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\begin{document}

\title{How plant traits respond to and affect vertebrate and invertebrate
herbivores---are measurements comparable across herbivore types?}



\author[1]{Gabrielle Lebbink}%
\author[2]{Anita Risch}%
\author[2]{Martin Schuetz}%
\author[1]{Jennifer Firn}%
\affil[1]{Queensland University of Technology}%
\affil[2]{Eidgenossische Forschungsanstalt fur Wald Schnee und Landschaft WSL}%


\vspace{-1em}



  
  \date{May 18, 2024}


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\selectlanguage{english}
\begin{abstract}
Examining how plant traits respond to and affect herbivory is a common
approach to exploring plant-herbivore interactions and their impact on
ecosystem processes and functioning. Despite plants being potentially
exposed to both vertebrate and invertebrate herbivores simultaneously,
fundamental differences in the ecology and evolution of these two
herbivore guilds results in them often being studied separately. A
synthesis of the literature is needed to understand the types of plant
traits examined and their response to, and effect on (in terms of forage
selection) vertebrate and invertebrate herbivory, and to identify
associated knowledge gaps. Focusing on grassland systems and species, we
found 139 articles that met our criteria: 40 invertebrate, 97 vertebrate
and 2 focussed on both vertebrate and invertebrate herbivores.
Invertebrate focussed research, research conducted in the Southern
Hemisphere and research on non-domesticated herbivores was significantly
underrepresented based on our search. Differences in study focus (trait
response or trait affect), along with considerable differences in the
types of traits examined, led to limited capacity for comparison between
the two herbivore guilds. For both invertebrates and vertebrates
however, plant traits related to growth, such as leaf nitrogen and
photosynthetic capacity, were often positively associated with
herbivory. Future research should prioritise understanding how
invertebrates, and the combined impact of both invertebrates and
vertebrates' respond to and affect plant traits. This review can be used
as a guide for future research to select plant traits which are commonly
measured either within one, or across both guild/s, as to help improve
comparability and the broader significance of results, while also
extending research breadth and knowledge.%
\end{abstract}%



\sloppy


\textbf{Introduction}

Plant-herbivore interactions link primary production and food webs. They
are the catalyst for the transfer of energy/nutrients between trophic
levels and the abiotic environment and are crucial to the shaping of
community dynamics and ecosystem function. Due to their fundamental role
in the web of life, plant-herbivore interactions have been the focus of
many fields of research, including ecology (such as, Risch et al.,
2018), evolution (such as, Johnson et al., 2015; Maron et al., 2019),
entomology (such as, Roohigohar et al., 2022; Zalucki et al., 2001) and
agriculture (such as, Christensen et al., 2013; Wari et al., 2019).
Questions range from fundamental knowledge building (Ramsey \& Wilson,
1997) and practical understanding, such as learning how to produce crops
or meat more efficiently, to testing and developing ecological theory,
such as understanding the role plant/herbivore interactions play in
ecosystem coupling and stability (Risch et al., 2018).

Because of the complexity of plant-herbivore interactions, it is easy
for knowledge silos to form within respective fields of expertise and
investigation---where for example, a plant ecologist may experiment in
different ways to an entomologist. A potential silo, perhaps easily
conceived, is that which develops between different herbivore guilds,
and specifically between vertebrate and invertebrate herbivores (Peisley
et al., 2015). Due to fundamental differences in their ecology and
evolution, these two broad taxonomic groups attract sometimes
contrasting interests, expertise and methodologies (Andrew et al.,
2022). This review aims to systematically investigate differences
between vertebrate and invertebrate focused research, specifically
regarding how herbivores respond to and affect plant traits.

Plant functional traits have been used as a `common currency'\selectlanguage{ngerman}¾ to
collate, compare and contrast response and effect correlations in plants
within and between different ecosystems and species (Lavorel \& Garnier,
2002; Suding et al., 2008). Functional traits also provide the
opportunity to discover generalities which arise out of complex
interactions between species within and across trophic levels (Carmona
et al., 2011; Lind et al., 2013). Further, using a common currency
correlated with function, lends well to exploring the influence of other
abiotic or biotic variables on traits and untangling their role in
modifying plant-herbivore interactions (Funk et al., 2017).

