------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 affect 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 the total
density of grasses and shrubs as well as their 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.
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