When species spread to new areas they are initially often limited to disturbed or anthropogenic habitats. In marine systems, non-native members of the sessile marine hard substrate community, known as the fouling community, have flourished on anthropogenic substrata such as floating docks and piers. However, many of these species have not yet escaped these beachheads to establish in offshore habitats or, sometimes, not even in surrounding estuarine habitats. The factors that have created a barrier to spread from these anthropogenic habitats have not been resolved. Therefore, we conducted an experiment to determine the effects of dispersal limitation and predation on the risk to surrounding habitats of being invaded by non-native species established on marina docks. We used a three-factor design, deploying seven unglazed ceramic panels per each treatment combination of 1) in versus outside a marina in Yaquina Bay, Oregon; 2) cage, no cage, or partial cage; 3) fixed near the substrata (benthic) versus floating 1m below the surface. We also moved panels of either adults or recruits from floating in the marina to any of the treatment combinations to test if they would have lower survival outside by the substrate.
Results/Conclusions
More species established on the panels suspended in the marina than on benthic panels or ones out of the marina, suggesting either recruitment limitation to or predation in the benthic and outside habitats. In addition, NMDS shows that the community on panels suspended in the marina (source community) only minimally overlaps with panels outside of the marina near the bottom, the available habitat for spread. There was no difference, however, between the cage treatments, which suggests that predators excluded by the cages are not responsible for the observed differences in species richness or composition. We cannot conclude dispersal limitation is primarily responsible for the difference because data on mesopredators (nudibranchs, crustaceans, and flatworms) that were found inside the mesh cage treatments suggest that their predation, especially on young recruits, may be interacting with dispersal to limit richness and affect benthic estuarine (out of marina) community composition. Hence, our study suggests that predation by small predators should be examined more thoroughly to establish its role in limiting colonization of natural benthic habitats by non-natives established on artificial suspended structures. Dispersal and predation by mesopredators may play an important role increasing lag time of spread and establishment in the wild for non-native species.