Species richness and floristic relationships between mesas and their surroundings in southern African Nama Karoo

Floristic composition of 14 mesas (i.e. flat-top mountains) in four different study areas was investigated in southern African Nama Karoo. The study areas were arranged along a latitude gradient. Two simple questions were asked: are mesas potential conservation islands (a) in terms of re-colonization potential and (b) in terms of species richness? Detailed vegetation surveys along a transect from the plains to the top of each mesa were summarized to obtain species composition for the three main habitats: plain, slopes and plateau. Floristic similarities between plains and mesa habitats were used to answer question (a), and species richness was used to answer question (b). Geographic position, high within-area variability and elevation were found to largely influence the findings, resulting in only few consistent trends. The main findings of this study were, however: (1) mesas can act as sources for re-colonization as well as havens for species adapted to mountain habitats. (2) Considering total species richness, mesa habitats were richer in species than plains in the northern, but not the southernmost study area. (3) Scaled by area, mesa plateaus emerged as the most species-rich in all study areas. (4) Smaller and medium-sized mesas showed greater re-colonization potential, i.e. shared more species with their surrounding plains. (5) Medium-sized to larger mesas supported plant communities more distinct from the surrounding plains. (6) A cut-off point of approximately 240 m above the surrounding plain was indicated for the shift from 're-colonization' to 'remnant' function. These findings stress the importance for the protection of mesa habitats in view of increasing human pressure on mountain habitats.