The Ecology & Evolution of Species Range Limits

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All organisms are limited in their geographic distribution, but how and why these range limits occur are poorly understood. What ecological processes enforce range limits over the short-term, and what genetic constraints prevent natural selection from enabling range expansion over the long term? This deficiency prevents realistic forecasting of whether species will shift their ranges or go extinct during climate change, and effective conservation strategies for Canadian species at risk, most of which occur in Canada at the northern limits of broader distributions south of the US border. My research seeks a better understanding of the ecology and evolution of range limits.A species’ distribution is thought to reflect its ecological niche: the conditions under which survival and reproduction are sufficient for sustainable populations. Areas where a species does not occur are simply outside of its niche. However, we recently analyzed studies that measured performance of species transplanted beyond their ranges and found that range limits did not coincide with niche limits in > 50% of all cases. Unfortunately, few studies were designed to provide definitive answers, so there is a clear need for more experiments. The dispersal capacity of a species may also constrain its distribution, but this remains speculative. We will combine large-scale geographic surveys, transplant experiments and genetic estimates of dispersal to test the relative contribution of niche limits vs dispersal to range limits.Long-term evolutionary stasis of range limits is thought to arise because range-edge populations lack genetic variation in traits that would allow persistence beyond the limit, or because natural selection for improved performance at the limit is continually thwarted by the immigration of genes from larger populations towards the range centre. We will test these hypotheses by combining transplant experiments and genetic analyses of gene movement and the adaptive potential of edge populations.Geographic range edges may be venues for significant evolution. For instance, some plants show striking range-edge changes in reproduction, often involving a shift from sexuality to asexuality. The loss of sex and thus genetic recombination at the range edge may reduce potential for further adaptation. On the other hand, asexuality can preserve unique combinations of genes advantageous in edge environments from being broken up by recombination. We will evaluate these hypotheses by comparing the performance of transplanted sexual and asexual genotypes in central vs edge habitats as well as testing for the genomic signature of long-term asexuality at range edges.This research will help answer fundamental questions at the forefront of biology and contribute to the conservation of rare species, the management of exotic invaders, and better forecasting of species’ responses to climate change; key environmental problems impinging on Canadian biodiversity. I will work with species proven to be excellent experimental subjects and experimental techniques we have used with success. The proposed research involves a range of highly transferable technical skills including modern genetic techniques, field biology in diverse ecosystems, experimental design, advanced statistical analysis and bioinformatics. As such it will provide trainees at all levels with research experiences and skills suited to their diverse interests and long-term career goals in academia, resource management and industry. Ongoing research partnerships with Parks Canada, Ontario MNR, California State Parks and US Park Service also broaden the scope of training and facilitate the transfer of information and skills between university researchers, government scientists and policy-makers.