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.