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Insights on Ecological Diversity Informed by Flora of Tropical Mountains of Southeast Asia
Rules are meant to be tested, and the discipline of ecology is no exception. For my dissertation I focused on testing ecological rules as applied to flora along tropical elevation gradients. One controversial rule is Rapoport's rule, which predicts that species’ range-sizes increase across elevational and latitudinal gradients, for diverse taxa and habitat types. First, I tested Rapoport's rule, for 54 plant families of tropical Southeast Asia. I found that the strength and significance of Rapoport's rule varied relative to the elevation where richness peaked, and by elevation distribution breadth, for each family. Next, I focused on richness and range-size trends at a smaller spatial scale, at Mt. Kinabalu. Using model selection, replicated for three soil association categories, I found more support for piece-wise regression models that incorporated vegetation zone boundaries. In addition, flora associated with ultramafic soil had distinct richness and range-size patterns. Next, I examined if it is possible to maintain a single ecological strategy along an elevation gradient, indicated by a suite of functional traits that remain relatively unchanged across different contexts, while still adhering to universal life history trade-offs. To do this, I used 169 species from the genus, Rhododendron, noted as having a "stress-tolerant" ecological strategy, with species occurring from sea-level to mountain summits. I found that the genus was able to maintain their stress-tolerant strategy, and a narrow set of associated traits, which is in contrast to the assumption that functional traits must change with elevation. The ability of Rhododendron species to occur across a wide elevation gradient, including expanding into resource-rich areas at lower elevations where they have reduced competitive ability, was most likely facilitated by species occupying new niches (transitioning from terrestrial to epiphytic growth forms) or by specializing in extremely stressful habitat types with reduced biotic filtering (ultramafic soil). I propose the "stable stress, novel niche" hypothesis as the explanation for how Rhododendron species can occur at different elevations. Overall this dissertation demonstrates that the exceptions to ecological norms offer an interesting perspective on the parameters which define rules regarding species range-size, richness, and functional traits along elevation gradients. ^
Whitman, Melissa, "Insights on Ecological Diversity Informed by Flora of Tropical Mountains of Southeast Asia" (2018). ETD collection for University of Nebraska - Lincoln. AAI10933332.