Invasion of Langebaan Lagoon, South Africa, by Mytilus galloprovincialis – effects on natural communities

Abstract

In 1992 the invasive mussel Mytilus galloprovincialis began establishing beds on the centre sandbanks of Langebaan Lagoon. This global invader had previously been restricted to rocky shores along the South African coastline. In order to investigate the effect of the invasion on naturally-occurring communities, a comparative study between invaded areas and areas clear of invasion was conducted. Communities in these areas differed significantly (ANOSIM, R = 0.685, P < 0.01). The biomass supported in invaded areas (53 262 g/m2), was significantly greater than that in clear areas (1133 g/m2), (Mann-Whitney, U = 225, P < 0.001). Invertebrate densities supported in invaded (6780.1 individuals/m2) and clear areas (835 individuals/m2) were also significantly different, (Mann-Whitney, U = 219, P < 0.001). Of the 66 species recorded, only 33 % occurred in both clear and invaded areas (often in very different densities), 41 % occurred exclusively in invaded areas, and 26 % were restricted to non-invaded areas. These figures indicate a replacement of the naturally-occurring sandbank communities by those more typical of rocky shores. It is thus recommended that the invasive mussel beds be removed to conserve the natural biota of the centre banks, which lie within a national park.