We found single-island endemic species richness to be most closely correlated to island maximum elevation. This was also observed for island group endemics, but the slope of the correlation was less steep. The primary importance of an island’s maximum elevation for endemic species richness has also been recorded for endemic orchids in the West Indies (Ackerman et al. 2007). Among the possible explanations for this relationship Selleckchem Birinapant are habitat diversity, human disturbance, habitat stability
and refugia during past climate change. Firstly, increased habitat diversity corresponds to increased availability of ecological niches to allow speciation of new endemic species. Kohn and Walsh (1994) and Ricklefs and Lovette (1999) reported a correlation between an island’s maximum elevation and its habitat diversity. Secondly, all islands that GSK1210151A support single-island endemics also support permanent human populations. However, we regard the possible conclusion that human disturbance and pressure induced speciation of single-island endemics as a logical error (cum hoc ergo Selleck GSK2118436 propter hoc) with no causality between the two. Thirdly, higher elevation is a precondition for long-term stable ecosystems such as cliffs which support plant assemblages with high proportion of narrow endemics. Fourthly, a large elevational range may allow the vertical migration during periods of climate change, allowing
the persistence of relictual populations of ancient species. The habitat diversity explanation assigns a major role to speciation through adaptive radiation, while the latter two explanations assign greater importance to the persistence of older species or to speciation through non-adaptive radiation. In the Aegean, there are documented examples of endemic species associated with non-adaptive radiation (see Gittenberger 1991 for land snails; Snogerup 1967a, b and Barrett 1996 for the genus Erysimum, Strid 1970 and Bittkau and Comes 2005 for the Nigella arvensis complex, Runemark 1980 for the Dianthus fruticosus complex, Snogerup et al. 1990 for Brassica, Turland 1992 for the Dianthus juniperinus complex). Non-adaptive radiation attributable to stochastic mechanisms such as genetic drift, acting on small isolated
populations, plays a primary role in speciation and endemism in the Aegean archipelago (Runemark heptaminol 1969, 1971a). However, it has to be stressed that these possible explanations are not mutually exclusive, and there is no reason to assume that they do not act synergistically to enhance endemic species richness. The relationship between total species richness on islands and environmental factors (mainly area, isolation, elevation and climate) has been studied extensively over the past century and a half (reviewed by Whittaker and Fernandez-Palacios 2007). Willerslev et al. (2002) reported that the ranking in relative importance of area, elevation and distance from mainland and other islands is the same for total and endemic plant species richness in the Galapagos.