Mechanisms of invasion resistance: competition among intertidal mussels promotes establishment of invasive species and displacement of native species

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 musselMytilus galloprovincialiswithin a Pacific Northwest invasion front. We examined the role of direct (interference) and indirect (exploitation) mechanisms of competition amongM. galloprovincialisand 2 native mussels (M. trossulusandM. californianus). As the fastest-growing 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 ofM. galloprovincialiswas 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 ofM. galloprovincialisconsistently led to both reduced growth and survival ofM. trossulus. Laboratory studies of mussel feeding and behavior revealedM. galloprovincialisto 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 gaveM. galloprovincialisa competitive advantage over the native mussels. Our results suggestM. galloprovincialismay have contributed to the displacement ofM. trossulusalong much of its historic southern range.