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In this article, a dynamic simulation model is presented, designed to add to the understanding of commercial livestock-grass-soil systems and, especially, the processes of desertification due to overgrazing. The model has two key innovative features: (i) the joint dynamic treatment of the three mentioned stock variables and (ii) the consideration of livestock dynamics determined by economic variables. The main possible behaviours of the resulting three-dimensional system are presented and it is shown that there are significant differences with respect to the subsystem constrained to livestock and grasses as considered in the literature to date. After analysing the model qualitatively, we are able to propose some early warning indicators of the risk of long-term desertification due to overgrazing. In this article, we illustrate the use of those indicators in three hypothetical, yet likely extensive livestock farming scenarios in Spain. (c) 2007 Elsevier B.V. All rights reserved.