Field data collection
Field data were collected identically at both sites across all years. During peak growing season each year (June/July), at each plot, we recorded plant species composition by visually estimating foliar cover across the entire 1 m2 area (i.e., calibrated to estimates generated by a 100 pin-point intercept frame) for each species to the nearest percent. We collected insects using a modified vacuum leaf blower according to the Dietrick Vacuum (D-VAC) method (Dietrick et al. 1960) between 11:00am-2:00pm at each sampling point. All insect samples were frozen after collection until processing. We took 3, 10 cm deep soil cores from the outer edge of each plot to assess soil microbial (bacterial and archaeal) taxonomic diversity (in 2019 at Wyoming and 2020 at Montana only) and homogenized all 3 cores in the field before freezing. We collected soil cores between 10:00am – 3:00pm at each sampling point and sterilized the soil probe and researchers’ gloves between plots with 70% isopropyl alcohol to avoid cross contamination.
In 2021 (late June) and 2022 (early June) at Montana only, we collected insect herbivory data according to Herbivory Variability Network protocols (Pearse et al. 2021). To calibrate our visual estimates of herbivory damage, we used the LeafByte application (Getman-Pickering et al. 2020) to ensure our estimates were accurate, and the same researcher collected all herbivory measurements across both sampling periods. In each of the 24 plots, we collected estimates of insect herbivory damage on 12 individuals. Six samples corresponded to invasive bromes (eitherB. arvensis or B. tectorum ) and 6 samples corresponded to abundant native species in the plot (native species were selected based on species composition data for each plot, so exact species selected differed between plots). We estimated both total plant herbivory damage and individual leaf damage on all individual leaves, randomly selecting 10 leaves per individual if the plant had greater than 10 leaves (all sampled individuals had at least 2 leaves).