Plant and pollinator community effects
To determine if pollinators carry more conspecific pollen at sites with
less heterogeneous plant communities (hypothesis 3), we characterized
plant species richness, diversity, and evenness using custom R scripts
(see data availability statement) and tested if site types differed in
these metrics using t-tests. We tested if plant species richness,
diversity, or evenness predicted the amount of conspecific pollen using
generalized linear mixed models with a Poisson error structure and site
as a random effect. To determine if pollinators carry more conspecific
pollen at sites with more heterogeneous pollinator communities
(hypothesis 4), we tested if insect species richness or diversity
predicted the amount of conspecific pollen grains found on pollinators
using generalized linear mixed models with a Poisson error structure and
site as a random effect. To evaluate if the abundance of floral
resources at a community level facilitates pollinator specialization
(hypothesis 5), we quantified the total number of flowers found along
each 90 m transect. We asked if the total number of flowers predicted
conspecific pollen amount using a generalized linear mixed model with
log-transformed flower number as the independent variable, site as a
random effect, and a Poisson error structure.
Surprisingly, we found the opposite of what we expected regarding
differences between urban and natural sites in the amount of conspecific
pollen carried by pollinators (see results). Therefore, we also explored
possible effects of the identities of individual plant and insect
species on pollinator short-term specialization. We hypothesized that
pollinators carry more conspecific pollen from invasive plants, which we
predict to be in greater abundance in urban areas. To examine if plants’
invasive status predicted the amount of conspecific pollen we determined
if each plant on which a pollinator was caught is invasive in California
using the
California Invasive
Plant Council Dataset , as well as theCalflora
Database. We determined if site types differed in the amount of
invasive plants sampled along transects, using a generalized linear
mixed model a binary dependent variable that reflected if a plant was
invasive or not, site type as the independent variable, site and plant
species as random effects, and a binomial error structure. We tested if
invasive status predicted conspecific pollen amount used invasive status
as a binary independent variable in a generalized linear mixed model
with average conspecific pollen amount within sites as the dependent
variable, site as a random effect, and a Poisson error structure.
To evaluate if differences in conspecific pollen amount were driven by
differences in the identity of the insect species found among sites, we
first determined which insects were common to all or the majority of
sites. Most insect species were found at five or fewer sites, with the
exception of one honey bee and one bumble bee species (Apis
mellifera and Bombus vosnesenskii ), which were found at 10 and
11 sites, respectively. We hypothesized that if rare species were
driving differences among site types, the change between urban and
natural environments in the amount of conspecific pollen carried would
be greater for common species. To test this hypothesis we modeled
conspecific pollen amount as a function of the interaction between
whether a pollinator was common or rare and site type, using a
generalized linear mixed model with site as a random effect and a
Poisson error structure.