Patterns of distribution, diversity and endemism of terrestrial molluscs in South Africa.

Molluscs are an important component of South Africa’s biodiversity. The assessment of distribution patterns and factors influencing the biogeographic distribution are an integral part of assessing the conservation status of molluscs and their conservation management needs. The existing terrestrial mollusc data from South Africa were assessed in terms of their value to biodiversity conversation planning and management. Although the data on terrestrial molluscs are incomplete and would be misleading in terms of identifying specific areas for protection, the data do illustrate significant patterns and trends of mollusc endemism and diversity, which can be used to improve biodiversity conservation and management efforts. The distribution of molluscs across the South African landscape illustrated ten broad biogeographical patterns. Two of these patterns reflected ancient distribution patterns of molluscs and consisted of molluscs of the Gondwanaland/southern relict and Laurasian origins. Three biogeographic patterns occurred across the eastern regions. These patterns were defined as the tropical/subtropical east African, subtropical east of southern Africa and east African afromontane patterns. The biogeographic patterns in the west consisted of the characteristic temperate ‘Mediterranean’ Cape centre and the arid regions of northwestern Cape, Namibia and parts of Botswana. An additional biogeographic pattern identified as the nama karoo/central west was recognised. The final two biogeographical patterns described taxa that were widely distributed and taxa that exhibited disjunct distributions. Twenty-six families and forty-three genera were associated with more than one biogeographical pattern. The dominant biogeographic pattern was the tropical/subtropical east African component. Twenty-one families and forty-eight genera were associated with this biogeographical pattern. The east African Afromontane pattern was also a conspicuous biogeographic element in South Africa. Fewer families and genera were distributed in the western and central regions. The distributions of terrestrial molluscs were influenced by a combination of various factors, which included the presence of rivers, the escarpment, altitude, humidity, precipitation, temperature and biomes. Rivers could possibly restrict the distribution of certain mollusc taxa but did not appear to be the dominant factor that influenced the distribution of molluscs across the landscape. In terms of the effect of temperature on the distribution of molluscs, the mean daily and mean annual temperatures appeared to have more of an influence on the distribution patterns than the mean daily maximum and minimum temperatures. Mean annual temperatures influenced the distribution of all families and genera. The mean daily maximum temperature appeared to have little or no effect on the distribution of mollusc taxa. Humidity and biomes also appeared to influence the distribution of taxa. The least inhabited biome was the succulent biome. Many mollusc taxa occurred in the wetter, warmer areas with high humidity levels. Areas of high species richness and high endemic species richness in South Africa were identified using two systems of endemism, one based on distinctive gaps in the frequency distribution of terrestrial molluscs in South Africa and the other based on an existing classification of invertebrate endemism (Hamer & Slotow, 2002). Areas of high mollusc species richness and endemism were also compared to areas of high millipede species richness and endemism. The total number of South African mollusc endemics was 370 (83 % of 447 indigenous species). The dominant mollusc families in South Africa were Achatinidae, Charopidae, Streptaxidae, Subulinidae and Urocyclidae. The first system of endemism identified 56 site endemics (species with only one locality), 50 local endemics (0 < maximum distance < 60 km) and 145 regional endemics (60 km < maximum distance < 330 km). The Hamer & Slotow (2002) classification of endemism classed 67 species as site endemics (maximum distance between localities < 10 km), 47 as local (11 km < maximum distance < 70 km) endemics and 59 as regional endemics (71 km < maximum distance < 150 km). The analysis of mollusc data, with both systems of endemism, showed similar areas of high species and endemic species richness. Quarter-degree grid cells with highest species richness