Pollination services are critical for many wild and crop plants; approximately 35% of all crop production is dependent on animal mediated pollination. Many wild bee species are declining rapidly across North America and Europe. Causes of this decline are thought to be a consequence of agricultural intensification and urbanization. Urbanization has created fragmented habitats with green spaces interspersed between impervious surfaces. In this study we assess the reproductive success and population growth rate of bumblebees in a natural to urban gradient to understand the impact of urbanization. We placed experimental nests in ten sites ranging from a natural to urban gradient (0% - 99% degree of urbanization); all nests were placed in urban community gardens except the one in the natural site.
Reproductive success and colony size were positively correlated with cumulative weight gain of the nests (p<0.05). Urbanization did not have an effect on the population growth rate of the nests as well as on forager activity (p>0.05). Population growth rate was also positively associated with the number of foragers entering the nest (p<0.05). Abundance of wax moth larvae strongly affected the growth rate negatively (p<0.01) and parasite diversity was found to be positively correlated with growth rate (p<0.05) Along with resource availability in the surrounding landscape, effects of parasitism also seem to have a large impact on the growth rate of the bumblebee colonies and both these drivers are likely responsible for pollinator survival and reproductive success regardless of the degree of urbanization. We argue that urbanization need not have a negative impact on the growth rate and reproductive success of bumblebee colonies and that urban community gardens may have the potential to provide suitable habitats for bees in highly urban landscapes.