Macropredators as shapers of invaded fouling communities in a cold temperate port

Ports are vulnerable to biological invasions because of the high shipping exchange and the abundance and diversity of artificial structures. These artificial environments provide new habitats for fouling communities, commonly hosting a large number of exotic species. The colonization of artificial structures is influenced by large and small scale processes, such as differences in recruitment time (that may vary with season and availability of new substrates) or predation. In cold temperate regions, with a marked seasonality, not only the starting time of colonization may influence fouling community structure but also the community of macropredators associated. This study addressed the following question: What is the effect of macropredators on invaded fouling communities started at different time in a cold temperate port? To asses this, the following experimental treatments were assigned to three plates attached to a fiberglass structure: exclusion, open cage and open plate. At the beginning of each season, seven of these structures were tied to port pilings, each one containing the three experimental treatments. We found that fouling communities are shaped by macropredators, such as sea urchins, gastropods and crabs, and that their effect on community structure depends on the time or season in which the colonization started. Exotic ascidians were absent in open plates and open cages when macropredators were present, and species diversity was highest when these predators were excluded. Our results suggest that predation at high latitudes in cold temperate regions might be more important than what is predicted in recent works. Although our study is conclusive on specific points, it also exposes a need for further research disentangling the processes regulating the colonization of artificial fixed versus floating structures at different latitudes.