted, but rather only the density of generalist herbivores including leaf-cutter ants ( Atta sp.) and howler monkeys (Chapter 5). In the final section (Chapter 6), I investigate how the insular communities of birds are structured. Based on 'assembly rule' models, I show that the communities are only weakly structured by competition and that community composition is primarily determined through selective extinction based on habitat requirements. |
