Invasive grass and litter accumulation constrain bee and plant diversity in altered grasslands

Ecologists consider biological invasions one of the primary drivers of global change. Many remaining grasslands in North America have extensive invasions of exotic grass species that have replaced native plant species. In the Northern Great Plains, exotic cool-season grasses Kentucky bluegrass (Poa pratensis) and smooth brome (Bromus inermis), paired with human alterations to the landscape and historical disturbance regimes, have resulted in functionally and structurally altered grassland plant communities. These changes may extend to impact species that rely on these plant communities, such as bees. Bees are ecologically diverse and serve important polli-nator roles but are at risk from the loss and change of floral and nesting resources in plant communities. Our objectives were to determine whether Kentucky bluegrass and smooth brome alter the bee and forb species richness in invaded Northern Great Plains grasslands and how litter accumulation, grass cover, the amount of bare ground, and forb species richness interact with bee functional traits. To do this, we surveyed 67 grassland sites from 2017 to 2020 with two bee-sampling methods (268 netting and bee bowl surveys total) and vegetation cover surveys at each site. We collected 20,559 bees from 201 bee species and observed 249 forb and shrub species in vegetation surveys. Bee richness and Shannon diversity were associated with greater forb richness while forb richness was significantly lower with thicker litter depths and higher with a greater coverage of all grasses other than Kentucky bluegrass and smooth brome. Bee trait ana-lyses showed varying relationships with plant community variables. Of these, litter depth and Kentucky bluegrass cover were associated with lower ground-nesting bee abundance while small-bodied bee abundance was positively associated with increasing bare ground. While our results support the close relationship between bee and plant diversity, we also found litter depth, in particular, contributed to the structure of these two communities. Specifically, Kentucky blue-grass and smooth brome are two exotic grass species associated with thatch-forming litter layers, especially under idle management that appear to simplify bee and forb communities. Our results make apparent the importance in maintaining structural and compositional diversity in invaded grasslands to support diverse bee communities.

Automated summary

Ecologists consider biological invasions one of the primary drivers of global change. Invasive grass species have replaced native plants in many grasslands of North America, impacting bee and forb species richness. The presence of Kentucky bluegrass and smooth brome, along with human alterations to the landscape, has resulted in alterations to grassland plant communities, affecting diverse bee communities. Litter accumulation, grass cover, bare ground, and forb species richness interact with bee functional traits, emphasizing the importance of maintaining structural and compositional diversity in invaded grasslands to support diverse bee communities.