Interactions among grasses, shrubs, and herbivores in Patagonian grass-shrub steppes

The Patagonian shrub steppe, as most Patagonian ecosystems, is dominated by tussock grasses and shrubs. Our main objective was to review the ways in which grasses and shrubs interact and how this affects the structure and functioning of the steppe and discuss the impact of grazing on them. Water is the main limiting resource in the Patagonian shrub steppe. Competition and facilitation control grass and shrub density and distribution. Introduction of sheep since the beginning of the century may have affected vegetation and ecosystem structure and functioning and modified the grass-shrub relationships. We use a simple conceptual model to suggest how sheep grazing can modify the result of competition and the grass/shrub balance in ecosystems that show different levels of degradation. Additionally, grazing may disrupt the current two-phase organization of vegetation and therefore may radically affect the ecosystem functioning of the steppe.