2.3) Data retrieval
To characterise each of the 139 relevant studies, we compiled
information for six main categories: publication data, geographic and
land-use data, herbivore data, plant data, trait data and sampling
design (Figure 1, Appendix A, B). All data was compiled using
information provided in the papers, except for climate information which
was obtained by overlaying study locations with ESRI World Terrestrial
Ecosystems, Temperature Regime data (Esri, USGS, TNC) (Sayre et al.,
2020). Land use at the study area were extracted from information
provided within the publications, although collated into four
categories: pastoral, natural area/minimal use, protected area and
research facility. These features were used as descriptive variables to
quantitatively summarise plant trait-herbivore research in grasslands
and to examine trends in a) the effect of vertebrate and invertebrate
herbivores on plant traits and b) the response of vertebrate and
invertebrate herbivore forage selection to plant traits. In scenario a)
plant traits are the response, while in scenario b) plant traits are the
effect and herbivore forage selection is the response. As some studies
included multiple herbivory treatments or multiple sites, there were a
total of 172 records from which we extracted trait and herbivore
responses.
Where trait information had been collected across multiple sites or
species, all trait responses are recorded. Plant traits were grouped a
priori into broad categories; morphological, biochemical, physiological
and phenological. A list of all traits and the broad category they fall
into can be found in Appendix C. The response of plant traits to
herbivory or the response of herbivore forage selection to the plant
traits was extracted from each article and recorded within the ordinal
scale; negative, positive and not significant. For example, a study
which found leaf nitrogen to be lower in grazed plants compared to
ungrazed plants, would be given a negative score. Alternatively,
if herbivore selection increased with leaf nitrogen, this would be given
a positive score. For traits with more than 5 responses recorded
across all studies, we calculated the proportion of trait responses
which were negative, positive or not significant.
As our study is focussed on reviewing and examining coarse differences
between vertebrate and invertebrate focussed herbivore research, we did
not to collect information on effect size. We do however present the
proportion and total number of studies which found positive, negative or
non-significant results. As such this study only accounts for
statistically significant results and not overall practical
significance.