| Summary: | Species coexistence theory attempts to explain and predict the mechanisms that promote species
diversity and the ecological consequences of this diversity. In this thesis I used observational and
experimental field studies to test the predictions of several coexistence theories, and developed
specific predictions in a theoretical study. The observational study was used to test general
predictions made by two mechanisms, neutral interactions and spatial niche partitioning, using
bromeliad-dwelling mosquito larvae in Costa Rica. Results from this study were only consistent
with spatial niche partitioning, and showed how local, within-bromeliad interactions could scale
up to meso-scale (among-bromeliad) distributions. The experimental study, based in the boreal
forest understory, used the standard rank-abundance relationships of plant species to test both
competitive and facilitative coexistence mechanisms that differentiate between the effects of
dominant species and species diversity. In particular, removals of a consistent biomass that
targeted one dominant or many low-abundance species were used in conjunction with seedling
additions to test the roles of different species, and species diversity, in limiting the establishment
of new species. High mortality of new seedlings in completely cleared areas indicated that
facilitation was important. However, small-scale disturbances (7% of community biomass
removed) either had no effect on seedling survival, or increased survival, indicating competitive
effects. These competitive effects were limited to a single dominant species, and were
inconsistent with current models of resource niche partitioning. The theoretical study used
computer simulations to investigate the effects of regional habitat heterogeneity on local
diversity in communities that differed in their connectivity (dispersal among patches) and
neutrality (niche overlap among species). The model suggested that dispersal and niche overlap
have synergistic effects on local diversity by increasing the size of sink populations, and likewise
destabilizing coexistence regionally. However, they have opposite effects on resource-use,
causing either positive or negative diversity to resource-use relationships. Together, the three
studies illustrate that different processes can scale up to general patterns of species distributions,
but that these processes have very different implications for understanding and protecting species
diversity and the functions this diversity may provide.
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