Facilitation and competition between invasive and indigenous mussels over a gradient of physical stress

The interactions between invasive exotic and indigenous species can have profound harmful effects on the recipient community; however, not all such interactions are negative. Facilitation is increasingly recognised as important in shaping natural communities and is believed to vary under different conditions. Earlier studies have shown that the indigenous intertidal mussel Perna perna initially facilitates survival of the invasive Mytilus galloprovincialis in the low mussel zone by providing protection against waves, but later excludes M. galloprovincialis through interference competition for space. Here, we examined interactions between these species in the mid and upper mussel zones, moving mussels to experimental plots in different combinations of densities and species. Mussels were left on the shore for more than a year and treatment effects on mortality, shell length and condition were compared. In the high zone, treatment had no effects and P. perna showed greater mortality than M. galloprovincialis, indicating that its exclusion from the high shore is due to emersion stress. In the mid zone, treatment had no significant effects on M. galloprovincialis, but multiple comparisons among treatments involving P. perna showed that facilitation occurred. P. perna survived better at higher densities, but survived even better when mixed with the physiologically more tolerant M. galloprovincialis. Length data indicated both inter- and intraspecific competition for P. perna in the mid zone. Whereas facilitation occurs strongly in the low zone (P. perna facilitates M. galloprovincialis) and weakly in the mid zone (M. galloprovincialis facilitates P. perna), the lack of facilitation in the high zone suggests that the probability of facilitation is not linearly linked to increasing physical stress. Instead it is likely to be hump shaped: relatively unimportant under conditions that are benign for a particular species, significant under more severe conditions, and overridden by physical stress under very harsh conditions.