Plants employ a variety of traits to defend themselves against
herbivory. These can be morphological (e.g., spine length, see Göldel et
al., 2016), phenological (e.g., lifeform, see De Bello et al., 2005) and
physiological (e.g., photosynthetic capacity, see Shen et al., 2019),
and usually are associated with herbivore avoidance or herbivore
tolerance (Núñez-Farfán et al., 2007). For example, plants might avoid
herbivory by being small and short (Wakatsuki et al., 2021), expressing
secondary metabolites (Jones et al., 2003) or being covered in spines
(Coverdale et al., 2019). Plants which tolerate herbivory might have a
fast growth rate and efficient nutrient acquisition strategies to allow
them to quickly regain photosynthetic tissue after feeding (Briske et
al., 1996). In general, plants with functional traits on the
conservative end of the leaf economic spectrum (e.g., relatively smaller
specific leaf area, lower nitrogen content, a slower assimilation rate)
are more tolerant of herbivores than those on the resource acquisition
end of the spectrum (e.g., relatively higher specific leaf area, higher
nitrogen, fast growth rate). Herbivory, in addition to plant
productivity, can moderate the abundance of species within this spectrum
(Wright et al., 2004).

Plant traits can be constitutive, i.e., present throughout a plants
life, or can be induced, i.e., expressed when herbivory takes place
(Barton, 2016; Züst \& Agrawal, 2017). An example of a constitutive
trait is the presence of plant spines. Traits such as this are expressed
all the time, although the degree of expression can vary with abiotic
and biotic factors, including herbivory (Hulshof et al., 2013). In this
way the expression of constitutive traits can also be induced. For
example, spine length (Young, 1987), or the expression of secondary
metabolites may increase (beyond their constitutive expression) in
response to herbivore attack (Huitu et al., 2014). Expression of induced
defence traits can occur immediately in response to herbivore attack,
such as the release of volatile organic compounds (VOCs), or over time,
such as the increased accumulation of carbon or silica within an
individual's leaves. Plant traits also can either respond to or affect
their abiotic and biotic environment (Funk et al., 2017). In the context
of herbivory, response traits `respond' to herbivore attack through an
induced response and effect traits can `affect' herbivory by attracting
or deterring herbivores. The capacity for plant traits to change in
response to short- and long-term changes in herbivory and other
perturbations creates the foundations for adaptation and speciation to
occur over longer evolutionary timeframes (Ackerly et al., 2000).

Plants are most of the time exposed to different types of herbivores
simultaneously, and therefore may express a suite of traits also known
as a `defence syndrome', to effectively defend against different types
of herbivory (Agrawal, Fishbein 2006; Moles et al. 2013). Vertebrate and
invertebrate herbivores for instance, vary in size, feeding strategy,
behaviour and ecology. Traits of the herbivores can then significantly
influence the type, duration and degree of damage experienced by the
plant and consequently the plants defence syndrome (Kotanen \&
Rosenthal, 2000b). Thorns, for example, are relatively ineffective at
reducing herbivory from small invertebrates such as aphids, but function
well against large browsing animals, like ibex or deer (Crawley, 2019).
Similarly, some plant secondary metabolites might deter invertebrate
herbivory, but may be ineffective against most vertebrate herbivores
(Marsh et al., 2020; Salminen \& Karonen, 2011). The type of herbivore,
as well as the research question and context, will therefore likely
influence the scientific lens researchers adopt and the traits chosen to
be measured when asking `How do plant traits respond to and affect
herbivory?'.

In this review, we aim to synthesise current evidence and understanding
of how plant traits respond to and affect (in terms of forage selection)
vertebrate and invertebrate herbivory. Further, we identify and discuss
any potential biases (i.e., taxonomic, geographic and climatic) and
knowledge gaps. We will focus our literature search on grasslands and
grassy woodlands. Grasslands cover \textasciitilde{}30\% of the Earth's
terrestrial surface (White et al., 2000) and herbivores are crucial to
their functioning, diversity and evolution (Axelrod, 1985; McNaughton,
1984). Grasslands are also important to the provision of food for
people, (Habel et al., 2013; Simon et al., 2009) and are important for
maintaining global carbon and nutrient cycling (Scurlock \& Hall, 1998).
Because of the pressures of human food production, many grasslands have
been extensively used and modified by humans and as a consequence the
persistence of many grassland species are under threat (Cousins \&
Eriksson, 2008; Deák et al., 2020; Scholtz \& Twidwell, 2022). To
conserve plants and animals within these important and widespread
ecosystems, we need to have a mechanistic understanding of the complex
functional relationships between plants and herbivores. By studying the
potentially disparate fields of vertebrate and invertebrate focused
studies, we aim to provide a more wholistic understanding of plant
trait-herbivore interactions in grasslands and highlight knowledge gaps
to guide future research.

