The influence of geographical and ecological factors on island beta diversity patterns

Aim To investigate the importance of various island characteristics in determining spatial patterns of variations in beta diversity for various animal groups. Location Analyses are presented for 10 animal groups living on the Aeolian Islands, a volcanic archipelago in the central Mediterranean, near Sicily. Methods Three hypotheses were formulated to explain patterns of beta diversity: the target-area-distance effect, stepping stone dispersal and island age. Matrices of inter-island dissimilarities were constructed under each hypothesis and correlated with matrices of faunal dissimilarities using Mantel tests. For the 'target-area-distance effect' hypothesis, inter-island dissimilarities were calculated using island sizes and distances to nearest mainland areas. For the 'stepping stone dispersal' hypothesis, inter-island distances were measured. Finally, for the 'island age' hypothesis, inter-island dissimilarities were calculated on the basis of the geological age of the islands. Cluster analysis was used to investigate inter-island faunal relationships. Results Support for a target-area-distance effect was found only for birds. For these highly mobile animals, inter-island distances had no significant effects on beta diversity. Birds are known to colonize islands by crossing large sea barriers and thus they can easily reach the Aeolian Islands, which are close to source areas (notably Sicily). Inter-island distances had a significant role in determining patterns of beta diversity in most invertebrates. For Mollusca, Opiliones, Chilopoda, Heteroptera, coprophagous Scarabaeoidea, and Tenebrionidae, even relatively short distances preclude invertebrates from colonizing an island regularly from the mainland, and most colonization probably results from inter-island faunal exchanges. Island age was proved to be important only for orthopterans. Main conclusions The origin of most of the Aeolian invertebrate fauna is quite recent, and species appear to have established on the islands predominantly by stepping stone dispersal. Birds, which are highly mobile organisms, follow more direct mainland-island dynamics. As further studies on other islands become available, comparative analyses will confirm whether the factors influencing variations in beta diversity in this study and their relationships with species dispersal ability are consistent across scales and geographical context.