Rangeland degradation around water-points under different management systems

[摘要] ENGLISH ABSTRACT:Over 70% of South Africa is too arid for crop farming and is used for commercial livestock ranching,communal livestock ranching, or game ranching. These management systems differ from each otherin a number of aspects, e.g. herbivore species, stocking rate, grazing regime, and managementstructure. The main question addressed in this study is: to what extent do the three managementsystems affect rangeland condition? Following the equilibrium paradigm of vegetation dynamics,communal livestock ranches are expected to have a greater detrimental effect on rangeland conditionthan other management systems because stocking densities and, consequently, herbivore impacts,are usually far higher than under the other two management systems. However, recent nonequilibriumtheories argue that vegetation dynamics in arid systems are mainly driven by rare andstochastic rainfall events. Biotic factors, such as grazing and herbivore diversity, are predicted to havelittle effect on rangeland condition. This leads to the prediction that herbivore impacts on aridecosystems are not density-dependent and, consequently, the high stocking densities commonlyrecorded on communal livestock ranches will not cause more rangeland degradation than othermanagement systems.Arid and semi-arid rangelands are characterised by high inherent spatial and temporal variation invegetation and soil parameters. Hence, differentiating between the effects of grazing managementsystems and natural variability caused by abiotic factors, such as rainfall, is difficult. This problem canbe circumvented by examining gradients of grazing intensity radiating from water-points (=piospheres). I examined changes in vegetation and soil parameters along 500 m-Iong grazinggradients on ranches in the semi-arid Northem Cape province, South Africa, with the three abovementionedmanagement systems. Analysis of the vegetation on these ranches demonstrated theexistence of grazing gradients around the water-points. Two characteristic zones described in otherstudies were identified, namely the sacrifice zone (almost complete denudation of the vegetation closeto the water-point) and the dense shrub zone (distal to the sacrifice zone). Communal livestockranching had a greater negative effect on rangeland vegetation than the two other ranching systems,particularly with regard to annual grass species diversity, percentage of bare soil, and shrub density.Within 100 m of the water-points, soil quality was negatively affected by commercial cattle ranching.Contrastingly, there were no significant effects of the other two management systems on soil quality inthe piosphere. The feeding of supplemental forage and nutrients may explain the greater impact of commercial cattle farming on soil quality. The results of this study are consistent with the predictionsof the equilibrium theory of vegetation dynamics because grazing impacts were density-dependent inthese semi-arid rangelands. However, the results presented here also indicate that foragesupplementation alters the spatial and temporal distribution of livestock in a manner that may havegreater negative impacts on soil quality than density of livestock per se.

[发布机构] Stellenbosch University