Endemic and alien vascular plant diversity in the small Mediterranean islands of Sardinia: drivers and implications for their conservation.
Islands are of high interest for conservationists, due to their great biodiversity within discrete territories. Nonetheless, several mechanisms of plant diversity patterns are unknown, especially for continental islands. In this paper, we explored how endemic and alien vascular plant species richness and the compositional dissimilarity of small Mediterranean continental islands vary according to factors related to human activities, geography/landscape, and climate. Actions for endemic plant conservation were also prioritised according to both endemic and alien components. To this aim, data of endemic and alien plant species for forty islands of Sardinia were considered. Species-area residuals, which express the actual species composition free of area-effects, were modelled using 19 variables related to anthropogenic, geographic/landscape and climatic domains (group of factors). The geographic/landscape domain appeared to be important for both endemic and alien species richness and compositional dissimilarity, while the climatic one was especially relevant for alien species richness. The anthropogenic domain was mostly important for the compositional dissimilarity of aliens and for the endemic species richness. Actions for endemic plant conservation were of high priority for 14 islands; 11 of which are located off the northwest coast of Sardinia. Our research confirms that the Mediterranean small islands of Sardinia are plant endemism hotspots prone to alien invasion. Our findings suggest that removing anthropogenic disturbances is not sufficient to prevent plant invasions. Other factors related to climatic and geographic/landscape domains are as or more important in determining endemic and alien plant composition.