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The communal areas of the semi-arid (200 mm) winter-rainfall Namaqualand region of South Africa are heavily utilised by a large number of subsistence farmers who keep sheep and goats. For several decades, farmers have maintained, on average, more than twice the number of animals on the rangeland than the number recommended by the Department of Agriculture for neighbouring commercial enterprises. As a result of the continuous high stocking densities, significant changes in the vegetation have taken place. Farmers are now heavily reliant on an annual flush of vegetation following winter rains to keep their livestock alive. Using a combination of empirical data and consensus agreement, a computer model is developed to describe the long-term climate, livestock and vegetation interactions on the communal rangelands and to investigate the impacts of a range of management strategies. The model suggests that the system (including livestock) is sustainable although not stable, and that its sustainability could be due to climatic variability. The model supports the view that when livestock numbers vary in a manner consistent with recorded observations herbivory has little long-term impact on productivity of the system. This supports recent views of rangelands in semi-arid and arid environments where non-equilibrium conditions are thought to dominate ecosystem processes.