1. The distribution of genetic variation can be interpreted to understand the timing and mechanisms of invasive species spread. Allee effects, positive relationships between fitness and density or number of conspecific individuals, can play a substantial role in determining the time lag between initial introduction and invasive spread and can produce genetic patterns in invading populations that can be interpreted to learn about factors affecting invasion mechanisms.
2. We examined the distribution of genetic variation in the invasive wetland grass Phragmites australis in the Chesapeake Bay, USA. We used microsatellite analysis to examine the reproductive mode (clonal vs. seed) by which the invasive haplotype of P. australis has spread and the distribution of genetic variation within and among brackish wetlands in nine subestuaries of the Chesapeake Bay. Watersheds associated with the subestuaries were dominated by forests, anthropogenic development or mixed forests and development.
3. Our results suggest that the invasive haplotype of P. australis has spread primarily sexually by seed, rather than clonally, and genetic diversity of patches within subestuaries increased while genetic similarity decreased with increasing development in the surrounding watershed.
4. This suggests a pattern whereby greater genetic diversity of patches may promote more rapid spread due to recruitment of multiple seedlings into a disturbed patch.
5. Synthesis. Evaluation of patterns of genetic distribution can help to identify factors affecting invasion in different environments and so inform management.