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.