Spill-back events followed by self-sustainment explain the fast colonization of a newly built marina by a notorious invasive seaweed

Biological invasions are occurring at increasing rates since the onset of the twentieth century. While ports and marinas have been identified as a major point-of-entry for the spread of marine non-indigenous species (NIS), their relationships with wild habitats however needs further scrutiny. We had the rare opportunity to monitor the real-time colonization dynamics of a newly-built marina by the notorious invasive kelp Undaria pinnatifida in the Bay-of-Morlaix, Brittany (France). Field surveys (> 20,000 individuals geo-localized) were combined with genetic analyses (10 microsatellite loci, N = 890 individuals) over 3 years (i.e., 6 generations in the study area). Regarding the colonization dynamics, a dramatic snow-ball effect was documented over time, with local density reaching up to 50 individuals per m after 2 years. Assignment tests showed that the primary colonizers came from neighboring populations established in natural rocky reefs. A shift towards a self-sustaining population was however observed the following year, with 44% of self-assignment. These processes are best explained by (i) life history traits, notably rapid growth and selfing, and (ii) natural dispersal within the marina combined with human-mediated dispersal—through leisure boating- over longer distances. Spill-over effects have been previously documented, and here also reported, to explain the expansion of U. pinnatifida from marinas to the wild. We showed that the on-going ocean sprawl also offers a perfect arena for spill-back events (i.e., spread from natural habitats to artificial structures), highlighting the need for careful surveillance of newly built infrastructures.