Urbanisation drives biodiversity loss, fitness decline and community shifts in cavity-nesting Hymenoptera

Urbanisation is reshaping landscapes worldwide, posing challenges to biodiversity and ecosystem function. Cavity-nesting Hymenoptera, including bees and wasps, deliver vital ecological services but are increasingly vulnerable to urbanisation. In this citizen science-based study, we deployed 286 trap nests across Leipzig and Hamburg (Germany) to investigate how urbanisation and landscape-scale environmental features influence the diversity, community composition, reproductive success, offspring survival and demographic status of these insects. Using Bayesian hierarchical and phylogenetically informed modelling, we found that urbanisation significantly reduced species richness, reproductive output and offspring survival in both guilds, despite stable nesting activity, suggesting potential ecological trap effects. For bees, urbanisation also reduced the probability of nests functioning as demographic sources. Environmental drivers shaped responses in taxon-specific ways: bee richness and survival increased with landscape diversity but declined with fragmentation, while wasp richness and nesting declined with higher temperatures, though survival increased with more green cover. Bee nests were more likely to serve as demographic sources in higher temperatures. Community composition responses also diverged; bee communities shifted via increased species turnover, while wasp assemblages remained relatively stable. Temperature was a major driver of compositional dissimilarity in both groups, whereas landscape diversity was associated with homogenisation in bees and diversification in wasps. Our findings demonstrate that cities filter cavity-nesting Hymenoptera through multiple ecological pathways, highlighting the need for differentiated conservation strategies that consider both habitat structure and taxon-specific responses.