Drivers of floristic richness in the Mediterranean: a case study from Tuscany

Unravelling patterns of species richness is a fundamental prerequisite to understand evolutionary and ecological dynamics, aiding to set efficient conservation activities. The Species-Area Relationship (SAR) is one of the most general patterns in ecology. Focusing on a Mediterranean area as case study, we investigated SAR and the drivers underlying species richness. We gathered data from all vascular floras published for circumscribed areas in Tuscany after 1970 and then we fitted SAR models based on Arrhenius’ power function for the whole established flora, and for native and alien species separately. SAR residuals, which express the actual species richness free of area-effect, were modelled through spatially explicit Generalised Linear Models using geographic, climatic, and anthropogenic explanatory variables. The most relevant predictor for species richness is the grain of the study area, while environmental drivers play a minor role. Topographic heterogeneity and the amount of precipitation show a positive effect on total floristic richness, while the spatial heterogeneity of annual temperature range shows a negative effect. Native and alien species richness are positively correlated, but different combinations of drivers are involved to explain the patterns of the two different species pools. A fundamental factor driving species richness is the insularity: islands host, proportionally, fewer native species and more alien species than mainland areas. Alien species richness is positively affected by landscape heterogeneity. Finally, we present for the first time a method to draft maps of SAR-predicted floristic richness, integrated by the influence of environmental drivers.