Impact of livestock grazing intensity on the plant diversity of species-rich montane grassland in the northern Drakensberg, South Africa
Shezi, Thamsanqa Alfred
Livestock grazing intensity is expected to impact on the plant species composition and plant diversity of Drakensberg grasslands. These montane grasslands are important for providing goods and services for the local and the national population, in part through supporting livestock on communal rangelands. Montane communal rangelands are generally heavily stocked, although grazing pressure is expected to be concentrated around kraals and to show a decreasing gradient with distance. The aim of this study was to determine the impact of livestock grazing on plant species composition and diversity of montane grassland in the northern Drakensberg (Thabo Mofutsanyane district). A grazing gradient from a kraal site to beyond the average foraging distance of cattle was examined. A second approach examined fence-line contrasts between communal rangeland and the protected areas of Golden Gate Highlands National Park (GGHNP) and Royal Natal National Park (RNNP). Nine transects were sampled (using 90 5 x 5 m plots) in order to define a gradient, ensuring relative uniformity of environment by sampling only spurs and crests. The fence-line contrast between GGHNP and QwaQwa was sampled using 40 adjacent pairs of plots (5 x 5 m), that of RNNP and communal rangeland using 20 adjacent pairs. The percentage cover of each species on each plot was estimated using the Domin scale. Soil from each plot was analysed for levels of P, K, Ca, N, Mg, Zn, Mn, Cu, exchangeable acidity, acidity saturation, total cations, pH (KCl), organic carbon and percent clay. The presence of a grazing gradient defined by distance was supported by a decrease in the amount of cattle dung with increasing distance from a kraal, described by a non-linear relationship. Distance was not confounded with altitude, solar radiation, or slope, nor with variation in soil physico-chemical properties as described by the first three axes of a Principal Component Analysis (PCA). An effect of grazing intensity was supported by a relationship between increasing distance and increasing species richness, an increasing abundance of graminoids, indigenous plants, and perennial plants, a decreasing abundance of shrubs, dwarf shrubs, annual herbaceous plants, and alien plants (especially Richardia brasiliensis and Hypochaeris radicata), and a change in herbaceous composition. However, most compositional variation was unexplained. The studies of fence-line contrasts provided some further support for an effect of grazing intensity. For the contrast between GGHNP and the QwaQwa communal area, species composition was different and GGHNP supported more endemic species. RNNP compared with the adjoining communal rangeland showed greater species richness and plant diversity, a greater abundance of grasses, dicotyledons and indigenous plants, and a lesser abundance of dwarf shrubs. In conclusion, a gradient study and fence-line contrasts both provided support for an effect of livestock grazing on the composition and diversity of montane grassland in the northern Drakensberg. However, despite a long history of communal livestock grazing a total of more than 320 indigenous plant species indicates these grasslands make some contribution to biodiversity conservation, but this contribution is compromised in the vicinity of kraals. Rangeland under reduced grazing intensity may therefore offer a means of maintaining the plant diversity of communal grassland grazing systems. The state of the grassland on the RNNP side of that fence-line contrast perhaps presents a benchmark grassland state that could be achieved in this communal grazing region if conservative livestock numbers could be maintained.
A Dissertation submitted to the Faculty of Science, University of the Witwatersrand, in fulfilment of the requirements for the degree of Master of Science, Johannesburg, South Africa March 2019