Changes in South African rocky intertidal invertebrate community structure associated with the invasion of the mussel Mytilus galloprovincialis

Since the establishment of the alien mussel Mytilus galloprovincialis in South Africa, several authors have studied its interactions with individual indigenous species. However, the broader implications of this invasion on the intertidal zone remain undocumented. This paper analyses the impacts of this mussel on the rocky-shore invertebrate community structure at Marcus Island on the west coast of South Africa. The effects of the invasion were linked to 3 key elements and were not consistently spread across the intertidal zone, but were focused within the mid-to-low shore. Firstly, physical stress in the mid-intertidal zones was ameliorated by the presence of M. galloprovincialis beds. Secondly, habitat complexity was increased where M. galloprovincialis replaced bare rock or less complex secondary habitat. Thirdly, habitat became less patchy as mussel beds blanketed the shore. Consequently, invertebrate density and species richness increased substantially, and community composition changed significantly in the mid-shore. Lower on the shore, significant changes in invertebrate community structure were driven by a switch from mono-layered beds of the small indigenous mussel Aulacomya ater to multilayered beds of M. galloprovincialis, despite no change in total species richness.