Effects of suburbanization on bee abundance, diversity, and plant-pollinator interactions

Background/Question/Methods

Land-use change is a dominant factor driving species distributions, abundances, and diversity. While recent research has sought to document how land-use change impacts native pollinators, little work has studied how changes in pollinator abundance or diversity alter the nature or outcome of plant-pollinator interactions.  We investigated the effects of one type of land-use change, suburbanization, on the abundance and diversity of bees in and around the Raleigh/Durham area of North Carolina.  In addition, we conducted an experiment to determine how changes in bee fauna associated with suburban development affect the degree of pollen-limitation of seed set in a native flowering plant, Gelsemium sempervirens.  We established five paired sites in naturally occurring forest (forested) and suburban forest (suburban).  Over two years, we sampled for bees using a combination of bee bowls and hand-netting, and in the second year, we experimentally manipulated pollination using supplemental pollen to test for pollen-limitation of seed set in one site pair.

Results/Conclusions

We found significantly higher bee abundance in suburban sites, more than double that of forested sites. However, we found no significant difference in rarefied bee species richness between suburban and forested sites. Supplemental pollen addition in one site pair resulted in no significant increase in proportion seed set for Gelsemium in suburban forest, while Gelsemium in natural forest set significantly more seed per fruit when provided supplemental pollen.  Taken together, these data suggest that suburban areas harbor significantly greater numbers of bee pollinators than contiguous forests, and that increased pollinator abundance in suburban forests may reduce the degree of pollen limitation of seed set in plants compared to naturally forested sites. Further investigation into the generality of this result and the mechanisms driving it will lead to a better understanding of the ecological impacts of urbanization on the nature and outcome of plant-insect interactions.