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.