Differences in species composition and diversity among Mediterranean grasslands with different history – the case of California and Spain

Species composition and diversity were compared among twenty Mediterranean annual grasslands in northern and central California and central and southern Spain, encompassing climatic gradients and local site variation in topography and soils Geographic proximity was more important than environmental factors such as climate, topography and parent material in predicting the species composition of these grasslands, with Californian and Spanish grasslands sharing only 9% of the species and geographically separated regions within each country sharing only 20- 32% of the species This importance of geographic separation in predicting species composition suggests a strong role of dispersal limitation in determining current community composition Mean species diversity was lower in Californian than in Spanish grasslands and was negatively correlated with cover of annual grasses that tended to be greater in California than in Spain However, there were few differences in species diversity among sites within either country and patterns of species diversity were unrelated to soil fertility or productivity We suggest that current grazing regimes contribute to the greater abundance of grasses and lower species diversity of Californian than of Spanish grasslands The apparent importance of dispersal limitation and grazing m explaining differences in species composition and diversity between Californian and Spanish grasslands and within each country suggest that the structure of these grasslands has been and will continue to be sensitive to human influence