Factors affecting plant species richness and endemism in the South Aegean (Greece)

The South Aegean island arc is ideal for the investigation of species richness patterns. We applied the species-area relationship for overall vascular flora and for endemic plants, as well as for different families, using available floristic data from 60 of its islands. Maximum altitude, shortest distance from mainland and from the nearest larger island, and habitat diversity were examined as potential predictors of species richness. Habitat diversity estimation was based on species light, temperature, moisture and soil salinity requirements, according to the Southern Aegean Indicator Values. The effectiveness of Choros model was also tested. Habitat diversity alone was more effective than area in determining the vascular flora of the 60 islands and of two endemic species categories. The Choros model was the most efficient one in shaping the number of South Aegean endemics. Area prevailed in the case of single island endemics. Our results indicated that both area and habitat diversity should be examined for a more thorough interpretation of richness patterns. Altitude contributed mostly to the prediction of species richness for total flora and for most subsets of endemics. Species-area relationships at the family level varied significantly in relation to the number of species within each family and the family distributional range within the study area. A strong correlation between intercept values arising from species-area relationships at the family level and total richness of these families in the South Aegean supports an ecological interpretation of the intercept as an indicator of the "capacity" of the studied area.