City climate and landscape structure shape pollinators, nectar and transported pollen along a gradient of urbanization

Abstract

Urbanization gradients influence both landscape and climate and provide opportunity for understanding how species, especially plants and pollinators, respond to artificially driven environmental transitions. Here, we investigated several aspects of pollination along an urbanization gradient in landscape and climate. We quantified wild hoverfly and bee abundances with trapping, standing crop of nectar with spectrophotometer, and the pollen transported by flower-visitors with DNA-metabarcoding, in 40 independent sites from seminatural to built-up areas in Northern Italy. Linear and nonlinear relationships were detected along the urbanization gradient. Pollinator abundances increased until 22% of impervious surface, dropping by 34% after that, and it also decreased with green patch distance and urban park size. Thus, suburban landscapes host more pollinators than highly cemented or seminatural areas. Moreover, pollinators diminished by up to 45% in areas with low temperature seasonality: urbanized areas likely posing thermic stress. Furthermore, the sugar mass available in nectar increased by 91% with impervious cover, indicating that city nectars were less consumed or flowers more productive. Furthermore, the species richness of pollen decreased by 32% in highly urbanized areas, and with a high incidence of exotic plants, hinting for anthropized plant communities. Synthesis and applications. Urbanization influences pollinator abundances, nectar resources and transported pollen. Pollinators are negatively affected by a thermally harsh climate in highly urbanized areas with isolated green areas and large parks. Suburban landscapes demonstrated the highest pollinator presence. In the city core, flowers contained more nectary sugar, while pollinators collected pollen from a small number of plants, mainly exotic. These findings highlight the influence of urban landscape structure and climate on pollinators and plants, showing that cities are heterogenous realities. Patterns from this study will serve as basis for pollinator-friendly planning and management of urban landscapes.