A biogeographic study of the KwaZulu-Natal sandstone sourveld patches within the eThekwini Municipal Area.

Abstract

KwaZulu-Natal Sandstone Sourveld (KZNSS) is an endangered vegetation type in South Africa. Approximately 68% of KZNSS is transformed, with remaining patches existing within an urban and suburban matrix. Fragmented patches of KZNSS found within the eThekwini Municipal Area (EMA) represent a conservation priority but are often not managed/ conserved appropriately as they are sometimes misclassified as Indian Ocean Coastal Grassland Belt (IOCGB) based on their close proximity to the latter vegetation type in a number of areas. This motivated the present study which involved characterising the flora of eight grassland patches within the EMA presently categorised as follows: three KZNSS sites (viz. Giba Gorge Environmental Precinct, Inanda Mountain and Springside Nature Reserve), three IOCBG sites (viz. Spyhill Open Space, Tanglewood Nature Reserve and Edgecliff Open Space) which are currently called Ecotonal given their close proximity to KZNSS, and two IOCGB sites (viz. New Germany Nature Reserve and Roosfontein Nature Reserve). Floristic surveys, which involved quadrats sampling were performed year round (c. 60% in the winter, 30% in the summer and the remaining quadrats performed in spring or autumn) until an 80% sampling effort was achieved. Additionally, transect sampling was performed monthly for a year at each site. Data from both methods were used to determine the vegetation composition and structure at each site. The below-ground flora of each site was also characterized by removing 30 – 35 soil samples after the two main flowering events (late November and early December as well as late April and early May) at each site and allowing germination to occur, with the resulting germinants being identified and quantified. Field observations on levels of disturbance and management practices at each were also considered. When data for quadrats and transects were pooled, 263 species were found to occur across the eight sites, with 110 of these being common to all three vegetation categories. Only one of the eleven endemic taxa characteristic of KZNSS were found across all three vegetation categories, while none of the endemic taxa associated with IOCBG were found, suggesting iv that identifying KZNSS or IOCBG based on endemic (i.e. diagnostic) species may be inappropriate at the sites investigated here. Further comparisons showed KZNSS and Ecotonal to contain more species than IOCBG, which were spread across more plant families (55 found in total), although IOCBG had one less site than KZNSS and Ecotonal. Diversity measures indicated that Ecotonal is more similar to KZNSS, with more species in common between KZNSS and Ecotonal than Ecotonal had with IOCBG. Additionally, diversity measures show very little differentiation between the dominant taxa of IOCBG from KZNSS. Cluster analyses and ordinations confirmed the current classification of Ecotonal sites as part of IOCBG, despite Ecotonal sharing more superficial similarities with KZNSS. Island biogeography theory‟s area and distance effects were not upheld – most likely due to the sites not being truly isolated from each other and a very localised spatial scale, the limited temporal scale (current status a result of the past 200 years), the lack of a true originating mainland and anthropogenic disturbance. Interestingly, the below-ground flora represented only c. 10% of the species found above-ground for all three categories, with Sørenson similarity index ranging from c. 15 – 22% as opposed to the 50% expected for southern African grasslands. This suggests that seedbank health and hence, regenerative potential may be poor at many of these sites and this may necessitate species reintroduction and habitat restoration at a number of these sites. Additionally, the effects of disturbance were evident across sites in all three vegetation categories, with disturbed sites (Edgecliff Open Space, Inanda Mountain and Roosfontein Nature Reserve) containing fewer species of conservation concern, fewer indigenous taxa in general and more alien plant taxa. Diversity indices also suggest that disturbed sites were more heterogeneous. However, disturbance agents such as herbivory appear to have had an enriching effect in term of the abundance of graminoids within the germinable soil seedbank. Alien taxa also occurred frequently in the below-ground flora but were not noted in the above-ground flora which suggests that given further disturbance/transformation, some of these sites are prone to alien plant invasion. The study suggests that IBT is not applicable to vegetation islands (grasslands in this case) with varying levels of transformation in urban matrices. The results confirm the high levels of transformation reported for sub-tropical grasslands within South Africa, specifically KZNSS, and highlight the need for floristic surveys to delimit different grassland types and in turn ensure their appropriate conservation and/or rehabilitation.