Identification of keystone species
Keystone species are defined here as plant taxa that play a major role
in providing a food source for the orthopteran herbivores (Millset al. 1993; Power et al. 1996). We identified keystone
species using custom R scripts, submitting each network, preliminarily
transformed into an igraph object (Csardi & Nepusz 2006), to a
sequential and random removal of plant species. Insect species were
considered to be extinct upon loss of all the plant species they feed
on. Plant species were removed until all insect species became extinct.
This was repeated for n*(n–1) simulations, with n equaling the total
number of plant taxa in the network. The mean number of secondary
extinctions caused by plant removal, to which we refer hereafter as the
keystone score, was then calculated for each plant species. To examine
the distribution and the keystone score of species within the plant
functional space at the low (<1050 m a.s.l.) and high
elevation (>2000 m a.s.l.), we performed a principal
components analysis on plant traits with the function dudi.pca from the
ade4 package (Thioulouse et al. 2018). We compared the
distribution of species with different keystone scores in the functional
space of plant traits of species at the lowest and highest elevation
sites. For both elevation classes, we extracted plant species based on
their presence within the corresponding elevation range. We further
determined the 10 species with the highest keystone scores for each
network, averaged their weights for each elevation class and placed them
in the functional space of plant trait.