Effects of large enemies on success of exotic species in marine fouling communities of Washington, USA

The enemy release hypothesis, which posits that exotic species are less regulated by enemies than native species, has been well-supported in terrestrial systems but rarely tested in marine systems. Here, the enemy release hypothesis was tested in a marine system by excluding large enemies (>1.3 cm) in dock fouling communities in Washington, USA. After documenting the distribution and abundance of potential enemies such as chitons, gastropods and flatworms at 4 study sites, exclusion experiments were conducted to test the hypotheses that large grazing enemies (1) reduced recruitment rates in the exotic ascidian Botrylloides violaceus and native species, (2) reduced B. violaceus and native species abundance, and (3) altered fouling community structure. Experiments demonstrated that, as predicted by the enemy release hypothesis, exclusion of large enemies did not significantly alter B. violaceus recruitment or abundance and it did significantly increase abundance or recruitment of 2 common native species. However, large enemy exclusion had no significant effects on most native species or on overall fouling community structure. Furthermore, neither B. violaceus nor total exotic species abundance correlated positively with abundance of large enemies across sites. I therefore conclude that release from large enemies is likely not an important mechanism for the success of exotic species in Washington fouling communities.