Factors affecting plant species richness and endemism on land-bridge islands – An example from the East Aegean archipelago

The native flora and a number of environmental factors were recorded for twenty islands in the eastern Aegean archipelago (Greece) in order to study the patterns of native and endemic plant species diversity as well as the effects of environmental factors in shaping these patterns at different levels of endemism. Stepwise regressions were used to test the effects of these factors on species richness of native and especially of endemic plants, at local scales. The performance of the Choros model was also compared with the separate effects of area and habitat diversity using simple linear regressions. The residuals from the species–area functions of native and endemic species were used as indices of relative island richness. Out of the 2238 species and subspecies recorded in total, 302 taxa (13.4%) are endemics (either single island endemics or endemic at a broader scale, namely Aegean, Greek-Aegean, or Anatolian–Aegean). All native, all endemic and Aegean endemic species similarities among islands were assessed and the three matrices were compared with the matrices of among-island geographical distances and island similarities based on components of habitat diversity using Mantel test. Among the factors tested, habitat diversity, of which elevation is another dimension, is the main factor affecting floral richness on east Aegean islands. The patterns exhibited by local endemics, though, are of a more idiosyncratic nature since this factor is the major predictor of species richness of most of the endemics levels (including single island endemics) except the Aegean endemics which are better predicted by elevation. The Choros model does not provide a better prediction of richness than habitat diversity alone. There is a positive correlation between native species similarities and geographical distances among islands, but no correlation between environmental and floral similarities.