A key goal in ecology is to understand the factors limiting species' distributions. Important range-limiting factors are often difficult to generalize, however, because organisms have many different life-history traits, evolutionary histories, and diverse interactions with other species. Climate is often implicated as the most important range-limiting factor in modern species distributions. Yet many species are not or not yet exhibiting range changes associated with anthropogenic climate change. A potentially important non-climatic range-limiting factor is dispersal limitation. Recently, some researchers have concluded that dispersal limitation is likely as strong a range limiting factor as climate. One way to tackle the limits to generalization is to investigate range limiting factors and patterns of range shift for well-chosen taxa in a comparative fashion to glean general principles. My research uses a comparative approach to investigate patterns of post-glacial colonization, factors involved in geographic range limitation, and species responses to future climates using genetic techniques, a field experiment, and a chamber experiment, respectively. All studies were conducted on species associated with the Garry oak ecosystem in the Pacific Northwest of North America and focused on four plant taxa: Quercus garryana var. garryana, the flagship species of the Garry oak ecosystem, and three Lomatium species, L. dissectum var. dissectum, L. nudicaule, and L. utriculatum. An overall conclusion from this dissertation is that related, co-occurring species provide an appropriate comparison for determining species- and trait-based generalization. Lomatium chloroplast genetic data suggest that abundance is important in determining the ability of long-distance seed dispersal. The field experiment shows that dispersal limitation is currently important in determining range boundaries for species no matter their regional abundance. The field experiment also shows that closely related species may differ in their competitive abilities and responses to competitors/facilitators. My genetic survey on Q. garryana provides evidence that generalizations about range changes in oaks as a taxonomic group seem to be relatively universal, no matter the historical landscape conditions. The chamber experiment provides evidence that some responses to global change will be unpredictable, making certain generalizations difficult. Given these findings, humans may consider accelerating species migration through purposeful translocation outside species' ranges to overcome dispersal barriers.