- The vegetation response to variation in rainfall totals along a climatic transect extending from a Mediterranean region to an extreme arid region was studied. A curvilinear relationship was suggested between annual rainfall, species richness, species diversity and dominance. The maximum number of species was calculated to be in the range of 900–950 mm, the maximum diversity between 750 and 800 mm, and the minimum dominance in the range of 600–700 mm. Annual species decrease logarithmically from the extreme desert to the Mediterranean region with a parallel increase of perennial herbaceous and woody plants. Wet and dry scenarios are considered as a framework to evaluate future vegetation responses to possible climate change.