Receptiveness of Foraging Wild Bees to Exotic Landscape Elements

Wild pollinators provide important services in both wild and human-dominated ecosystems, yet this group may be threatened by widespread anthropogenic landscape change. We explored the responses of wild bees to exotic floral species and novel habitat in a fragmented, suburban landscape, using pollen grain identification. Pollen loads from bee specimens collected in 13 suburban grassland fragments in Denver, Colorado were sampled and compared with a pollen reference collection. Averaged across two seasonal sampling rounds, 45% of the bee-borne pollen grains were identified to the species level. Wild bees in this system were very receptive to using alien plants for pollen foraging, at least 45% of pollen sampled from bee specimens consisted of non-native pollen grains. During peak flowering in early summer, bees obtained at least 32% of their pollen resources from within-fragment sources and at least 7.5% from surrounding suburban residential yards. In midsummer, within-fragment sources represented 58% of pollen sampled while yards dropped to 1.5%. These bees appear to be more accepting of exotic floral species than of exotic habitat types (yards). The advantages and disadvantages of pollen load analysis for movement studies are discussed.