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).