Despite increasing awareness of the importance of environmental variation in ecological systems, there are still many open questions about how variation affects population dynamics. I studied several aspects of environmental variation and the roles it plays in population dynamics, particularly for species with complex life histories. My first chapter addresses spatial variation in the effects of native insect herbivores on the population dynamics of an invasive thistle, Cirsium vulgare. It shows that while the underlying population dynamics of the thistle were predicted to vary substantially across space in the absence of herbivores, native insects were able to drive thistle populations to negative growth rates across the study region. My first chapter treats populations as isolated entities, with no dispersal between different populations. However, there is often dispersal between disjoint populations, creating metapopulations, isolated population patches connected by dispersal. Therefore, my second chapter incorporates multiple populations and dispersal to test for the importance of within-patch heterogeneity for metapopulation persistence, using a whitebark pine metapopulation as a model. I found that for whitebark pine, within-patch heterogeneity had minimal effects on metapopulation, and the metapopulation was predicted to be stable with or without the observed within-patch heterogeneity. My final chapter also used the whitebark pine system to test for shifts in elevation distributions due to climate change. Whitebark pine populations were predicted to be buffered against climate change because they do not fill their climatically-determined fundamental niche, and they can therefore tolerate warmer and drier climates than they experience in their current range. Thus, environmental variation can play an important role in population dynamics, but different sources of variation might be important at different scales of organization.