Wildlife following people: A multidisciplinary assessment of the ancient colonization of the Mediterranean Basin by a long-lived raptor

Abstract

1. Modern humans widely shaped present ecosystems through intentional and unintentional geographical redistribution of wildlife, both in historical and pre-historical times. However, the patterns of ancient human-mediated indirect changes in wildlife range are largely unknown, and the mechanisms behind them remain obscure. 2. We used a multidisciplinary approach to (a) reconstruct the process of colonization of the Mediterranean Basin by a long-lived bird of prey, the Bonelli's eagle (Aquila fasciata), and (b) test the hypothesis that this colonization was unintentionally favoured by anatomically modern humans through a release of competition by dominant species, primarily golden eagles (A. chrysaetos). 3. The fossil record of Bonelli's eagles in the Mediterranean Basin was restricted to the last c. 50 ky. This timing matches the period of modern human presence in Europe. Distribution modelling showed that Bonelli's eagles find more suitable conditions in interglacial periods, while glacial maxima are largely unfavourable unless in coastal refugia. In agreement with this, all Bonelli's eagle's fossils were found in coastal areas, and demographic inference from genetic data revealed a drop in the effective population size by around the last glacial maximum. 4. In today's communities, we found a strongly asymmetric competitive relationship between (subordinate) Bonelli's and (dominant) golden eagles, with the former occupying far more humanized areas than the latter both at the landscape scale and the local (i.e. nesting cliff) scale. Moreover, the nesting habitat overlap analysis indicated that, in the absence of the other species, a notably higher population of Bonelli's eagle, but not of golden eagle, could be expected. 5. Our findings are consistent with the human-mediated competitor release hypothesis, by which anatomically modern humans could have unintentionally favoured the large-scale colonization by Bonelli's eagles of a previously competitively hostile Mediterranean Basin. Reconstructing the role of ancient humans in shaping present ecosystems may help to understand the historical, current and future population trajectories of competing species of conservation concern under the ongoing scenario of global environmental change. It also illustrates how human-mediated apparent competition may promote large-scale redistribution and colonization of wildlife, including long-lived species.