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.