Two independent models concerning the effects of grazing on vegetation have gained wide acceptance in the last decade: Westoby et al.'s state‐and‐transition (S– T) model, and Milchunas et al.'s generalized model of the effects of grazing on plant community structure and diversity (MSL model). These two prevailing models, as they stand, are conceptually divergent. The MSL model implicitly assumes that, at a given site, for each grazing intensity there is a single equilibrium situation with a single diversity value. The S–T model suggests that rangeland dynamics include irreversible transitions and alternative equilibria. Here we propose a modification of the original MSL model, to encompass a wider range of real situations and to place it within the context of the S–T model. The four extreme cases proposed in the original MSL model are revisited, taking into account that (1) the “moisture” gradient can be generalized as a “productivity” gradient; (2) the selective pressure of herbivores on systems with long history of grazing has fluctuated over time, allowing the development of different pools of species adapted to low or high grazing intensities; and (3) systems with long evolutionary history of grazing have developed resilience mechanisms that allow reversible shifts in floristic composition with changes in grazing intensities. The grazing intensity vs. diversity curves thus postulated for systems with a long evolutionary history of grazing are similar to those proposed by the original MSL model because resilience mechanisms allow for reversible changes associated with grazing intensity. In contrast, the curves postulated for systems with short evolutionary history of grazing include different alternative branches, indicating irreversible transitions, because resilience mechanisms to grazing were not fully developed. By incorporating these modifications, the divergence between the original MSL and S–T models can be resolved. A set of published examples from real systems is presented and compared with the predictions of the modified model. The modified MSL model is applicable to a wider range of real situations than the MSL model in its original formulation.

Summary 1.In some ecosystems there is a positive feedback between forage quality and grazing intensity. This involves three components of plant tolerance to grazing: functional traits, herbivore selectivity and response to grazing. We analysed the relationships between these components at species and community levels in Patagonian steppe grasslands. 2. We measured plant functional traits [height, specific leaf area (SLA) and foliar toughness] and estimated sheep selectivity and grazing response indices for 35 plant species. Sheep selectivity indices were obtained from microhistological and species' availability data, and grazing response indices from species' abundances in sites with contrasting grazing intensities. We performed correlations and multiple regressions among the three types of variables across the pool of 35 species. 3. To analyse data at the community level, we computed weighted averages of traits and sheep selectivity indices for 34 floristic samples taken from each side of 17 fence lines with contrasting grazing intensities. Correlations between mean trait values and sheep selectivity across the 34 samples, and paired comparisons of those variables between sides of the fences, were performed. 4. Taller plants had leaves with lower SLA and/or higher toughness. Short species of intermediate toughness were selected more often by sheep, while SLA was not related to sheep selectivity. Short species with high SLA increased with grazing, while toughness and sheep selectivity were unrelated to grazing response. 5. At the community level, short swards with high average SLA had high selectivity indices and were more abundant on the most intensively grazed sides of fence lines. Leaf toughness was unrelated to other traits or to sheep selectivity, and showed no significant response to grazing. 6. Synthesis and applications . Intensive grazing can increase the forage value of grasslands by the creation of lawns dominated by tolerant species. However, results from this study showed that some plant species that were avoided by grazers also increased, indicating a potential risk of a shift in composition of grazing lawns towards states of low forage value. This suggests that periodic resting of lawns could be a good management strategy to favour palatable species, thereby minimizing the risk of undesirable shifts in the overall species composition.

Aim  To determine how the distribution and cover of different vegetation types are affected by physical factors and livestock in a mountain range with a long evolutionary history of grazing. Location  Upper vegetation belt of the Córdoba mountains (1700–2800 m a.s.l., 31º34′ S, 64º50′ W) in central Argentina. Methods  Using GIS, we analysed the relationships of plant cover types to physical features (physiography and topography) and indicators of accumulated livestock pressure (distance to human settlements and roads) through multinomial logistic regression. We predicted a present vegetation map which was validated with a real map. We then constructed two maps simulating minimum and maximum values of accumulated livestock pressure for the whole area. Map comparisons allowed evaluation of the possible influence of livestock, both in extension and intensity. Results  Both physical features and livestock pressure influenced the occurrence of vegetation units. The overall accuracy of the predicted map at the pixel level was low (26%) indicating low habitat specificity of the vegetation units. We suggest that some part of the unaccounted for variance was due to livestock pressure patterns that were not fully captured by our indicators. Our models proved adequate for predicting the total percentages of vegetation units at coarser scales. The extrapolations showed that under a history of low livestock pressure, such as in sites far away from human settlements and roads, the area would be dominated by woodlands, tussock grasslands and natural rock outcrops. Under a history of heavy livestock pressure, in turn, rock exposed by erosion, tussock grasslands and natural rock outcrops would dominate. Main conclusions  Vegetation units showed low habitat specificity, and were associated with accumulated livestock pressure, indicating that livestock and its associated activities are important factors structuring the landscape and have important consequences for the integrity of the ecosystem. Results suggest that although this system evolved with large herbivores, it has experienced irreversible degradation processes, and intensification of current domestic livestock pressure is likely to lead to even more land degradation.