City layers: Vertical stratification of wild bees and the structure of urban ecological resilience

Urban environments can support diverse pollinator communities, yet their vertical stratification remains underexplored. This study examines how bee abundance, species richness, and functional diversity vary across ground, mid-story, and canopy levels in a temperate city.
We sampled wild bees at three heights and assessed how ecological traits and environmental factors, including impervious surface area, green space, and tree cover, influence vertical patterns. Bee abundance and richness varied across strata, with peak abundance in the canopy and higher species richness at both ground and canopy levels. Impervious surfaces did not directly reduce overall abundance and richness, nor did they eliminate bees; instead, they appeared to alter vertical patterns with activity tending to shift upward. Vertical space use was shaped by species traits, particularly body size, nesting habits, and social strategies. Strong interactions between height and land cover further influenced species composition and functional diversity along the vertical gradient.
Recognizing this vertical structuring offers new insights for biodiversity-focused city design. Our findings show that pollinator communities respond to vertical stratification, shaped by both traits and urban landscape features. This has direct implications for multi-layered greening strategies, such as tree canopy enhancement, rooftop habitats, and stratified plantings, that can improve habitat quality and ecological function in dense urban settings. The study contributes to growing evidence that vertical green infrastructure can support pollinator resilience and inform sustainable planning under intensifying land-use and climate pressures.