How city traits affect taxonomic and functional diversity of urban wild bee communities: insights from a worldwide analysis

Land-use change, including urbanization, is known to affect wild bee (Hymenoptera: Apoidea) diversity. However, while previous studies have focused on differences across local urbanization gradients, to the best of our knowledge, none focused on differences among cities at a wide geographical scale. We here used published data for wild bee communities in 55 cities across the globe, in order to explore how city traits (population density, city size, climate and land-use parameters) affect both taxonomic (diversity, distinctness, dominance) and functional (body size, nesting strategy, sociality, plant host specialization) profile of urban bee communities. By controlling for sample size and sampling effort, we found that bigger cities host few parasitic and oligolectic species, along with more above-ground-nesting bees. Cities with highly fragmented green areas present a lower proportion of oligolectic species and a higher proportion of both social species and large-bodied bees. Cities with more impervious surfaces seem to host a lower proportion of below-ground-nesting bees. Hotter cities present both a lower richness and diversity, with functional diversity highest at intermediate precipitation values. Overall, it seems that high levels of urbanization—through habitat modification and the “heat island” effect—lead to a strong simplification of the functional diversity of wild bee communities in cities. Our results may help explain the previously observed variable response of some bee community traits across local urbanization gradients.