Coastal Invasions: The South African Context

Abstract

AbstractIn total, 95 marine alien species are known from the South African coast, of which 56 have spread from their points of introduction to become invasive. While just over half of these alien species are restricted to harbours, 45 invasive species have been recorded in natural habitats. The association between marine alien species and harbours reflects the importance of shipping as a pathway for introducing novel marine biota. In the South African context, 91% of introductions have been linked to this mode of transport, with the majority originating from the North Atlantic Ocean. The most invaded region is the Southern Benguela ecoregion along the west coast, where 67 alien species have been detected, with the number declining towards the east. The drivers of this spatial pattern are not yet fully understood, although an interaction between vector strength and compatibility of climate between recipient and donor harbours is likely to play a role. Three species, the Mediterranean Mussel Mytilus galloprovincialis, the Chilean Mussel Semimytilus algosus and the Pacific Barnacle Balanus glandula, have become abundant and widespread along the open coast, and are dominant on wave-exposed rocky shores along the west coast. Here, their sequential invasions have altered intertidal community structure, predominantly through their high abundance and resultant alteration of habitat complexity. Furthermore, the potential threat posed by alien biota to the effectiveness of marine protected areas (MPAs) is increasingly being recognised. Baseline surveys of 19 South African MPAs have revealed the presence of 22 alien species from eight phyla. The highest number of alien species (9) has been noted in Langebaan Lagoon (along the west coast), while Sixteen Mile Beach and Helderberg MPAs (along the west and south coasts, respectively) remain the only MPAs free of alien species. Dedicated research effort in the last two decades has undoubtedly provided valuable baseline knowledge on the status of marine invasions in this region. This is expected to provide a solid basis upon which effective evidence-based management will be developed in the future.