Increasing demand for cropland derived and herbaceous bioenergy production is expected to alter future agricultural landscapes and has the potential to impact native pollinator communities. However, baseline data on pollinator communities is lacking in many intensive agricultural regions. To determine how increased bioenergy production could affect native bee communities we surveyed bees across 32 focal grassland sites in the most agriculturally intensive region of northeastern Colorado. We sampled bees using a combination of bee bowls, vane traps, and netting in 12 actively grazed rangelands and 20 Conservation Reserve Program (CRP) grasslands that spanned five counties expected to be impacted by future bioenergy production. We estimated local floral resources at each site (forb richness and cover) and also extracted remotely sensed land-cover data at multiple scales around each site. We explored both local and regional factors driving bee abundance and species richness using linear modeling and used stepwise AIC model comparison to identify the best fit model.
Results/Conclusions
Overall, CRP plantings supported 30% more bees and 24% more bee species on average than rangeland sites. Site type (rangeland vs CRP) was the largest factor driving variation in bee communities. In addition, the amount of roadside edge habitat within 1,600 m was negatively related to both bee abundance and richness; although, the response of the bee community to landscape factors was guild specific, depending on bee body size and the scale at which cropland and edge habitat were measured. Local forb species richness was also nearly twice as high in rangeland sites and positively related to bee abundance and richness. However, given that the largest differences in CRP and rangeland sites was likely a reduced biomass associated with grazing, the loss of floral resources through consumption by grazers across a landscape is likely related to reduced bee abundance and richness. Repeated reductions of plant biomass in perennial bioenergy cropping systems (due to harvest) could therefore have impacts on bee communities similar to intensive grazing. Moreover, increased edge habitat associated with the development of roads and more intensely cropped agroecosystems could negatively impact certain guilds of bees. Ultimately, these results suggest that mitigating the impacts of future agricultural intensification should continue to include protecting habitat through conservation programs and moderating the intensity of harvest in herbaceous agricultural systems.