Trait effect on herbivore selection
In contrast to response traits which capture changes to plant traits
with herbivory, effect traits potentially inform us more about the
plants constitutive mechanisms to avoid or tolerate herbivory. Trait
effect on vertebrate herbivore selection was most frequently recorded
for leaf nitrogen, leaf fibre, total phenols, tannins, and SLA (Table
2). In contrast, trait effect on invertebrate herbivore selection was
most frequently recorded for leaf hairs, LDMC, SLA, leaf nitrogen and
leaf lignin (Table 2).
Like the trait response results, there was a high proportion of studies
which found a non-significant effect of plant traits on herbivore
selection (Figure 7). Where there was a significant effect, leaf
nitrogen and SLA was mostly positively associated with both vertebrate
and invertebrate herbivore selection. For invertebrates’ leaf LDMC and
leaf lignin was commonly negatively associated with selection. These
results are in line with the dominant ecological paradigms which
highlight leaf nitrogen and leaf tenderness (high SLA, low LDM, low leaf
lignin) as important influencers of herbivore selection for plants that
are more nutritious and palatability (Coley et al., 1985;
Perez-Harguindeguy et al., 2016).
Leaf fibre and total phenols are often postulated to be negatively
associated with palatability, as fibre can make the leaf tough and hard
to digest and phenols can be toxic to both vertebrates and invertebrates
(Salminen & Karonen, 2011). We found support for this within the
reviewed vertebrate focussed studies (Figure 7) ; however, two studies
also recorded a positive effect of these traits on herbivore selection
(Egea et al., 2014; Hjalten et al., 1996). As these studies allude to,
and as we have discussed earlier, phenols are a diverse group of
compounds which are not just involved in defence but contribute to
several other metabolic processes. Without identifying the exact
compounds, it is difficult to attribute their role to defence. Further,
as found in the study by Egea et al. (2014), some species, such as
goats, are capable of eating tannin-rich plants due to their ability to
neutralise the negative effects of tannins, a type of phenolic compound
(Allegretti et al., 2012). Some studies did look at specific phenol
groups and in particular total tannins, where the dominant effect on
vertebrate herbivore selection was negative. Tannins are a large group
of water-soluble phenols which when ingested can bind to and precipitate
proteins and micronutrients and reduce nutritional gain. Herbivores have
been recorded selecting tannins rich forage to self-medicate against
gastrointestinal parasites (Villalba et al., 2010), although this
observation was not recorded in this review.
Leaf hairs were only investigated in relation to invertebrate herbivory
and only in the context of herbivore selection. Where a significant
response was recorded, most studies found leaf hairs to be negatively
associated with invertebrate selection (Figure 7). One study, however,
found leaf hairs positively influenced selection by a specialist
herbivore, but negatively influenced selection by generalist herbivores
(Lau et al., 2008). This highlights the importance of accounting for
species co-evolution when assessing plant trait-herbivore interactions.
Although not found within the reviewed studies, evidence from other
studies suggest leaf hairs can also be induced by herbivory (Björkman et
al., 2008; Tian et al., 2012). It may be that leaf hairs are also be an
important inducible response in grassland species.
Table 2. The most common (top 5) and frequently measured (top
5) traits across and within all vertebrate and invertebrate studies. The
study level reflects the most common traits examined, irrespective of
frequency the trait was measured across and within all studies (e.g.,
ignoring site and species replicates). The response level reflects the
most frequently measured traits, considering within study site and
species replicates.