Pollination in suburban landscapes: Increased bee abundance does not benefit native plants

Background/Question/Methods

While the overall effects of land-use change on bee abundance and diversity appear negative, increasing evidence suggests that bees may benefit from some moderate levels of human development, especially in forested ecosystems. How changes in land-use affect the performance of bee-pollinated plants, and the mechanisms involved, are still relatively unexplored. Pollination may be highly susceptible to ecological perturbations, and variation in the abundance or composition of pollinator communities between plant populations has been shown to affect the degree of pollen limitation of seed set and subsequent plant reproductive success. To determine whether greater bee abundance associated with forests in suburban development translated into increased pollination services, we experimentally manipulated pollination using supplemental hand pollinations for three native, bee-pollinated plants: Gelsemium sempervirens, Oenothera fruticosa, and Campsis radicans. If greater bee abundance in suburban forests results in greater pollination services, we predicted plants growing in suburban forests would exhibit less pollen-limitation of fruit and seed set compared to plants growing in natural forests.

Results/Conclusions

We found that all three flowering species were pollen-limited for some measures of female plant reproduction but that pollen limitation did not vary with suburbanization. Hand pollinations increased fruit set, proportion seed set, and seed set per fruit in Gelsemium; seed set per fruit and proportion seed set per fruit in Oenothera; and fruit set and seed set per fruit in Campsis. Contrary to our prediction, the effect of pollen supplementation did not vary between forest types for any species, suggesting all three were equally pollen limited in suburban and natural forests. These results raise potential concerns, as they suggest that increased bee abundance may not translate into increased pollination services to native plants growing in suburban forest remnants. Continuing research on the role of alternate floral resources and floral herbivores will provide mechanistic insight into how factors other than pollinator abundance affect pollination and reproduction of native plants growing in suburban and natural forests.