Plant species richness, biogeographic and conservation interest of the vascular flora of the satellite islands of Sicily: patterns, driving forces and threats.

The vascular flora has been investigated on about 60 of the approximately 100 islands and islets of the Sicilian archipelago. This paper provides detailed information on the number of exclusive, rare, or threatened vascular plants living on these islands and islets. Focusing on the 18 most-investigated islets, we evaluate the extent to which species richness, rate of endemism, number of alien plants, and number of terrestrial habitats have been influenced by 1) geographical setting, 2) geological history, 3) geo-pedological variability, 4) bioclimate, 5) number and patchiness of local plant communities, and 6) natural and human disturbance history and regime. Special attention is directed to the rarefaction and extinction of many interesting species that live or have lived only on these islands. Rarefaction and extinction are mostly linked to the increase in anthropogenic disturbance, which have caused the destruction, degradation, and/or fragmentation of many plant communities and especially those associated with rocky or sandy shores and temporary ponds. Interestingly, in some cases extinction has resulted from a reduction in certain human activities. For example, many noteworthy species are disappearing with the abandonment of traditional land uses that created low-impact agro-ecosystems, new microhabitats (e.g., stone walls), and open semi-natural habitats (including vineyards, cereal crops, olive groves, and fallows subject to extensive grazing). Finally, we emphasize the need for current information about species turnover, invasion processes, and demographic patterns of the vascular flora of those islands and islets that support the most fragile communities and/or species.