Shifting dispersal modes at an expanding species' range margin.

While it is generally recognized that noncontiguous (long-distance) dispersal of small numbers of individuals is important for range expansion over large geographic areas, it is often assumed that colonization on more local scales proceeds by population expansion and diffusion dispersal (larger numbers of individuals colonizing adjacent sites). There are few empirical studies of dispersal modes at the front of expanding ranges, and very little information is available on dispersal dynamics at smaller geographic scales where we expect contiguous (diffusion) dispersal to be prevalent. We used highly polymorphic genetic markers to characterize dispersal modes at a local geographic scale for populations at the edge of the range of a newly invasive grass species (Brachypodium sylvaticum) that is undergoing rapid range expansion in the Pacific Northwest of North America. Comparisons of Bayesian clustering of populations, patterns of genetic diversity, and gametic disequilibrium indicate that new populations are colonized ahead of the invasion front by noncontiguous dispersal from source populations, with admixture occurring as populations age. This pattern of noncontiguous colonization was maintained even at a local scale. Absence of evidence for dispersal among adjacent pioneer sites at the edge of the expanding range of this species suggests that pioneer populations undergo an establishment phase during which they do not contribute emigrants for colonization of neighbouring sites. Our data indicate that dispersal modes change as the invasion matures: initial colonization processes appear to be dominated by noncontiguous dispersal from only a few sources, while contiguous dispersal may play a greater role once populations become established.