SPECIES-DIVERSITY PATTERNS IN MEDITERRANEAN GRASSLANDS

A field experiment involving herbivore exclusion, ploughing, and the combination of both was carried out over a period of 4 - 5 yr in Mediterranean grasslands located along an elevational gradient. The empirical results provide a general hierarchical framework for understanding patterns of plant species diversity in these grasslands. In grazed grasslands, plant species density decreased as altitude increased, and this pattern was maintained through time. The reduced seasonality along the climatic gradient is suggested as the extrinsic, indirect control factor. Ploughing caused species loss, but after 4 yr the original diversity was recovered in most grasslands. Our hypothesis is that a negative feedback mechanism regulates species increase towards a characteristic level. A trend of species density reduction was observed in ungrazed grasslands. Plant-herbivore interaction is considered to be essential for maintaining species diversity in grazed grasslands and for the recovery of diversity in mechanically disturbed grasslands.