Field data collection
To study variation in the plant–insect trophic network with elevation,
we established six elevation transects that covered the diversity of
environmental conditions of the Swiss Alps, differing in local climate
and bedrock – i.e. in the areas of Bex, Calanda, Faido, Grindelwald,
Martigny and Salgesch (Supporting information, Appendix S1, Fig. S1).
Each transect was divided into eight sites, spanning elevations from 578
to 2417 m a.s.l., located on average 240 m of elevation apart from each
other. Sites were chosen to be open grasslands with a limited impact
from anthropogenic activities. At each site, we defined a 10 m x 10 m
survey plot representative of the homogeneous composition of the
surrounding vegetation. Orthopteran surveys were conducted under sunny
weather conditions during the summer at insect peak activity times. We
focused on Caelifera and Ensifera suborders that are known to feed on
living plant material (Baur et al. 2006). We identified
orthoptera by visual inspection, caught on average 10 individuals per
species, kept them in falcon tubes for c. 2 h, for collecting fecal
excretions, before releasing them again all at once. We performed the
vegetation surveys in a 9 m2 circular plot located in
the most homogeneous zone of the 100 m2 plot and
searched for additional rare species within the 100 m2plot. We used temperature as the main environmental variable that
changed along the elevation gradient (Appendix S1, Fig. S2). Soil
temperature data were collected for half of the sites per transect using
temperature loggers (DS1921G-F5 HomeChip, Newton Longville, England)
that were parameterized at a 0.5°C resolution with a sampling rate of
240 minutes, wrapped in parafilm, protected by a silicone capsule and
buried 4 cm deep in the ground at each site (from October 2017 to
October 2018). Summer soil temperature was extrapolated for unmonitored
sites by linear regression (Appendix S1, Fig. S3). To study the keystone
plant species, we measured plant functional traits that related to
physical resistance or nutrient content: specific leaf area (SLA), leaf
dry matter content (LDMC), force required to pierce the leaf lamina
(punch), and carbon-to-nitrogen ratio (C/N). We sampled well-developed,
healthy leaves to measure the traits of all species with a minimum of
three replicates across their elevation range (76% of the total number
of surveyed species). SLA and LDMC were measured using standard
procedures (Pérez-Harguindeguy et al. 2013). Punch was calculated
using a digital force gauge (IMADA CO., LTD. Toyohashi, Japan),
following Sanson et al. (2001). C/N was determined by dry
combustion of ground leaf material (4mg +/- 0.2mg) of intraspecific
replicates pooled to equal weight using an elemental analyzer (NC-2500
from CE Instruments, Wigan, Lancashire, United Kingdom). The collection
of trait data was completed with published datasets (Kattge et
al. 2011; Körner et al. 2016; Descombes et al. 2017).