Long-term impacts of livestock grazing in the Succulent Karoo: A 20-year study of vegetation change under different grazing regimes in Namaqualand

Livestock grazing is a key form of land-use around the world. Sustained, heavy grazing pressure, however, changes the vegetation structure of arid and semi-arid regions, often resulting in the local extinction of perennial, palatable species. It also causes changes in species composition through its impact on the recruitment of palatable and unpalatable rangeland species. Fence-line contrast studies are often used as natural experiments, since they contrast the long-term impact of herbivory between two different land tenure regimes and grazing intensities. This study used a fence-line contrast approach to investigate the long-term impact of high grazing pressure at a site in Namaqualand, South Africa. Forty pairs of permanently marked plots, separated by a fence (with communal grazing land on one side and commercial rangelands on the other) were established in 1996 to study the long term impacts of livestock grazing. Vegetation structure and composition of the plots were analysed in 1996 and 2006. The plots were resurveyed in 2016 as part of an on-going monitoring programme in the area. The main objective of this study was to investigate changes in the vegetation structure between the overgrazed communal rangelands and the relatively lightly grazed commercial rangelands over the 20 year period. All species in the paired plots were identified and assigned to 10 growth forms based on their height and life histories (e.g. annuals, perennials, geophytes). Differences in the average % cover as well as the number of species in each of the growth forms were compared across treatments. Differences in shrub volume, mean number of adults and the number of seedlings for each of five key indicator perennial shrub species between treatments and over time were also assessed. The five-indicator species covered a range of palatabilities from unpalatable and semi-toxic (Galenia africana), moderately palatable (Ruschia robusta, Eriocephalus microcephalus) and highly palatable (Tripteris sinuatum, Pentzia incana). Monthly rainfall totals and livestock grazing pressure for locations on the communal side of the fence, recorded by an on-site assistant, were also related to the changes recorded in rangeland composition over the study period. Results indicated that there was a decline in total vegetation cover in both commercial and communal rangelands in 2016 relative to the two earlier sampling periods. The patterns observed appeared to have been influenced strongly by the reduction in annual plant cover which was especially noticeable on communal rangelands. This in turn was probably as a result of the low rainfall experienced in the seven months prior to the 2016 sampling period. The number of species in the remaining growth forms did not differ between treatments and over time. Galenia africana had the largest volume of the perennial shrub species although considerable variation exists between treatment and years. The average number of individuals of mature G. africana shrubs was significantly greater in the communal plots at all time steps. The relative patterns of abundance of the different growth forms between land use treatments were maintained across sampling periods. Continuous heavy grazing over long periods in spatially constrained grazing systems changes the vegetation from being perennial to annual-dominated. This is analogous to switching from a predictable equilibrium system with forage reserve being transferred from one year to the next to one driven more directly by seasonal rainfall. Such rangelands are more characteristic of disequilibrium systems which are more unpredictable in terms of forage supply and livestock production potential. Vulnerable people who are not buffered from economic shocks will be more severely affected by the variability in forage supply for their livestock. The impact of sustained heavy grazing over long periods of time, therefore, has important consequences for people's livelihoods