The Anatolian diagonal revisited: Testing the ecological basis of a biogeographic boundary

Abstract

The Asian part of Turkey, i.e. Anatolia, is the region where three of the world's 35 biodiversity hotspots meet, and interact: the Caucasus, Irano-Anatolian, and Mediterranean basin hotspots. One of the most distinctive biogeographic features that helps in understanding the biodiversity of Anatolia is the Anatolian diagonal, which has long been recognised as a biogeographic boundary between the central and eastern Anatolian floras and faunas, but the processes (i.e. historical, ecological or some combination of these) responsible for its origin and maintenance have not been well understood. The aim of this study was to assess whether the Anatolian diagonal corresponds with a significant environmental barrier. I used for this purpose ecological niche modelling and associated comparative metrics. First, I created virtual records in the Anatolian part of the Irano-Anatolian hotspot, and split these records into two groups: those occurring to the west and to the east of the Anatolian diagonal. Then, I examined whether the Anatolian diagonal is associated with a steep environmental gradient. It was found that the Anatolian diagonal is indeed associated with a steep environmental gradient, and therefore corresponds with a significant environmental barrier. This steep gradient associated with the Anatolian diagonal is mainly in temperature seasonality. The models did not cross-predict each other, either at the Last Glacial Maximum or at the present, suggesting that during at least the last glacial-interglacial cycle, many of these populations or taxa were excluded from the other side of the Anatolian diagonal by environmental rather than non-environmental reasons (i.e. dispersal, competition).