| Study of the determinants of species' geographic distributions has a rich tradition in ecology and evolution, and understanding these determinants is becoming increasingly important in the face of climate change. While we know many range limits are set by abiotic stress, species interactions can also be important drivers of range limits. However, we lack any well-tested predictive framework for when and where each of these two broad classes of factors will most commonly set range limits.;A long-standing, but still nearly untested, hypothesis suggests that abiotic stress most often sets range limits in seemingly stressful areas, such as arctic, high-alpine, or arid systems, with species interactions having more influence in apparently benign environments, such as the tropics, low-elevation, or mesic places. In my dissertation, I experimentally tested a fundamental assumption of this hypothesis: namely, that the relative importance of species interactions and abiotic stress for population performance varies systematically with abiotic stress. I tested the relative importance of abiotic stress vs. three species interactions (herbivory, neighbors, and pollinators) for population dynamics of a model plant species in central Kenya, Hibiscus meyeri, across a sharp aridity gradient.;I find broad-scale support for Darwin's hypothesis, with stronger effects of herbivores, neighbors, and pollinators on population growth rate in mesic areas v. arid areas. Interestingly, I find universal competitive effects of neighbors (rather than the switch from facilitative to competitive with increasing rainfall predicted by recent theoretical and empirical work). This work suggests that species interactions might be critical drivers of range limits only in unstressful regions of a species range.;This work also has implications for projecting shifts in species' distributions. While in some cases, leaving biotic interactions out of species' distribution models reduces accuracy, the vast majority of projections of shifts in distributions with climate change do not include such interactions. This work suggests that species distribution modelers should include species interactions in their predictions only in abiotically benign portions of a species range. |
