Pollinator abundance and richness
Overall, urbanisation had a negative effect on pollinator abundance (d= -0.42; 95% CI = [-0.7, -0.15]; P = 0.003) and richness (d= -0.66; 95% CI = [-0.96, -0.36]; P < 0.001; Fig. 2). The heterogeneity of the effect sizes was large and statistically significant for both abundance (Qt = 1450.11, df = 227, P < 0.001) and richness (Qt = 908.31, df = 146, P < 0.001). The taxonomic identity of the pollinator was not an important moderator of the effects of urbanisation on pollinator abundance (Table 2). Although this moderator was not significant, urbanisation had a significant negative effect on the abundance of Lepidoptera (Fig. 2a). The taxonomic identity of the pollinator was an important moderator of the effects of urbanisation on pollinator richness (Table 2). The richness of Diptera, Hymenoptera, and Lepidoptera was negatively influenced by urbanisation (Fig. 2b). When the Hymenoptera were divided into bumble bees, honey bees and other wild bees, the results were qualitatively similar. We did not detect an effect of urbanisation on bumble bee, honey bee or other wild bee abundance (Fig. S1).
The climatic region (tropical vs. non-tropical) did not explain the heterogeneity of the effects of urbanisation on pollinator diversity (Table 2). However, pollinators studied in non-tropical regions were more severely influenced by urbanisation than in the tropics (Fig. 2a). The origin of the pollinator was not an important moderator of the effects of urbanisation on pollinator abundance (Table 2), however, it was an important moderator of the effects of urbanisation on pollinator richness (Table 2). Urbanisation had a negative effect on native pollinator species richness and a positive effect on non-native pollinator species richness (Fig. 2b).
Flowering plant richness had a positive effect on pollinator richness (F = 5.838, P = 0.019; Fig. 3a, Table S2) and a positive effect, although not statistically significant, on pollinator abundance (F = 3.490, P = 0.065; Fig. 3b, Table S2). Flowering plant abundance did not affect pollinator biodiversity (Fig. S2, Table S2). Furthermore, urbanisation was not found to influence the richness and abundance of flowering plant species (Fig. S3, Table S3).