Impacts of an invasive brome grass across multiple trophic levels of rangelands

Morgan Frost, Biology, University of North Carolina at Greensboro, Greensboro, NC and Sally E. Koerner, Department of Biology, University of North Carolina Greensboro, Greensboro, NC

Background/Question/Methods
Invasive species harm biodiversity and ecosystem services, threatening the integrity of native ecosystems. Mixed grass prairies, utilized as working rangelands, are susceptible to damage from invasive plants. Of particular concern in rangelands are invasive annual brome grasses, as they are widespread, able to outcompete native species, and are poor-quality forage for livestock. Currently, however, the effects of invasive brome grasses on other trophic levels, such as soil microbes and insects, are not well understood, particularly in mixed grass prairies. In this multi-year field experiment, we assessed how Bromus arvensis, a well-established invasive brome grass across US rangelands, changes the native plant, insect, and soil microbial communities across gradients of invasion in Fort Keogh Livestock and Range Research Laboratory in Miles City, Montana. To do this, we established 8 invasion gradients with three invasion levels – low, moderate, and high. Each gradient has 6, 4m2 plots, with two at each invasion level. All pretreatment sampling occurred in summer 2020. In October 2020, we sprayed a pre-emergent herbicide across half of the plots per gradient in order to experimentally remove B. arvensis, with post-treatment sampling scheduled for summer 2021. Here, we present the results of the pre-treatment year, summer 2020.
Results/Conclusions
Non-brome plant community richness significantly decreases with increasing B. arvensis invasion, as does total native species abundance. While these findings are not surprising, the magnitude of native species decline from high to low invasion (~20%) is of concern for forage availability. Interestingly, we found that arthropod abundance and community composition may not be affected by invasion level, compounding the herbivory pressure left on the remaining native plants. Highly invaded areas thus have potentially enhanced insect and cattle pressure as the herbivores are forced to focus on the remaining native plant community, heightening insect-cattle competition for forage. These findings suggest that highly invaded rangelands may need to be managed in different ways than their native rangeland counterparts and that while insect herbivory in native rangelands may be negligible, increased invasion may shift this type of herbivory to higher intensities.