Where is the UK's pollinator biodiversity? The importance of urban areas for flower-visiting insects

Abstract

Insect pollinators provide a crucial ecosystem service, but are under threat. Urban areas could be important for pollinators, though their value relative to other habitats is poorly known. We compared pollinator communities using quantified flower-visitation networks in 36 sites (each 1 km(2)) in three landscapes: urban, farmland and nature reserves. Overall, flower-visitor abundance and species richness did not differ significantly between the three landscape types. Bee abundance did not differ between landscapes, but bee species richness was higher in urban areas than farmland. Hoverfly abundance was higher in farmland and nature reserves than urban sites, but species richness did not differ significantly. While urban pollinator assemblages were more homogeneous across space than those in farmland or nature reserves, there was no significant difference in the numbers of rarer species between the three landscapes. Network-level specialization was higher in farmland than urban sites. Relative to other habitats, urban visitors foraged from a greater number of plant species (higher generality) but also visited a lower proportion of available plant species (higher specialization), both possibly driven by higher urban plant richness. Urban areas are growing, and improving their value for pollinators should be part of any national strategy to conserve and restore pollinators.

Keywords: networks; pollinators; urban.

Figure 1.

Mean (a–c) flower-visitor abundance and (d–f) visitor species richness per month per site ± 1 s.e. across the 12 cities for the three landscape types (urban, farmland and nature reserves). Landscape types significantly different from one another are indicated by different letters. Marginal (adjusted) means from the GLMMs, back-transformed to the original scale, are plotted, with standard errors based on the posterior distributions of the regression coefficients using a simulation approach implemented with the R package arm [43]. Results are shown for (a,d) all visitors combined, (b,e) bees and (c,f) hoverflies. Full GLMM results for all taxa are given in table 1.

Figure 2.

Numbers of rare, intermediate and common visitor taxa found in (a) the whole dataset and (b) individual sites. Urban sites are shown in dark grey, farmland sites in light grey and nature reserves in white.

Figure 3.

Mean site-level values ± 1 s.e. for (a) visitor generality, (b) plant generality, (c) visitor specialization (d′), (d) plant specialization (d′), (e) network specialization (H2′), (f) flowering plant richness, (g) native flowering plant richness, (h) non-native flowering plant richness, (i) total flower visits, (j) native flower visits and (k) non-native flower visits. Landscape types significantly different from one another are indicated by different letters. Full GLMM results are given in electronic supplementary material, appendix S7. Marginal (adjusted) means from the GLMMs, back-transformed to the original scale, are plotted and standard errors based on the posterior distributions of the regression coefficients using a simulation approach implemented with the R package arm [43].