Origin and evolution of the Mediterranean dry grasslands.

In the Mediterranean region, three main structural types of dry grasslands can be recognized: those dominated by perennial caespitose grasses (wintergreen perennial dry grasslands, Lygeo-Stipetea), those dominated by thermoxerophilous therophytes (ephemeral dry grasslands, Tuberarietea guttatae), those dominated by chamaephytes, thistles and relatively small hemicryptophytes (summergreen perennial dry grasslands, Festuco-Brometea). All three kinds are commonly represented all over the Mediterranean territories, especially in the dry most areas. Relevant physiognomic, adaptive and floristic differences are featuring the three mentioned typologies. Lygeo-Stipetea and Festuco-Brometea may gradually shift towards Onopordetea acanthii and Poetea bulbosae-vegetation through overgrazing, while Tuberarietea guttatae may shift towards Stellarietea mediae-vegetation through ruderalization. The origin of the Mediterranean dry grasslands must be viewed as a result of the gradual shifting from humid subtropical to seasonally dry climatic conditions, with the meaningful gap represented by the Messinian disruption. Most of the species forming the savanoid dry grasslands derive from the Paleotropic Tertiary flora (and currently have a Saharo-Arabic distribution), while the summergreen pasturelands are chiefly constituted by species deriving from the Arcto-Tertiary flora (and currently have a South-European distribution). For the ephemeral dry grasslands, a relatively recent origin can be assumed, as a consequence of the evolutionary trend of the Mesogean flora towards short-lived, small, dry-seeded and polichorous growth forms, currently having an Irano-Turanian distribution. The ephemeral dry grasslands are characterized by the highest species richness and the most noticeable ecological versatility. They are included among the most diverse Mediterranean vegetation types, both for the number and evenness of species. All these annual communities have a limited resistance to perturbations and a very low resilience, so they are rather unstable: their floristic composition may change year by year. Therefore, a great part of the Mediterranean plant diversity turns out to be precarious and ever-changing. In spite of this, most of the species forming the Mediterranean ephemeral dry grasslands have broad distribution ranges and a key ecological function over a great part of the Mediterranean lands. Their evolutionary success have been boosted by a positive feedback switch with the human activity, that in the Mediterranean region begun since the Paleolithic. The ephemeral dry grasslands produce many food items for the humans (legumes, oily seeds, cereals, bulbs); moreover, they take advantage from the human disturbance, that increases the ecological fitness of these versatile but poorly competitive plant communities. The positive feedback switch settled down as follows: raised human population → increased impact (clearings, fires, generic disturbance) → decreased interspecific competitiveness → increased ecological fitness of the ephemeral dry grasslands → raised human population. This positive feedback led to the transition towards the Neolithic culture and towards an extensive agri-pastoral landscape, that begun with the selection and domestication of the most productive breeds. But if the feedback at issue lasts for several centuries, it may progressively deplete the soil fertility, because a great part of the produced biomass is taken away by the agri-pastoral practices and used (directly or indirectly) to feed the human societies, leading to an irreversible land degradation (i.e. desertification), that actually affects large areas of the Mediterranean lands. While searching the best compromise between exploitation and conservation, the Mediterranean man was forced in many cases to migrate from his homeland, as a consequence of the decreased land productivity due to the over-exploitation of natural resources. The Mediterranean dry grasslands, namely those dominated by annuals, often represent an useful indicator of the past human mistakes. The therophytic abundance can be therefore used to assess the threat of desertification in the Mediterranean lands.