Authors: Testolin, Riccardo; Zannini, Piero;Chiarucci, Alessandro;

Small islands represent a hotspot of plant diversity within the global diversity hotspot of the Mediterranean basin. These areas are particularly vulnerable to anthropogenic disturbance, species invasions, and the adverse effects of climate change. In this context, nano-hotspots (i.e., islands with higher species richness than expected simply by area) can be identified and prioritized when planning conservation actions. In this study, we employed a large dataset of about 5000 vegetation plots located in 53 central Mediterranean small islands of different size, degree of isolation, morphology, geological origin, and climate. By drawing on classical biogeographic theory, we used the species-area relationship to identify alpha (plot-level) and gamma (island-level) hotspots of plant species richness. To assess the drivers underlying the observed patterns, we modeled species richness at the two spatial scales against a set of environmental and biogeographical predictors. Both alpha and gamma nano-hotspots are mainly located in southern Italian archipelagos, although with some discrepancies between the two spatial scales. After accounting for the area, gamma diversity is mainly explained by island morphology, while alpha diversity by island origin and local contingencies, with a marginal effect of climate in both. Our work explores the conservation implications of classical biogeographic theory across scales.