Interference or exploitation? Competitive interactions among native and invasive Mytilus spp

In rocky intertidal communities, competition has been identified as an important driver of community structure. Competitive interactions are also a recognized mechanism of community resistance to exotic species invasions. The wave exposed shores of northern California mark an invasion front for the exotic mussel Mytilus galloprovincialis.  Despite overlapping settlement patterns with two native congeners, Mytilus trossulus and Mytilus californianus, adult M. galloprovincialis are often rare while M. californianus dominate the intertidal landscape. This study investigates if exploitative and/or interference competition among native and invasive Mytilus spp. are important post-settlement forces that may be constraining the establishment of M. galloprovincialis in Bodega Bay, CA. Field and laboratory manipulations of Mytilus spp. composition and density indicate that inter-specific competition does not limit the growth or spread of M. galloprovincialis as growth rates of the invader were often an order of magnitude greater than the two native mussels.  Results from laboratory feeding trials, growth studies, and behavioral observations further suggest that competition between M. galloprovincialis and M. trossulus may contribute to the displacement of M. trossulus. Investigations into the specific mechanisms of displacement suggest that both exploitative and interference competition among mussels may be important determinants of northern California intertidal community structure.