Are dispersal and dormancy alternative strategies for overcoming environmental variability?

Dispersal and dormancy serve as strategies for persistence in varying and uncertain environments and are critical to ecological models of biodiversity maintenance. Theories of specific ecological scenarios that favor dispersal, dormancy, or their covariance are rarely tested empirically, particularly in response to realistically complex patterns of spatiotemporal environmental variation. To resolve these complexities, we collected 20 populations of Vulpia microstachys, an annual grass native to California, from the field and grew them in a greenhouse, and on the offspring generation measured seed dispersal ability and seed dormancy rates. We hypothesized that seed dormancy rates, but not dispersal abilities, would be highest in populations from more productive, temporally variable sites, causing dispersal and dormancy to evolve independently-in other words, we leveraged evolved differences among populations to identify what ecological strategy (i.e., dispersal, dormancy, or both) is most likely to evolve at different parts of a variability gradient. Our data suggest that both dispersal and dormancy evolve to combat different axes and scales of spatial heterogeneity and can evolve independently (thus, they are not forced to covary). Most surprisingly, seed dormancy appears to have evolved as a strategy for overcoming microgeographic heterogeneity, an outcome that to our knowledge has not been considered by theory; we confirm the plausibility of this conclusion with a simulation. In sum, we provide much needed empirical data on the evolution of ecological strategies for coping with environmental variance, as well as a new perspective on the ecological function dormancy provides in heterogeneous landscapes.