Environmental and geographical controls on species distributions: A case study on proteas of the Cape Floristic region

Date of Completion

January 2006


Biology, Biostatistics|Biology, Ecology




The high plant species diversity of the Cape Floristic Region of South Africa (CFR), one of the world's biodiversity hot spots, is characterized by extraordinarily high levels of species turnover across the region. Using neutral ecological theory, I showed that species relative abundance patterns in CFR woody plant species are consistent with high levels of speciation and low levels of migration of individual plants among subregional communities---and that these migration rates are much lower than those for tropical rain forest trees. Much species turnover in the CFR arises from low overlap in the distributions of sibling species. I used three complementary methods to explore the causes for this geographic pattern in one species group, the White Proteas. First, I used Bayesian hierarchical statistical models to predict species distributions; these models suggested that the species are not completely filling their potential distributions, and that the species are likely physiologically capable of coexisting. Second, I used transplant experiments to test the performance of seeds and seedlings of the species within and outside their current distribution limits. These experiments demonstrated ecological differences among the species, but support the conclusion of the statistical models that the species are capable of coexisting. Water use efficiency, as measured by carbon isotope analyis, appears to be an important basis of ecological differences among the species. A separate competition experiment suggested that intea-specific competition at the seedling stage is stronger than inter-specific competition, and thus that competition likely does not contribute to the non-overlapping distribution pattern. Finally, I used microsatellite markers to investigate migration and dispersal rates. Population genetics analysis demonstrated little genetic differentiation among populations, suggesting ongoing bird-delivered pollen flow and/or more continuous distributions in the recent past. Small genetic neighborhoods within populations, by contrast, imply short mean gene dispersal distances, probably due to limited seed dispersal. Together, the results of this investigation suggest dispersal limitation plays a key role in shaping species distributions in the CFR and elevating regional diversity levels.^