Redefining the Griqualand West Centre of endemism
Frisby, Arnold Walter
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The Griqualand West Centre (GWC) of plant endemism in the Northern Cape was defined and described in 2001 according to geological features and limited floristic data. Approximately 40 plant species were proposed to be endemic to the GWC in an area larger than Lesotho and similar in size to provinces such as KwaZulu-Natal and Mpumalanga. As these political areas each contain one or more centres of endemism with hundreds of endemics, the existence of a GWC could be questioned considering its large size and few endemics. To test for the presence of the GWC, data regarding the plant species present in the region and their distributions were sourced from the Plants of Southern Africa database, literature surveys and herbaria with specimens collected from the region. After mapping the species distributions, 24 plant species were found to be restricted to the Griqualand West region. The disjunct distributions of two near-endemic taxa indicated a link between the GWC and two other centres of plant endemism, namely Gariep and Sekhukhuneland. Based on the distributions of species deemed to be floristic elements of the Griqualand West region, links were identified with both the Nama-Karoo and Savanna biomes. When the total distributions of all GWC endemic and near-endemic species are considered, the resulting boundaries of the GWC are extensive, covering an area even larger than the existing unresolved boundaries. Thus, this study proposed the concept of ‗core area‘ when assessing large centres with few endemic species. In this method, distant outlier populations (>100 km) of endemic or near-endemic species are discarded during the demarcation of the Centre. The proposed boundaries of the GWC by means of concept of core area differs from the 2001 borders in two ways, namely that the duneveld in the northern and north-western section of the GWC is not included in the new boundaries and that it is extended eastwards towards Kimberley to include thornveld. Two GWC endemics may be edaphic specialists, having only been recorded on soils rich in calcium, namely Nuxia gracilis (near-endemic) and Rennera stellata. The floristic patterns in Griqualand West were identified with cluster analysis. Resulting floristic clusters were compared to the extent of various abiotic variables to infer links with the flora. As previous studies have suggested, geology was a major factor dividing the flora of Griqualand West according to areas dominated by siliciclastic rock, limestone and dolomite, mafic and felsic formations, and schists. However, topography and climate also had a major influence on the flora, with divisions on similar geology being ascribed to mean rainfall and temperature or geomorphology (plateau, mountain or valley). A number of GWC endemics were restricted to three of the floristic clusters present within the core area boundaries, which allowed the description of three sub-centres of endemism, namely the Northern Ghaap Plateau and Kuruman Hills, Southern Kalahari and Langeberge, and Southern Ghaap Plateau and Kimberley-Prieska valley sub-centres.
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