Patterns of distribution, diversity and endemism of terrestrial molluscs in South Africa.
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Date
2007
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Abstract
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 overlapped with grid cells with the highest number of endemic species. However these grid cells coincide with areas that have been intensively sampled and this bias limits the application of the data in conservation planning. The patterns of endemism for molluscs and millipedes within the provinces differed, indicating that the inclusion of a single taxon in conservation planning would inadequately reflect the diversity of invertebrates in South Africa. A preliminary list of specific priority endemic sites for terrestrial mollusc conservation was identified. It is essential that the existing data on invertebrates be evaluated and used to identify key patterns and trends in invertebrate diversity as this will allow for the inclusion of invertebrates in biodiversity conservation planning and management. The analysis of the existing mollusc data identified bio geographical patterns that are important to conservation planning both at the local and national level as well as commonalities and differences between molluscs and millipede distributions. The analysis also highlighted the importance of municipal areas for conservation of hotspots of diversity, particularly in the eastern coastal areas of South Africa.
Description
Thesis (M.Sc.)-University of KwaZulu-Natal, Westville, 2007.
Keywords
Mollusks., Mollusks--South Africa., Theses--Biological and conservation sciences.