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).