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Notes on Species Coexistence, Invasion and Ecosystem Function

Gilbert, B. 2008. PhD Dissertation, University of British Columbia, Vancouver, BC


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.