Patch-level floral diversity and structural complexity drive differences in bumblebee richness and composition across different urban green space types

Abstract Habitat loss and land-use intensification threaten pollinator populations globally. Urban ecosystems can provide significant floral resources and are increasingly seen as a refuge for key pollinator groups such as bumblebees ( Bombus spp.). However, the associations between bumblebee assemblages and characteristics of urban green infrastructure (GI) are still poorly understood. Urban green spaces are diverse in character and reflect a variety of planning and management options. Understanding the ability of different types of urban GI to support bumblebee populations is needed to assist urban planning in the delivery of nature recovery and ecosystem services. The characteristics that determine the degree to which different GI types promote bumblebee abundance and richness is of even greater importance. We sampled bumblebee assemblages across a typology of urban GI (parks, cemeteries, and allotment gardens), capturing a range of floristic and structural attributes. Controlling for surrounding neighbourhood greenness, we found significant differences between GI types in terms of bumblebee richness. Allotments and cemeteries exhibited comparable mean species richness (4.87 ± 0.39 and 4.03 ± 0.39, respectively), significantly higher than that supported by parks (2.27 ± 0.43). Our study revealed non-linear associations between bumblebee species richness and site floristic diversity with evenness across vegetation height classes emerging as a driver of bumblebee diversity. At the community-level, bumblebee species were clearly differentiated by their unique responses to plant richness and structural complexity. The findings of the study underline the merits of providing small pockets of floristically and structurally complex GI for sustaining urban bumblebee populations. These aspects should be prioritised in the design of urban GI.