## 4 Discussion
Rainfall levels ensuring optimal production for one vegetation type will result in suboptimal production for the other types, and vice-versa; conversely, the temporal sequence of production peaks along the xeric-mesic gradient will buffer the loss of production of any given vegetation type through the aforementioned increase in the production of other vegetation types, which not only increases the peak but also expands the breath of the phenological curves. Hence, ensuring a balanced access to the different vegetation types may hold the key to ensuring the supply of a diverse set of forage resources to meet the herbivore guld's dietary needs [57], thus extending the availability of resources for herbivores, facilitating the maintenance of biodiversity patterns and processes [58], and increasing its resilience to climatic conditions predicted by climate change scenarios [59]. This may require, however, a wise choice of stocking rates, tailored to the productivity offered by each vegetation type, in particular, the most limiting type at each management unit (see below). Model results also indicated that the impact of ungulate density (i.e., grazing pressure) on plant production differed strongly between wild and domestic species. These distinct effects may, at least partially, arise from the strong difference in stocking rates when expressed in terms of animal biomass. Although population densities were comparable among the four different herbivores (averaging 10.15 and 8.58 individuals/km 2 for wild and domestic ungulates, respectively), the body mass of cattle and horse is four- to six-fold higher than the body mass of the two cervid species present in the study area, thus resulting in much larger biomass stocking rates of domestic ungulates (1.05 t/km 2 of cervids vs. 4.06 t/km 2 of livestock). However, this does not seem to be the only reason behind this discrepancy because linear, negative effects of both cattle and horses on vegetation were also exerted at low densities (i.e., if responses were comparable to those found for wild ungulates, non-linear effects with flat responses at low densities would be expected). Differences in foraging intensity, diet choice, and behavior (including the proportion of non-consumptive plant damage, e.g., by trampling in soft soil) probably combine with higher consumptive rates (related to the larger body mass) to cause the negative impact of domestic, but not wild, herbivores on vegetation. The positive effects of wild ungulates on several vegetation types (either at all densities, or at the low-density part of the concave downward curves) may result from facilitation mechanisms, e.g., eliminating shading by less productive plant parts, removing apical dominance, and favoring regrowth; [60; 61] or may simply reflect indirect, non-causal effects (e.g., displacement away from areas grazed by herbivores with more severe impacts). Among the domestic herbivores, cattle and horses showed similar effects (i.e., linear negative responses of similar slope) but affecting different vegetation types, probably indicating a trophic niche segregation between these two species. Both species caused similar, moderately negative effects on the grassland, the most suitable type of pasture for them. However, cattle showed strong negative effects on bulrush marsh vegetation, whereas horses did so for the saltmarsh vegetation. Preliminary data on habitat use and knowledge shared by local ranchers suggest that this difference may reflect different foraging choices by these two herbivores, with cattle being able to extensively exploit the bulrush marsh and horses feeding primarily in saltmarsh vegetation (both before bulrush emergence and after a few weeks of growth, when bulrush plants become unpalatable).