Enhanced urban roadside vegetation increases pollinator abundance whereas landscape characteristics drive pollination

Abstract

Roads cause pollution, habitat degradation and fragmentation, while recent studies report that roadside vegetation can benefit plant and insect diversity. However, the factors that drive effects of roadside vegetation on pollinator abundance and pollination need further attention. Our main interest in this study was to evaluate urban roadside enhancement by identifying direct and indirect effects of local resource availability and landscape characteristics on pollinator abundance and quality of pollination, by using functionally different phytometer plants. Thus, we established 34 species-rich wildflower patches along urban roads, and monitored flowering species and floral density at the local scale. At the landscape scale we measured the amount of impervious surface within a radius of 500 m around the wildflower patches, as well as edge density of all major land-use types, and distance to the city center. As control, we used 34 patches with existing species-poor roadside green. The abundance of pollinators, i.e., honeybees, wild bees, and hoverflies, was sampled with colored pan traps. Pollination was monitored based on fruit number and weight of three phytometer species, i.e., Fragaria × ananassa, Ranunculus acris, and Trifolium pratense, that differ in flower morphology and color, and attract different pollinators. Pollinator abundance was positively related to local richness of flowering plants and floral density. At the landscape scale, pollinator abundance was positively related to the proportion of impervious surface but negatively to edge density. Phytometer-specific effects on pollination were moderated by landscape characteristics: Fruit mass of F. x. ananassa was highest in the inner city, fruit number of R. acris was lower in areas with high edge density, whereas in T. pratense fruit number increased with edge density, and was negatively related with the proportion of impervious surface. There were no indirect effects of local and landscape factors via pollinator abundance on pollination. The study of the three phytometer species revealed that roadside enhancement can directly support fruit production in wild plants and crops in cities. Thereby, different landscape characteristics influenced the quality of fruit set, which also depended on individual plant reproductive adaptations. This stresses the importance of using a set of complementary phytometer species in pollination experiments, especially to gain insights into ecosystem functioning at coarser spatial scales.