Two long-term mechanistic models of grazing systems in the semi-arid succulent Karoo have been used to study factors that influence vegetation changes, livestock productivity and sustainability of the ecosystem. In this region of low and highly variable rainfall, goats and sheep feed on vegetation comprising perennial shrubs and annuals. A previously published model of the Namaqualand system (the "standard" model) explicitly simulates three guilds of perennial shrubs, a guild of annuals, forage consumption, growth of goats and goat reproductive and survival rates. The model also simulates variable rainfall and predicts that, if no steps are taken to control the goat population, stock numbers will vary widely between years and the population of the different plant guilds will fluctuate. Keywords: Rangeland dynamics, Semi-arid regions, Vegetation change, Goat production.