Floral resources of an invasive shrub alter native bee communities at different vertical strata in forest-edge habitat

Abstract

Disturbances associated with intensive agriculture facilitate the spread of invasive plants that become dominant along habitat edges in fragmented landscapes. Many invasive woody plants offer large quantities of floral resources to bees, but little is known about how invasive plants affect the use of flowering resources in the forest canopy and understory by native bee communities. We sampled the bee community at vertical strata along forest-agriculture edges that varied in density of a dominant invasive shrub Lonicera maackii before, during, and after the flowering period. We also recorded diameters and species identities of woody stems. Bee and woody plant abundance, diversity, and life-history traits were then spatially and temporally compared in response to L. maackii density. Overall, we found that L. maackii structured bee communities through its floral resources by altering bee species composition and supporting greater abundances and species richness of bees during and after its flowering period. We also demonstrated a diverse and abundant bee community up to 16 m high in the forest canopy that is supported by floral resources of native shrubs and trees but sensitive to woody shrub invasion. These findings collectively suggest that this invasive shrub structures the bee community in favor of species that use its own flowers and competes with co-flowering woody species at different vertical strata. Our study emphasizes the importance of forest trees and shrubs for bees in agricultural landscapes and demonstrates potential risks for early-season bees as well as those that cannot use floral resources of this invasive shrub.