Herbivore pressure in the common garden
We conducted a common garden experiment at Henan University, Kaifeng, China in 2020 (34°30’ N, 114°10’ E). We selected the 12 most common non-native herbaceous plant species in the field survey and compared herbivory on these species and 12 co-planted native congeners (Table S4). Specialist herbivores were absent in our system. Thus, these species provided an explicit test of how non-native plant species cope with generalist herbivores (see Müller-Schärer et al., 2004; Joshi & Vrieling, 2005).
In the field survey, we found no significant effect of site on leaf damage for most non-native species or native congeners (Table S2 and S3). Therefore, for controlled experiments we collected seeds from 2 to 3 individuals of each non-native species and their native congeners at each site and mixed them. We sowed seeds in trays (20×30 cm) and placed them in the greenhouse. Similar sized seedlings of all species (about 5 cm in height) were used for the following experiments.
We removed standing vegetation by hand from an agricultural field at Henan University and established 168 1×1 m plots grouped into 6 blocks. Plots within each block were separated by 1.5 m and blocks were separated by 3 m. Then, the 28 plots within each block were randomly assigned to one of 14 pairs of non-native species and their native congeners. We transplanted four seedlings into each plot spaced 25 cm apart. All plants were exposed to natural herbivores.
Three months after planting, we recorded herbivore abundance and measured herbivore biomass as described above and also evaluated the percentage of damaged leaf area instead of the percentage of damaged leaves for all plants. We clipped 20 leaves from each plant and calculated damaged leaf area and total leaf area using Image Proexpress V.6.0 (Media Cybernetics, Inc., Bethesda, MD, USA). We used the average of herbivore biomass and percentage of damaged leaf area of the four plants in each plot for analysis.