Plants
A total of 31 plant families and 134 plant species were examined across all studies (Figure 4; Appendix B). Plant traits were mostly measured at the community level using mean traits values for dominant species (70 records total: 59 vertebrate and 11 invertebrate) or from species within the Poaceae family (grasses) (37 records total: 26 vertebrate and 11 invertebrate studies, 37 and 19 species respectively). Research involving plant functional traits are often inherently biased towards dominant species within the community due to the need to achieve adequate sampling and replication across treatments and because of the strong influence dominant species have on ecosystem function (Grime, 1998). The focus on species from the Poaceae family may also be due to the focus on domestic vertebrates who predominantly eat grass. For invertebrate focussed studies, plants from the family Asteraceae were also frequently examined, totalling eight studies and 13 species overall. This finding is again likely a matter of their dominance within grasslands, as the most commonly studied Asteraceae are disturbance tolerant and fecund, such as the common dandelion (Taraxacum officinale ) and common milk weed (Asclepias syriaca ). Invertebrate focussed studies also often examined the response of flowering traits (such as flower number) to herbivory. As Asteraceae species generally flower frequently and have easily observable flowers, this may be another reason for their focus, along with Fabaceae which were also commonly examined in invertebrate focussed studies.
Most plant species examined were native to their study range (Appendix B). Six studies examined plants invasive to their study range: 3 from America, 1 from Australia and 2 from New Zealand. As most studies were from the Northern Hemisphere, mainly China and the European Union, this result is unsurprising as these regions contain few non-native species. In contrast, in the southern hemisphere many non-native species are dominant due to a strong history of Eurasian plant introductions (Cook & Dias, 2006). Understanding the relationship between invasive non-native plant traits and herbivory is important for understanding factors associated with their spread and dominance and may help to predict and manage the impact of invasive plants. Future plant trait-herbivore research should work to further unravel these interactions between herbivory and plant invasion.