Specific edge effects in highly endangered Swartland Shale Renosterveld in the Cape Region

Résumé En

The critically endangered Renosterveld shrubland of the Cape Floristic Region in South Africa is one of the most transformed vegetation types in the world. The mostly small patches that remain after the extensive fragmentation are potentially vulnerable to edge effects and therefore, information on the extent of edge effects in this vegetation type is urgently required for conservation planning. To provide this information, we studied edge effects at two sites in each of five larger fragments of Swartland Shale Renosterveld near Cape Town surveying the vegetation composition along three 10 m wide and 200 m long belt transects. There was little indication of edge effects among the dominant woody species. However, abundance and/ or species richness of the petaloid monocotyledonous plants, which contribute a disproportionally high fraction of the vegetation type’s high biodiversity, as well as the ferns, had a clear negative correlation with edge proximity. For these taxa, effects did not level off at the end of the transect at 200 m. This extent of impact is much larger than those reported in most other studies on edge effects among plants. In contrast, species indicative of high disturbance levels generally decreased within the first 30 m. Consequently, Swartland Shale Renosterveld fragments would be likely to benefit from being enlarged to over 400 m width, while corridors or stepping stones should have a width of over 60 m to minimize major edge effects.