3.5) Perspectives and limitations
We have presented an overview of the literature on plant trait-herbivore
interactions in grasslands across the globe. Specifically, we highlight
how vertebrate and invertebrate focussed studies differ in the types of
traits examined, and their response to, and effect on herbivory. It is
clear from the results of this review, that there is a particularly
limited understanding on the impacts of invertebrates or the combined
impacts of invertebrates and vertebrates on plant species in grasslands.
Given the importance of grasslands for diversity, carbon cycling and
human food production this is concerning. Considering that insect
populations are declining across the globe (Eggleton, 2020), this
highlights a particularly important area of future research to
appropriately predict changes to ecosystem functioning.
Another important finding from this review is the significant bias
towards domestic livestock and researching their response to, and effect
on plant traits. Due to the dominant use of grassy ecosystems across the
globe for livestock production, livestock potentially have a
proportionally greater effect on plant traits and consequential
ecosystem functioning than wild native vertebrate herbivores. Native
herbivores usually exist within these pastoral landscapes however, and
understanding how these species interact with domestic livestock to
affect plant traits will provide a more wholistic understanding of
herbivore impacts on grassland species. Further, due to differences in
evolutionary history, plants likely respond differently to native
compared to non-native herbivores, and this may be reflected in their
trait response (Diaz et al., 2007).
As with most functional trait research, results from this review are
biased toward the dominant plant species within grassland communities.
This bias may be challenging to avoid, as rare species likely do not
offer adequate replication, but fundamentally limits our understanding
of plant-herbivore interactions. For instance, dominant plants within
grazed grassland communities are likely to be selected for their traits
which confer grazing tolerance or avoidance. In contrast, species which
occur at low frequencies (one definition of ‘rare’) may do so because
they are highly palatability and have been selectively grazed.
Alternatively, they may be inherently ‘rare’, due to low fecundity and
dispersal. Subordinate and rare species can have significant influence
on ecosystem functioning (Baer et al., 2002; De Vries et al., 2011;
Grime, 1998) and understanding their interactions with herbivory is an
important area of future research.
Although there were some similarities between vertebrate and
invertebrate focussed studies in terms of the plant traits examined,
there is still considerable disparity, which reduced the capacity for
comparisons. Differences in the potential scale, intensity and
specificity of herbivory inflicted by these two groups is important to
understand the differences in the types of plant traits being chosen to
be researched. In saying this there is considerable capacity to expand
plant trait-herbivore research to increase comparability of results
between vertebrate and invertebrate herbivores. For instance,
investigating the relationship between VOCs and vertebrate herbivory or
examining how invertebrates influence plant morphology using longer-term
experiments would be an interesting area for future research. Overall,
we suggest there is also scope to shortlist the types of traits being
examined to improve comparability of results, but still remaining
adaptable to change as more is learned.
Despite considerable effort to ensure all relevant studies were included
in this systematic review, we recognise the search terms used may not
have been sufficient to find species-focussed studies that did not
specify that their focal species occurred in grasslands. For instance,
for some laboratory or glasshouse studies the fact that their focal
plant species naturally occurs in grasslands may have been irrelevant to
their study question. Overall, we found perhaps surprisingly few studies
which adhered to the criteria of this systematic review. Many
potentially relevant studies were excluded because they examined how
plant traits responded to various intensities of herbivory but failed to
include a no herbivory control. Other key reasons for exclusion were the
use of categorical traits and the use of cropping species.
Overall, there is exciting opportunity for future research to explore
interactions between vertebrate and invertebrate herbivores and
grassland plant traits. This will add to a growing body of literature
aimed at better understanding the complex relationships between plants
and herbivores. As the primary pathway for nutrient cycling and movement
across trophic levels, these relationships are crucial for the
functioning of agricultural and ecological systems alike. By improving
our understanding of these relationships, we contribute to the
development of theories around plant herbivore interactions and in doing
so help to guide the development of better research questions and the
building of knowledge.