Impacts of grazing systems on Nama Karoo phytodiversity

Abstract

The study was carried out on two adjacent farms on the plains of the Nama Karoo near Beaufort West. The impacts of three grazing treatments (a) zero grazing (b) non-selective grazing (c) conventional grazing, on plant diversity and certain vegetation parameters were compared. Unpredictable and variable rainfall and major disturbance events such as droughts drive vegetation change in the Nama Karoo. Major recruitment events are rare and can determine Karoo vegetation composition for many years. The diversity of plant species plays an important role in determining vegetation composition during major recruitment events and following drought or disturbance such as grazing. Grazing can influence the composition, abundance and seed production of Karoo plants and in so doing influence the future abundance of desirable and undesirable forage species. These changes may only become evident over long periods, but small changes in vegetation as a response to grazing treatment can accumulate considerably over time.On the farm Elandsfontein, studies have shown that non-selective grazing leads to a higher plant turnover rate, resulting in more vigorous and productive plants, and improved ecosystem functioning. However there is no evidence of this grazing system promoting or reducing plant diversity. The aim of this study was to test whether the non-selective grazing system promoted or reduced plant diversity compared to no grazing and conventional grazing. The hypothesis was that there were no differences between the grazing treatments in terms of plant diversity or any of the vegetation parameters measured. To evaluate this hypothesis, plant data were collected from the three grazing treatments using the Modified-Whittaker vegetation sampling method. The method was further modified for this study to allow for accurate abundance measurements rather than estimates, and an increased area for recording species richness. Using various diversity indices that incorporate species richness and the proportional abundance of species, plant diversity values for each treatment were obtained. No differences in terms of plant diversity were found between the treatments. A significant difference between treatments was found in the density of plants, particularly in perennial grasses and shrubs. Canopy cover percentage did not differ for individual species or as total cover between the treatments.