Livestock impact assessment and restoration strategies in the semi-arid Karoo

Doctoral Thesis


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University of Cape Town

Karoo rangelands exhibit spatial and temporal patterns that have important implications for the livestock industry. Spatially, there are gradients, often abrupt, in ecosystem structure and functioning, while plant composition and productivity are highly variable over time. A predictive understanding of these patterns, and the processes that cause them, is a prerequisite for developing appropriate restoration strategies. This thesis comprises several studies that attempt to relate vegetation patterns and processes to restoration strategies in southern Africa's Succulent- and Nama karoo ecosystems. One hypothesis is that small-scale changes in soil physical and chemical properties are responsible for the fine-scale patterning evident in winter-rainfall Succulent karoo ecosystems. Alternatively, these patterns could be the result of area-selective grazing by livestock. To evaluate these hypotheses, plant and soil data were collected along soil- and grazing gradients radiating from a watering point in a Succulent karoo landscape. Results indicated that properties influencing soil hydrology and nutrient status are important determinants of pattern, and that long-term area-selective grazing can permanently change some of these properties. The hypothesis that the stasis of severely degraded patches in this biosphere is a consequence of poor water infiltration and seed limitation was tested in a restoration experiment. It appeared that natural seed availability is not limiting, but water infiltration has to be improved to initiate the restoration process. Removal of shrub material in long-ungrazed and moribund areas on the outer perimeter of the biosphere, had a positive impact in releasing resources for more seedlings and young growth, but did not alter plant species richness. Stocking rate, composition and management of livestock profoundly influence the dynamics and composition of summer-rainfall Nama karoo vegetation. Proponents of non-selective grazing (NSG) argue that the periodic concentration in high densities of livestock in small areas, followed by long resting periods, improves vegetation composition as a consequence of low grazing selectivity, and enhances vegetation productivity and soil ecosystem processes as a result of intense hoof-action, dunging and urination impacts. Despite its application in farming systems, no studies have yet tested the predictions of NSG. I evaluated the effects of NSG on the soils and vegetation of Nama karoo rangeland in a fully replicated experiment. NSG did not alter the fertile-patch matrix, but improved soil infiltration. Plant compositional and cover changes could not be related to NSG. Rainfall was a much stronger driving force. I also explored the economics of NSG at the farm scale under different rainfall and stocking scenarios. An ecological-economic model predicted that NSG would be a viable option in higher rainfall (>200mm) areas because of the forage buffering capacity which enables the manager to maintain livestock through unpredictable droughts. Restoration strategies for the Succulent Karoo have to focus on the resource-retention capacity of the soils. Livestock can reduce this capacity; low-stocking, flexible farming systems are therefore recommended for these more fragile ecosystems. Livestock in the more resilient Nama Karoo can be managed in a NSG system that can lead to an improvement in ecosystem functioning and maintain productivity in times of drought.

Includes bibliographical references.