Gradual replacement of wild bees by honeybees in flowers of the Mediterranean Basin over the last 50 years

Abstract

Current evidence for generalized pollinator decline largely originates from mid-latitude regions in North America and Europe. Unacknowledged geographical heterogeneity in pollinator trends, in combination with geographical biases in pollinator studies, can produce distorted extrapolations and limit understanding of pollinator responses to environmental changes. In contrast to the severe declines experienced in some well-investigated European and North American regions, honeybees seem to have increased in recent years over large expanses of the Mediterranean Basin. Since honeybees can have negative impacts on wild bees, the hypothesis was formulated that if honeybees are actually increasing in the Mediterranean Basin, then an extensive, biome-wide alteration in the composition of bee pollinator assemblages may be currently underway there, involving a progressive reduction in the importance of wild bees as pollinators. This hypothesis was tested using a large data sample gathered from published investigations on the composition of bee pollinators of wild and cultivated plants conducted between 1963-2017 in the Mediterranean Basin. Over this period, honeybee colonies increased exponentially and wild bees were gradually replaced by honeybees in flowers of wild and cultivated plants. Mean estimated proportion of wild bees at flowers roughly quadruplicated that of honeybees at the beginning of the period considered, the proportions of both groups becoming roughly similar fifty years later. The Mediterranean Basin is a world biodiversity hotspot for wild bees and wild bee-pollinated plants, and the ubiquitous rise of honeybees to dominance as pollinators could in the long run undermine the diversity of plants and wild bees, as well as their mutualistic relationships in the region.