Conclusions
Our results document that local-scale biotic processes, such as facilitation by the shrubs, are important determinants of diversity patterns. In general, shrubs are known to provide refugee for species in harsh conditions, such as high aridity or overgrazing. Furthermore, we argue that the discrepancy in the literature on changes in plant-plant interactions may be partially explained by differences in plant life strategies of species in the conditions of the combined effect of biotic (grazing) and abiotic (aridity) stress. Therefore, in the arid region, drought-escaping species like ephemerals and ruderals (R-selected) and species tolerating stress (S-selected) but avoiding herbivory are highly dependent on the facilitation under the canopy of shrubs. However, in sites without severe aridity, canopy of the target shrub protected competitive species (C-selected) in the conditions of high grazing intensity (low aridity/high grazing). On the contrary, on sites without intensive grazing and severe aridity (low aridity/low grazing), facilitative effects of the shrub turned to neutral or even competitive.