Hyperdiverse insect group indicates forest encroachment a threat to the Mediterranean biodiversity hot-spot

The Mediterranean Basin represents a global biodiversity and sociocultural hotspot. Currently, cessation of traditional agropastoral land use, followed by encroachment of open landscapes by scrub forests, are potentially threatening the endemic biodiversity, which has been earlier documented for several taxa over a range of scales. This view is contested by the “forested Mediterranean hypothesis”, tracked back to antiquity, which considers the encroachment as a return to pristine pre-cultural conditions, and has far-reaching policy implications, including support for afforestation, and neglect of non-forested habitats in conservation planning. We sampled macro-moths, a hyperdiverse insect group, from 150 plots distributed from southern Greece to Bulgaria and North Macedonia, and analysed the material (641 species, 42,136 individuals) with respect to the biogeography of the constituent species. The three land cover categories – grasslands, semi-open formations, scrub forests – did not differ in numbers of species or individuals. Moths with small Mediterranean and Pontic ranges were associated with grasslands and semi-open formations, whereas scrub forests were mainly inhabited by northern species with wide distribution ranges. This implies that substantial numbers of Mediterranean Basin endemics inhabit open savannas in which they probably evolved, that historical land uses allowed persistence of endemic biota long into the modern era, and that the current forest encroachment, and policy initiatives supporting landscape closure, represent direct threats to a global hotspot of diversity and endemism.