Body mass decline in a Mediterranean community of solitary bees supports the size shrinking effect of climatic warming

The long-known, widely documented inverse relationship between body size and environmental temperature ("temperature-size rule") has recently led to predictions of body size decline following current climatic warming ("size shrinking effect"). For keystone pollinators such as wild bees, body shrinking in response to warming can have pervasive effects on pollination processes, but there is still little evidence of the phenomenon because adequate tests require controlling for climate-linked confounding factors (e.g., urbanization, land use change). This paper tests the shrinking effect in a diverse community of solitary bees from well-preserved habitats in the core of a large nature reserve undergoing fast climatic warming but not disturbances or habitat changes. Long-term variation in mean body mass was evaluated using data from 1,186 individual bees (108 species, 25 genera, 6 families) sampled over 1990-2022. Climate of the region warmed at a fast rate during this period and changes in bee body mass verified expectations from the size shrinking effect. Mean individual body mass of the regional community of solitary bees declined significantly, shrinking being particularly intense for female individuals and large-bodied species. As a consequence, the pollination and mating systems of many bee-pollinated plants in the region are likely undergoing important alterations.