Impacts of urban agriculture on pollinator communities: a case study in Bologna (Italy)

Insects are responsible for pollinating most wild flora and food crops worldwide along with contributing to many ecosystem services. However, global declines in pollinator populations jeopardize providing these ecosystem services in both managed and natural ecosystems. Urban agricultural sites are a growing component in cities and harness the capacity to increase the sustainability of cities by serving as a source of biodiversity conservation, reducing their ecological footprint and improving the general quality of life. However, the amount that urban agriculture contributes to biodiversity conservation is still poorly understood and requires elaboration. In this study, wild bees (Hymenoptera: Apoidea) and hoverflies (Diptera: Syrphidae) were surveyed across a newly established urban farm in Bologna, Italy, over four years (namely two years before and two years after the establishment of the farm), to test whether the introduction of complex vegetation positively impacted local pollinator communities. Sampling was conducted via observational plots, to construct plant-pollinator networks, and pan traps, to acquire a broader idea of the diversity of pollinators at the farm. The structures of plant-pollinator networks were explored to understand the interactions between trophic levels and the ramifications of pollinator declines. The results underline the necessity for greater focus on non-bee pollinators, in this instance, syrphid flies, as providers of pollination services, especially in urban settings where bee declines are drastic. Further, slight differences in the community composition of pollinators were detected before and after urban farm establishment, indicating that urban farms could be a green infrastructure that facilitates pollinator conservation. As urban agriculture fosters noteworthy environmental, economic and social value, it is integral to understand the ecosystem services that fundamentally sustain it and determine which conservation strategies to adopt to provide suitable areas of habitat and refuge for pollinators across cities.