Opportunities for studying propagule pressure using gene flow reveal its role in accelerating biological invasions.

When an alien species establishes at a new location, it must spread to become an invader. The extent to which propagule pressure promotes the spread of invaders, especially at local scales, is often difficult to quantify because it requires a reliable measure of, and variation in, rate of spread, and of propagule pressure across similar areas. In this issue of Molecular Ecology, Mairal et al. (2022) make use an unique system of paired sub-Antarctic islands, one with very infrequent human activities, and another inhabited by scientists, to assess the role of propagule pressure and anthropogenic disturbance in the introduction and spread of a major global invader, Poa annua L., to and on the islands. Genetic admixture between different genetic clusters is virtually absent from the little-visited island, while the inhabited island experienced more introduction events, but also significant admixture between genetic clusters. Detailed distribution maps of P. annua spanning more than 50 years allowed the authors to link genetic diversity to residence time. The nature of the system, and the multifaceted approach used by the authors, allows for new insights into the mechanism by which propagule pressure results in the spread of invasive species.