Winners and losers at enhanced urban roadsides: Trait-based structuring of wild bee communities at local and landscape scale

Pervasive urbanization contributes to biodiversity declines globally, and with urbanization, road densities increase, amplifying habitat degradation and landscape homogenization. However, as a major part of urban green space, roadside vegetation permeates the urban fabric and, if enhanced, can be used to support insects, such as wild bees. To analyze local and landscape-scale effects of enhanced urban roadsides on wild bee communities and to identify bee traits affected by urban filtering, we established 78 wildflower patches with a native seed mixture along five major roads in Munich (S Germany). Species-poor roadsides with regular management were used as control. During a three-year experiment, we sampled wild bees with pan traps, monitored roadside vegetation, and analyzed landscape diversity and perimeter-area ratio of green space within a radius of 500 m. We gathered information on wild bee morphological, behavioral, phenological, and foraging traits and calculated their abundance, species richness and functional dispersion. Wild bee abundance, richness and functional dispersion increased with vegetation diversity. Functional dispersion was positively correlated with landscape diversity and distance from the city center, indicating trait-based urban filtering. A fourth corner analysis revealed that small, short-lived, and univoltine bees were disadvantaged at urban roadsides. While univoltine bees were discriminated in areas of high landscape diversity, large bees prevailed in the city outskirts, and solitary bees were associated with green space edges. We conclude that enhanced roadsides contribute to urban wild bee diversity and that identifying functional groups affected by urbanization can help to draw recommendations for effective conservation.