Potential for conspecific pollen transfer
We examined the potential for conspecific pollen transfer as follows. For each pollinator, we counted the number of pollen grains that were morphologically identical to grains obtained from the anther slide on which the pollinator was caught (hereafter ‘conspecific pollen grains’), from a subsample of 500 pollen grains. To do so, we began examining each slide at the upper left corner, and we recorded the species of each grain until either 500 grains were encountered (N = 355 slides) or all the pollen on the slide had been counted (N = 158 slides; Table S1). We asked how different our estimate of the proportion of conspecific grains from a subsample of 500 might be from the proportion calculated using all grains. To do so, we randomly selected 10 pollinator slides that had more than 500 total grains and counted and identified all grains (59,189 grains). The correlation of the proportion of conspecific grains estimated from the subsample of 500 grains and that estimated from all grains was 0.87 (cor.test function in R; P < 0.001).
We examined if pollinators from natural sites carried a greater amount of pollen grains that were conspecific (hypothesis 1) using generalized linear mixed models with a Poisson error structure implemented using the lme4 package (Bates, 2014) in R (R Core Team, 2013). The number of conspecific grains was the dependent variable, site type (urban or natural) was the independent variable, and sampling site was a random effect. We determined if urban and natural sites differed in species diversity or richness of pollen on pollinators using linear mixed models with site type as the independent variable, and sampling site as a random effect. We used likelihood ratio tests (LR tests) to evaluate the significance of the independent variable of interest by comparing nested models with and without that factor. In the results section we report estimates from the best model chosen via backward model selection (hereafter “Est.”), and the P-values from likelihood ratio tests comparing nested models.