Effects of mussels on competitively inferior species: competitive exclusion to facilitation

Intertidal mussel species often provide a secondary substrate for competitively inferior species, while excluding them from the primary substrate. To evaluate the net effect, we conducted field experiments that specifically focused on interactions between mussels Septifer virgatus (Miegmann) and algae species. Mussels affected the abundance of 7 algal species differentially, with effects being positive, neutral, or negative. The red alga Porphyra yezoensis grew more abundantly on mussel shells than on rock surfaces. Mussels facilitated recruitment intensity of this species, resulting in increased adult cover on the shells. In contrast, the green alga Monostroma angicava grew less abundantly on mussel shells than on rock surfaces. Mussel shells did not modify recruitment intensity of this alga, but did inhibit its frond growth, and would thus seem to reduce adult cover. Modifications of grazer density by the mussels did not affect either of these algae species. The results indicate that the net effect of mussels on competitively inferior species is not grazer-mediated, and varies from species to species.