Native predators limit invasion of benthic invertebrate communities in Bodega Harbor, California, USA

Consumption by native predators is an important means by which assemblages repel invasion by non-native species. This resistance may be compromised as the diversity and abundance of species at upper trophic levels continues to decline globally. We investigated whether consumptive biotic resistance by mobile benthic fauna could explain compositional differences between a highly invaded fouling community and a nearby rocky subtidal community in Bodega Bay, California, USA. Surveys revealed little overlap in the sessile and mobile invertebrate communities of the 2 habitats, with the fouling community dominated by non-native tunicates and bryozoans. Through a series of predation experiments, we established that many mobile species, particularly sea stars, crabs, and chitons, consume settlers and adults of non-native sessile invertebrates, and we observed these mobile species to be more abundant and diverse at the rocky subtidal site in field surveys. In a field experiment, we found that transplanted non-native sessile invertebrates (both settlers and adults) survived at the rocky subtidal site when cages excluded benthic predators but showed reduced survivorship when these predators were allowed access. Our results suggest that variation in the intensity of predation by native mobile consumers likely contributes to the dominance of non-native sessile invertebrates in the fouling community and the relative absence of these species from the rocky subtidal. Benthic marine invaders are often more abundant on floating anthropogenic structures than on the adjacent benthos. Our results suggest that one possible explanation may be the more abundant and diverse community of native consumers present in benthic rocky habitats.