The Biogeography of Enid Landsnails on the Aegean Islands

The distribution in the Aegean archipelago of one family of landsnails, the Enidae, is described. The relationship between the number of species on an island and (i) the island's area, (ii) the island's distance from the nearest mainland, and (iii) mean size of each species, is examined. The number of species per island is not correlated significantly with any of these three factors. There is a good correlation between the distribution of landsnails and the topography beneath sea level around the island. Some species are found only off the coasts of Turkey or of Greece, on islands included within the 100 fathom isobath; one subspecies is found on twenty-four islands, all within the area included within the 200 fathom (364 m) isobath; and other species are found on those islands which lie beyond the 200 fathom (364 m) isobath. This suggests that, rather than representing island biogeography in its dynamic sense (recent dispersal over a hostile habitat), the recent Enid fauna represents a relict continental biogeography (past dispersal over a continuous, non-hostile habitat), and that such continental dispersal occurred in periodY when, as the relative level of the sea dropped as compared to that of today, isolation between certain islands, was abolished. The known geological history of the Aegean archipelago agrees well with such an hypothesis. From the recent distribution pattern it appears that the Enidae invaded the Aegean in three distinct waves, corresponding to three different past sea-levels. The fact that over 800% of the sixty-three islands, regardless of each island's area or proximity to mainland, have only one species of Enid each, whereas Turkey is inhabited by fifty-five Enids, suggests that today there is almost no immigration from the Turkish mainland. There also appears to be no immigration from Greece. The fact, on the other hand, that almost every Aegean island, down to the most remote and minute of these, is inhabited by Enidae of one species or another, suggests that the rate of extinction is very low. The number of species on an island thus appears to be the combined result of almost no immigration and almost no extinction.