DOI: 10.5061/dryad.jdfn2z3b0
title: Wild bees and hoverflies respond differently to urbanisation, human population density and urban form

description: While urbanisation contributes to global biodiversity declines,
 flower-rich urban habitats may provide beneficial pollinator habitats. We
 investigated the potential of urban residential areas to contribute to
 pollinator diversity by analysing wild bee and hoverfly species richness
 and composition of species assemblages of summer-active species, sampled
 in 53 gardens across urban and rural landscapes of Malmö, the regional
 capital of Sweden’s southernmost county. Species richness differed between
 urban and rural gardens, and between four urban residential types (ranging
 from low human density and high vegetation cover, to high human density
 and low vegetation cover), and taxonomic groups responded differently.
 Solitary bee species richness was higher in urban than rural gardens,
 driven by a higher richness in low-density urban gardens compared to both
 high-density urban gardens and rural gardens. In contrast, bumblebee
 species richness was higher in rural than urban gardens, whereas
 differences among the urban types were less clear. Hoverfly species
 richness was consistently higher in rural gardens than any urban garden
 type. Species richness of all groups was negatively related to human
 population density at the landscape scale (radius 500 m), but unrelated to
 vegetation cover. This indicates that population density affects
 pollinator habitat quality through associated green space management and
 design. Rural and urban wild bee species assemblages consisted of
 different species (significant species turnover), whereas urban hoverfly
 assemblages were a subset of rural ones (significant nestedness). Species
 nestedness of hoverflies, but not bees, increased with human population
 density. We show that urban areas can complement the regional wild bee
 species pool, mainly caused by large variation in tenure and management at
 small spatial scales, while urbanisation drives a systematic loss of
 hoverfly species. We suggest alternatives to improve dense residential
 areas for pollinators.
description: We surveyed pollinators in residential gardens of neighbourhoods
 that differed qualitatively in morphological type relevant to urban
 planning, as well as quantitatively in vegetation cover and human
 population density at spatial scales relevant for pollinators. We focused
 on wild bees (Apoidea) and hoverflies (Syrphidae) but also included
 soldier flies (Stratiomyidae), because although often not treated as
 pollinators they have a similar life history to hoverflies.
 We surveyed pollinators using pan-traps for
 30 days during summer 2017, from 2nd July until 2nd August. Three
 pan-traps (one yellow, one blue, one white) 15 cm in diameter and 5.5 cm
 deep, sprayed with UV-fluorescent paint (Sparvar Leuchtfarbe), were placed
 together on a black plastic tray in each garden in connection to
 vegetation typical of each garden, clearly visible and without shade or
 with only light shade for part of the day. Each bowl was half-filled with
 50% propylene glycol and a drop of non-scented detergent. Traps were left
 in gardens, and contained liquid, for the whole period and emptied every
 10 days (i.e. three times). Insects were preserved in ethanol until pinned
 and identified to species by entomologists at the Biological Museum, Lund
 University. We pooled data per pollinator taxon (bumblebees, solitary bees
 and hoverflies) at the individual garden level (three pan-traps) and
 calculated species richness, giving us three data points per garden. One
 urban site (multifamily open yard) was dropped from analyses because
 pan-traps were tampered with, leading to reduced sampling effort,
 resulting in 39 urban gardens. Two rural gardens were not surveyed for
 flowers because of time constraints, and were thus not included in models
 where flower data was a predictor, resulting in 12 rural gardens in those
 models. We surveyed plants in each
 garden twice during the study period. Each time we surveyed
 50 m<sup>2</sup> each of lawn/grass and flowerbeds/ornamental
 shrubs, depending on the characteristics of each garden. Hence, in total
 100 m<sup>2</sup> per garden was surveyed. However, in 12
 urban gardens less than 50 m<sup>2</sup> of either habitat was
 available, and for these we surveyed the entire habitat whilst assessing
 the total area surveyed to be able to control for area in analyses. We
 noted all flowering entomophilous plant species and estimated the total
 number of “flower units”, i.e. an easily countable structure that requires
 pollinators to fly between units.
description: Land use gradients (proportion vegetation) and human population
 per sealed surface are only available for rural sites. Hoverflies include
 both Syrphidae and Stratiomyidae (soldierflies), as they are both flower
 visiors with similar life histories.