Variation within species drives differences in population dynamics, interactions between species, and the functioning of complex ecosystems. Fittingly, understanding the factors that govern this intraspecific variation remains a central goal of ecology and evolutionary biology. Local processes such as adaptation tend to increase divergence among distinct populations, while regional processes such as dispersal and gene flow tend to homogenize those differences. My research addresses how heterogeneity is maintained despite the movement of individuals around a landscape. Specifically, I use an experimental host-parasite system to propose and test mechanisms contributing to ecological differentiation. Initially, I found that variation in colonizer traits makes the order in which they arrive to a new habitat important. Intraspecific priority effects (IPEs) occur when early arrivers limit the growth of late arrivers, and drive context-dependent differences in growth among populations (Chapter 1). These effects of variation in individual traits and arrival order extended to alter interspecific competition and host/parasite interactions. My research indicates, then, that the process of community assembly depends not only on the traits of a dominant species in the environment, but also which individuals of that species get there first (Chapter 2). The relatively short-term importance of IPEs, however, may wane over time as individuals continue to disperse among populations. I tested the consequences of repeated dispersal, and found that they depend heavily on the expected fitness of migrants in their new habitat. That is, dispersal from a common source can increase ecological heterogeneity among populations if migrants have different effects in different environments (Chapter 3). The context-dependent effects of dispersal suggested an underlying trait-based mechanism. Specifically, I hypothesized that the effects of increasing trait variance in a population (e.g. via immigration) depend on the relationship between a population’s trait mean and the local environmental optimum. I found that increasing trait variance helps populations with suboptimal trait means, but harms populations already well-suited to the local environment, doubling their disease burden (Chapter 4). Overall, my research identifies novel ways in which intraspecific variation contributes to its own maintenance, limiting the ability of individual movement among populations to homogenize ecological and evolutionary differentiation.