Effect of shipping traffic on biofouling invasion success at population and community levels

The relationship between shipping and invasion success in marine waters has rarely been demonstrated empirically. In commercial ports, greater shipping activity is expected to increase invasion success at both the community and population levels by altering the diversity of exotic species discharged (colonization pressure) and the number of introduction events (propagule number). This study sought large-scale correlations between metrics of shipping activities and exotic fouling species diversity at the community (species richness evaluated using standardized port surveys) and population (genetic diversity) levels. The richness of exotic fouling species was evaluated at the community level by sampling 15 commercial ports in marine temperate Canadian waters. At the population level, we investigated genetic diversity of the model fouling species Botryllus schlosseri in five commercial ports, by identifying 262 individuals at the mitochondrial cytochrome c oxidase subunit I gene and eight polymorphic nuclear microsatellites. Measures of community- and population-level richness were related to various measures of propagule number and colonization pressure related to ship ballast and hull biofouling. Patterns of exotic fouling diversity are consistent with the hypothesis that increased shipping arrivals increase establishment success at both the community and population levels. As expected for fouling exotic species, ballast water was a less important vector; introduction success was best correlated to the number of arrivals of non-merchant ships from close regions, suggesting that this vector is important for successful invasion of this group of organisms.