Understanding interactions between invasive species and recipient communities is essential to determining whether invasive species will become established and spread. In this study, we explored the role of competition and the specific mechanisms of interaction in limiting the spread of the Mediterranean bay mussel Mytilus galloprovincialis within a Pacific Northwest invasion front. We examined the role of direct (interference) and indirect (exploitation) mechanisms of competition among M. galloprovincialis and 2 native mussels (M. trossulus and M. californianus). As the fastestgrowing organisms are often competitively dominant in space-limited systems, such as rocky intertidal communities, we used changes in relative performance (growth and survival) in monocultures and polycultures to assess interactions among mussels. Performance of M. galloprovincialis was always greater than that of the 2 native species of mussels in both field and laboratory manipulations of species composition and density, indicating that interspecific competition did not strongly limit the growth or survival of the invader. Moreover, the presence of M. galloprovincialis consistently led to both reduced growth and survival of M. trossulus. Laboratory studies of mussel feeding and behavior revealed M. galloprovincialis to be a robust interference competitor. The invader restricted movement, smothered and interfered with filter feeding of the 2 native species of mussels. Rather than limiting invasion, interference competition gave M. galloprovincialis a competitive advantage over the native mussels. Our results suggest M. galloprovincialis may have contributed to the displacement of M. trossulus along much of its historic southern range.

Trophic relationships among native and exotic species produce novel direct and indirect interactions that can have wide-ranging community level effects and perhaps confer invasion resistance. We investigated whether native predators have the potential to directly limit the spread of the exotic mussel Mytilus galloprovincialis or mediate interactions among native and invasive mussels at a rocky intertidal invasion front in northern California. Lower survival of M. galloprovincialis in transplanted cultures exposed to predators indicated that the invader was more vulnerable to predators than the numerically dominant native M. californianus. Survival and per capita mortality rate in monocultures and polycultures did not vary for either M. galloprovincialis or M. californianus, suggesting that predator-mediated apparent competition and associational defense did not occur. Complementary laboratory feeding trials determined which among 3 intertidal predators preferred the exotic to 2 native species of mussel. The whelk Nucella ostrina was most selective, consuming the thinner shelled mussels (M. galloprovincialis and the native M. trossulus ) rather than the thickershelled native species M. californianus. The crab Cancer antennarius and the sea star Pisaster ochraceus showed no preferences among mussel species. N. ostrina were commonly observed among field-transplanted mussels; thus whelk predation may be especially important in limiting the establishment of the invasive mussel. However, 15% of M. galloprovincialis remained intact in the field after 1 yr, suggesting that predation alone may not inhibit establishment of the invader. A tenuous balance between larval settlement and early post-settlement predation likely characterizes the invasion front.