Title : Drivers of alpha and beta vascular plant diversity in a continental archipelago (Ionian Islands, Greece)
The Mediterranean Basin is one of the major global terrestrial biodiversity hotspots, while most of the Mediterranean Islands belong to the two Greek archipelagos, namely the Aegean and the Ionian. Although the factors affecting plant species richness and endemism are well studied in the Aegean Sea, regression modeling techniques have not been applied to disentangle the role of biogeographical factors in shaping plant assemblages in the Ionian archipelago. Moreover, the majority of biogeographical studies in the Greek Islands have been focused on alphadiversity patterns, while there are only two macroecological studies investigating factors affecting beta-diversity in the Aegean archipelago. The aims of the present study in the Ionian archipelago are: 1) to investigate the drivers of native and endemic plant species richness, 2) to analyze beta-diversity and its separation into its two components (turnover and nestedness) and finally, 3) to identify if there are any islands that can be regarded as plant diversity hotspots. According to our results, area is confirmed as the most powerful single explanatory variable of island plant species richness for all species richness metrics, namely Native (N), Total Endemics (TE) and Ionian Endemic taxa (IoE). Mean annual precipitation and temperature, as well as shortest distance from the nearest island are also significant predictors of vascular plant diversity in the Ionian archipelago. Finally, our hotspot analysis revealed four small islands (i.e. Kalamos, Ereikousa, Antipaxoi and Atokos) with exceptionally high plant diversity, while the aforementioned islands belong to the “NATURA 2000” network of protected areas. In conclusion, the minimal isolation and the close proximity of the Ionian Islands to the mainland source have resulted in islands that still behave as parts of a continuous land mass. The rather strong environmental and the reduced dispersal filtering (due to the recent formation of the Ionian Islands) have led to islands with homogenized plant communities and to floras principally comprising common species, also explaining the low proportion of endemics. Kalamos island emerged as the most important diversity and endemism hotspot.