From pollen provision to pollinator: Species-specific sterol assimilation by wild bees in urban landscapes

Abstract

Human-driven landscape change, particularly urbanization, is reshaping pollinator communities, yet the functional traits that mediate species persistence remain poorly understood. Dietary specialization is commonly used to predict species vulnerability. However, conventional measures based on taxonomic plant host breadth may not reflect pollinators' actual nutritional needs, especially in cities where exotic and native plants coexist. Because insects cannot synthesize sterols de novo, they must obtain these essential lipids from their diet, making sterol requirements a critical and overlooked axis for understanding pollinator persistence. Here, we analysed 22 sterols in 274 pollen provision samples and 1267 bee individuals from four common wild bee species, ranging from specialists to generalists, sampled following urban intensity gradients across five European cities. We reveal that regardless of host–plant specialization levels, all species relied on a shared set of four abundant sterols, commonly found in plant pollen, suggesting that sterol generalization may contribute to their urban persistence. Nonetheless, the full sterol profiles, both composition and concentration, varied between the bees' bodies and their pollen provisions, as well as across species and cities. This suggests that wild bees do not simply mirror their diet, but rather assimilate sterols in a species-specific manner. The most specialized bee, Chelostoma florisomne, showed more uniform sterol use and was often absent in highly urbanized areas, possibly due to frequent mowing of its main host, Ranunculus flowers. These findings underscore the limitations of using solely on plant host composition in pollen for predicting pollinator responses to urban intensity and highlight the importance of nutritional needs, such as sterols, in shaping ecological niches and guiding conservation strategies. Read the free Plain Language Summary for this article on the Journal blog.