Aim This research aimed to study if a transitional character can be postulated for the tenebrionid beetle (Coleoptera, Tenebrionidae) fauna of the Aegean Islands, and to identify ecological and historical factors responsible for observed patterns. Location Eastern Mediterranean, Aegean Islands. Methods A total of 32 Aegean Islands and 166 taxa (species and subspecies) were included in this study. In order to assess whether the Aegean tenebrionid fauna is mainly equilibrial or relictual, the importance of different eco-geographical variables (i.e. area, latitude, longitude, distance to the nearest island, distance to the mainland) in determining species richness and endemicity levels was analysed by using several statistical methods. These included single regressions, multiple regressions, and partial correlations. Cluster analysis, multidimensional scaling (MS) and discriminant function analysis (DFA) were used to quantify the similarities between the islands. Regression lines were used to study variations in the proportion of Balkan and Anatolian species. Parsimony Analysis of Endemicity (PAE) was used to study possible relationships between hierarchical species assemblages and palaeogeographical reconstructions. Results Island area accounted for most variability in species number. Distances to the nearest island and to the mainland were not identified as of any statistical importance in affecting species number. The proportion of Balkan taxa sharply decreases from west to east, whereas the Anatolian taxa follow an opposite trend. The proportion of endemic taxa appears uncorrelated with island present isolation, while the proportion of subendemic taxa appears to be affected by present distance to mainland and interisland distances. Cluster analysis, MS and DFA, as well as PAE, revealed a clear faunal discontinuity between the western and central Aegean Islands on one side, and the islands close to the Anatolian coast on the other side. This discontinuity, consistent with the persistence (from Messinian to Pleistocene) of a sea barrier between these two groups of islands, strongly supports the importance of Pleistocene island configurations in determining present distributional patterns. Main conclusions The tenebrionid fauna of the Aegean Islands appears to be relictual. Most of the tenebrionid species have probably colonized the Aegean Islands by means of land-bridges during Pleistocene falls in the sea level. The palaeogeography of Pleistocene island groupings is identified as responsible for present levels of endemicity and distributional patterns. An overall transitional character can be observed for the tenebrionid fauna in the study area. However, in accordance with the persistence of a deep sea barrier between the western and the eastern islands, these two groups appear to harbour very distinct faunas.
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