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

By Marcos Moleón, Eva Graciá, Nuria García, José M. Gil-Sánchez, Raquel Godinho, Pedro Beja, Luís Palma, Joan Real, Antonio Hernández-Matías, A. Román Muñoz, Eneko Arrondo, and José A. Sánchez-Zapata.

Read the full paper here.

Understanding what we observe in nature today often requires looking to the past. The Mediterranean Basin is home to the Bonelli’s eagle (Aquila fasciata), a species that has garnered significant attention from scientists, conservationists, and the general public. In our study, we propose that the presence of this species in the region is relatively recent and that its colonization was unintentionally facilitated by early Europeans. By compiling multiple sources of evidence and integrating various disciplines, our objective was to reconstruct the puzzle of the ancient colonization of the Mediterranean Basin by the Bonelli’s eagle and to propose a plausible mechanism that explains the observed patterns.

We found that Bonelli’s eagle fossils were confined to the last 50 thousand years. This timeframe aligns with the arrival of the first anatomically modern humans in Europe. Surprisingly, this suggests that Bonelli’s eagles, a species heavily constrained by low temperatures, began colonizing the Mediterranean Basin during a period considerably cooler than the present. Genetic data confirmed that the population of Bonelli’s eagles was very small during the last glacial maximum, which occurred approximately 27 to 19 thousand years ago.

But, why did Bonelli’s eagles arrive around 50 thousand years, and not earlier during preceding inter-glacial periods that were climatically favorable for this species? To address this question, we also investigated the competitive interactions between Bonelli’s and golden eagles (A. chrysaetos) in present-day ecosystems. Our observations revealed that Bonelli’s eagles faced limitations due to the dominance of golden eagles. In the absence of golden eagles, we could expect a proliferation of Bonelli’s eagle pairs; however, the reverse scenario does not hold true. Nevertheless, Bonelli’s eagles exhibit significantly higher tolerance towards human presence, enabling them to inhabit humanized areas that are free from their larger competitors.

Thus, our findings align with the human-mediated competitor release hypothesis, by which anatomically modern humans could have unintentionally favored the large-scale colonization by Bonelli’s eagles of a previously competitively hostile Mediterranean Basin. In other words, Bonelli’s eagles could not have established in the Mediterranean until the arrival of humans, which indirectly created an increasing number of patches free from competition with larger eagles. Paradoxically, the initial advantages of living near humans have transformed into an ecological trap for Bonelli’s eagles in contemporary times. Presently, the populations of this species in the western Palearctic are significantly jeopardized by threats such as electrocution and habitat degradation, which stem from the extensive intensification of human activities.