Indirect relationships between invasive grasses and bee communities in the Northern Great Plains

Disruptions to natural fire and grazing processes, paired with other human activities, have resulted in prevalent Kentucky bluegrass (KBG; Poa pratensis) and smooth brome (SB; Bromus inermis) invasions across the Northern Great Plains. Cool-season invasive grasses serve as unstable forage for cattle and can decrease native plant richness, potentially affecting bee communities relying on floral resource availability. Past investigations into the influence of invasive plants on bee communities has been restricted to invasive forbs. We sampled bees and estimated canopy cover of KBG, SB, and individual forb species at 38 grassland sites in North Dakota and tested generalized linear models of invasive grass cover predicting forb and bee diversity measures. Smooth brome had a significant negative relationship with both forb and bee richness and diversity (p<.05). Kentucky bluegrass had a positive relationship with forb richness (p=.035) and notable negative relationship with bee abundance (p=.056). Other grass cover had a significant positive relationship on forb richness (p=.0001) and diversity (p=.038). Nonmetric multidimensional scaling ordination analysis results displayed relationships of grass and forb measures on bee communities defined by taxonomic genera and functional traits. Invasive grasses are associated with honeybee and bumblebee components of the bee community. Our results show invasive grasses reduce bee abundance, richness, and diversity depending on species and may create an environment suitable for particular subsets of the bee community. These novel investigations further our understanding of how invasive grasses impact the ecological services provided by bees and support the need to manage invasive grasses.