Rethinking urbanization effects on pollinators: a comprehensive review

Source abstract

Abstract Urbanization exposes biodiversity to multiple, spatially heterogeneous environmental pressures, and pollinators provide informative models for evaluating these effects. We conducted a bibliometric and systematic review of 200 studies published between 2006 and 2024 to examine how urbanization affects pollinators, the response patterns reported, and persistent knowledge gaps. To provide a structured synthesis, we classified 328 environmental predictors into five non-exclusive, mechanistically informed dimensions—habitat loss and fragmentation (D1), resource modification (D2), thermal and abiotic stress (D3), biotic pressure (D4), and chemical contamination (D5)—and examined whether the consistency of reported responses differed among dimensions across 2,233 predictor–response relationships. D1 comprised 66.6% of all variables and generated the largest number of relationships; its high conflict rate (34.0%) was associated with the inclusion of both built-area and natural-area predictor subtypes. D3 exhibited the highest predictor-level conflict rate (60.0%), whereas D4 showed the highest proportion of negative effects (38.7%), a pattern concentrated in studies using honeybee abundance or managed-bee pressure as predictors. Among 19 studies evaluating pollination services, 52.9% reported divergent directions between service and diversity outcomes; Apis/Bombus -dominated studies showed 75% positive service outcomes, compared with 67% negative outcomes in studies without managed-bee dominance. The entire evidence base on urban pollination services originated from non-tropical cities. We organize these findings into a four-axis heuristic framework and three testable hypotheses, providing a structured basis for future empirical and quantitative evaluation of the sources of heterogeneity.