Wild bees and hoverflies respond differently to urbanisation, human population density and urban form

•The value of urban gardens for pollinator diversity differed between taxa.•Wild bees benefitted from gardens in low-density urban, compared to rural, areas.•Hoverfly species richness was consistently higher in rural than urban gardens.•Species richness of all groups was negatively related to human population density.•Wild bees showed turnover of species assemblages, while hoverflies showed nestedness.