3.1) Overall publication trends and biases
Our final dataset comprised 139 articles from 68 journals and spanning
the year range from 1985 to 2022. Of these, 40 investigated invertebrate
herbivory and 97 investigated vertebrate herbivory. Two studies
investigated both vertebrate and invertebrate herbivores, however only
one examined the combined effect of both herbivore guilds on plant
traits. Despite diverse and abundant invertebrates’ assemblages within
grasslands, relatively poor representation of invertebrates within plant
trait-herbivore research aligns with trends in ecological research more
generally (Eisenhauer et al., 2019).
Research on vertebrate herbivores focussed more on the response of plant
traits to herbivory (76% of vertebrate studies and 51% of invertebrate
focussed studies). In contrast research on invertebrate herbivores
focussed more on the effect of plant traits on herbivore forage
selection (24% of vertebrate and 48% of invertebrate studies
respectively; Table 1). These differences in focus may reflect
experimental biases, such as conducting appropriately replicated
selection experiments is easier with invertebrates or may reflect the
perceived importance of plant traits in influencing herbivore forage
selection among these two guilds of herbivores. Vertebrates are
polyphagous, which suggests that they are less affected by declines in
food quality in comparison to invertebrate herbivores (Oduor et al.,
2010). Similarly, vertebrates can generally move more easily between
food sources increasing their available food options and perhaps making
initial food selection strategies less important. Vertebrates also have
greater bite and chewing capacity than invertebrates, perhaps making
some morphological traits such as leaf hairs or leaf toughness less
relevant to their forage selection (Kotanen & Rosenthal, 2000b).
Nevertheless, chemical cues and morphological traits are regularly
studied for understanding vertebrate forage selection in grasslands, and
this is highlighted in the 24 studies analysed in this review (Appendix
C).
Table 1. Summary of the 139 articles which adhered to the
criteria of this review. ‘Study focus’ details the total number of
studies which examined trait response or effect. ‘Taxonomy’ details the
total number of plant and herbivore orders/families/species examined
across and within all studies. ‘Plant traits’ details the total unique
traits examined across and within all studies.