Plant species richness hotspots and related drivers across spatial scales in small Mediterranean islands

Small islands represent a common feature in the Mediterranean and host a significant fraction of itsbiodiversity. However, the distribution of plant species richness across spatial scales—from local communities(alpha) to whole islands (gamma)—is largely unknown, and so is the influence of environmental, geographical, andtopographical factors. By building upon classic biogeographic theory, we used the species–area relationship andabout 4500 vegetation plots in 54 Central Mediterranean small islands to identify hotspots of plant species richnessand the underlying spatial determinants across scales. To do so, wefitted and averaged eight species–area modelson gamma and alpha richness against island area and plot size, respectively. Based on positive deviations from thefitted curves, we identified 12 islands as cross‐scale hotspots. These islands encompassed around 70% of species andhabitat richness, as well as almost 50% of the rarest species in the data set, while occupying less than 40% of thetotal island surface. Byfitting generalized linear mixed models, we found that gamma richness was mainly explainedby island area and was weakly related to mean annual temperature (positively) and annual precipitation(negatively). As for alpha richness, after accounting for the idiosyncratic effect of habitats and islands, plot size andgamma richness remained the only significant predictors, showing a positive relationship. This work contributes tothe understanding of the patterns and drivers of plant diversity in Central Mediterranean small islands and outlines auseful methodology for the prioritization of conservation efforts.