Biological invasions are occurring at increasing rates since the onset of the th 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 three years (i.e., 6 generations in the study area). Regarding the colonization dynamics, a dramatic snowball effect was documented over time, with local density reaching up to 50 individuals per m after two 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 ongoing 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.
Biological invasions have caused the loss of freshwater biodiversity worldwide. The interplay between adaptive responses and demographic characteristics is expected to be important for the resilience of populations to biological invasions, but the interaction between these factors is poorly understood. The native freshwater gastropod Amnicola limosa is distributed along spatial variation in impact from an invasive molluscivorous fish (Neogobius melanostomus), as well as calcium concentrations, limiting the distribution of this invader (refuges). We investigated the potential for genetic adaptation of A. limosa to the invasive predator and the low calcium habitats. We conducted pooled whole-genome sequencing of twelve gastropod populations from the Upper St. Lawrence River, complemented with a laboratory reciprocal transplant of wild F0 A. limosa to measure survival and fecundity in treatments of water calcium concentration (low/high) and round goby cue (present/absent). We quantified gene flow between the habitat types to test how population structure might interact with adaptation. We found that low calcium, uninvaded habitats could act as refugia for the gastropods from the invasive fish and provide migrants to declining invaded gastropod populations through strong gene flow (i.e., demographic rescue), which also maintained genetic diversity (i.e., genetic rescue). However, we also detected signatures of divergent selection between habitat types and evidence of low fitness of individuals from refuge populations in both habitat types. This suggests that migrants from refuges could introduce maladapted alleles to recipient populations in high calcium, invaded habitats, thereby reducing fitness via outbreeding depression and producing conflict between demographic, genetic, and evolutionary rescue.
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