Biogeographical patterns in the fauna associated with southern African mussel beds

Macrofauna communities colonizing intertidal mussel beds were sampled at seven sites between Swakopmund (Namibia) and Salt Rock (KwaZulu-Natal). Mean mussel cover, length, biomass and bed depth were all low in the southeast (former Transkei). Faunal abundance and biomass were minimal along the south coast, increasing up both west and east coasts. Species diversity and richness showed the reverse trends, peaking at south coast sites and dropping off to the west and east. Multivariate analysis grouped the faunal communities into four distinct clusters, corresponding to the recognized Namib, Namaqua, Agulhas and Natal biogeographical provinces. The low mussel biomass and cover in the Eastern Cape can be attributed to intense human exploitation, but the low faunal abundance and biomass along the south coast was unexpected, as was the negative correlation between faunal abundance and diversity. Over recent decades intense exploitation has resulted in marked declines in mussel cover along the east coast, while the M. galloprovincialis invasion has resulted in dramatic increases in mussel cover to the west. The results given here indicate how these changes might have affected the wider intertidal communities in these regions.