RESULTS
Pollinators at urban sites carried more conspecific pollen than
pollinators at natural sites (Est. = 0.33; P = 0.02; Figure 1).
Relative to natural sites, individual pollinators at urban sites carried
pollen from fewer plant species (Est. = -0.52; P = 0.027), and
the species diversity of pollen on individual pollinators was lower in
urban sites (Est. = -0.17; P = 0.03). At the site level, urban
and natural sites did not differ in the total number of plant species
from which pollen was found on pollinators (t = -0.71, df = 8.1,P = 0.5), the number of plant species on which pollinators were
caught (t = 0.55, df = 6.2, P = 0.61), or plant species richness
(t = 0.87, df = 6.8, P = 0.42), diversity (t = 1.3, df = 7.2,P = 0.24); or evenness (t = 1.1, df = 36, P = 0.29).
Across sites, pollinators exhibited negative frequency dependent
foraging, with plant relative abundance predicting pollinator preference
for a given plant species (Est = -0.34, P = 0.001). Across sites there
was an inverse relationship between conspecific pollen amount and plant
species richness (Est. = -0.057, P = 0.0496), diversity (Est =
-0.38, P = 0.002), and evenness (Est = -0.79; P = 0.013).
Urban sites had lower insect species richness than natural sites (mean
urban = 2.8; mean natural = 7.5; t = -3.5, df = 8.3, P = 0.008).
Urban sites also had lower insect species diversity than natural sites
(t = -3.9, df = 9.0, P = 0.0036). However, the amount of
conspecific pollen was predicted neither by insect species richness
(Est. = -0.029; P = 0.26) nor species diversity (Est = -0.22,P = 0.11).
Floral abundance at the site level was positively associated with the
amount of conspecific pollen (Est = 0.13, P = 0.0014), and the
number of flowers per plant was higher in urban areas (Est. = 1.8,P = 0.046). Urban sites also tended to have a higher total number
of flowers but this trend was not significant (Est. 1.5, P =
0.06). There was substantial variation among sites in the species
richness of invasive plants (Table 1). Urban sites contained fewer
invasive plant individuals (Est. = -1.6, P = 0.028), but
contained more flowers from invasive plants (Est. 1.2, P< 0.001). Furthermore, the increase in the number of flowers
per plant individual in urban sites was greater for invasive plants
(Interaction est. = 1.2, P < 0.001; Figure S2). Pollinators
carried more conspecific pollen when caught on invasive plants in both
types of sites (Est = 0.07, P < 0.001; Figure 2), and
the difference in conspecific pollen amount between urban and natural
sites was lower for invasive plants (Interaction est. = -0.058; P< 0.001).
Common pollinators comprised a greater amount of the pollinator
community at urban sites than at natural sites (Est = 4.7, P< 0.001). Differences between urban and natural sites in the
amount of conspecific pollen carried by pollinators was different for
common and rare pollinators. Common pollinators carried more conspecific
pollen than rare pollinators (Est. = 0.38; P < 0.001),
and the increase in the amount of conspecific pollen carried in urban
environments was greater for common pollinators (Interaction est. =
0.12; P < 0.001; Figure S3).