We structured our review around the following three questions:

1. Are their geographic, taxonomic or climatic trends evident within
grassland plant trait-herbivore literature?

2. What plant traits are measured in vertebrate and invertebrate
herbivore focussed studies?

3. How do plant traits respond to and affect (in terms of forage
selection) vertebrate and invertebrate herbivory?\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1.} Procedures used in systematic literature review for
articles examining herbivore effect on or response to, in regard to
forage selection, plant traits in grassland ecosystems. Key data
retrieved from each article is also shown within their categories,
publication, geographic and land use, plant, trait and sampling details.
A full list of the type of data collected is available in Appendix A and
B. \textsuperscript{1} Plant trait response to
herbivory.\textsuperscript{2} Herbivore selection response to plant
traits.

\textbf{2) Methods}

\textbf{2.1) Search Strategy}

Our literature search and screening processes are shown in Figure 1.
Using a specific set of keywords and criteria the following databases
and internet search engines were searched for literature investigating
how plant traits respond to and effect vertebrate and invertebrate
herbivores in grasslands across the globe.

\emph{Databases and Internet searches}

SCOPUS (\emph{http://www.scopus.com})

Web of Science Core Collection (http://www.webofknowledge.com )

Google Scholar (scholar.google.com)

\emph{Search terms and combinations}

Searches were conducted in English and using the default date range for
each database/ search engine (1980s to present for SCOPUS and Web of
Science (WOS); 1970s onward for Google Scholar). To ensure a
comprehensive search, we selected search terms related to herbivores and
herbivory, plant functional traits and grasslands or other grass
dominated systems (e.g., grassy woodlands) or the species that occur in
these systems. The following search terms were utilised in each of the
online databases and searches.

TS=((herbivor* OR graz* OR brows * OR feed* OR forag*) AND (``plant
trait*'' OR ``functional trait*'' OR ``leaf trait*'' OR ``foliar
trait*'' OR secondary* OR defens* OR defenc*) AND (grass* OR grassland*
OR ``grassy woodland*'' OR meadow * OR pasture* OR prairie* OR monocot*
OR graminoid* OR pampa* OR savanna* OR savannah OR campos OR cerrado OR
espinal* OR fynbos OR veldt* OR veld* OR llanos OR downs OR tundra* OR
pustaz) )

Using the field tag function in SCOPUS and WOS these terms were searched
within the title, abstract and key words of these databases.

Search results from databases were exported to Endnote citation manager
(Clarivate 20.3) and cleaned for duplicates. To check for additional
results not within these databases, the first 150 papers (sorted by
relevance) were exported from Google Scholar.

\textbf{2.1) Search results}

The initial search in WOS and SCOPUS yielded 4282 and 4082 respectively.
To narrow down our search results we confined returned results to the
field of ecology, plant sciences, environmental sciences, biodiversity
conservation and entomology in WOS and environmental science and
agricultural and biological sciences in SCOPUS. This returned 2757 in
WOS and 3694 in SCOPUS, in addition to the 150 from Google Scholar.
After duplicates were remove a total of 2928 papers were screened using
the below screening process and criteria.

\textbf{2.2) Article screening and criteria}

To ensure articles were relevant and aligned with the review aims, only
articles which adhered to the below criteria were included. We started
with the title (to exclude obviously irrelevant articles), then the
abstract, then full text. At conclusion of the screening process 139
articles remained (Appendix F) and were included for data collection

Criteria

\emph{Article type:} Article documents an empirical investigation. This
excludes reviews. Article is peer reviewed and in English.

\emph{Question:} Article examines how vertebrate or invertebrate
herbivory affects or responds (in terms of forage selection) to plant
traits.

\emph{Subjects:} Include one or more herbivore/s and plant trait/s and
be within a grassland/grassy woodland ecosystem or on a plant species
which occurs in these ecosystems.

\emph{Study design and treatments:} Article included a no herbivory
control treatment, either through herbivore exclusion, herbivore
addition or natural herbivory gradients which included areas of no
herbivory.

\emph{Excluded studies:} Articles reporting on simulated grazing.
Articles with only categorical traits. Articles which focussed on plant
species which are crops. Articles which artificially modified plant
traits and measured herbivore response (e.g., application of silica onto
plants).

\emph{Rules for multiple treatments or time periods:} Where articles
report results from over multiple time periods, the most recent
comparison was recorded. Where articles report results from multiple
grazing intensities only difference between the control and highest
grazing intensity were used.

\textbf{2.3) Data retrieval}

To characterise each of the 139 relevant studies, we compiled
information for six main categories: publication data, geographic and
land-use data, herbivore data, plant data, trait data and sampling
design (Figure 1, Appendix A, B). All data was compiled using
information provided in the papers, except for climate information which
was obtained by overlaying study locations with ESRI World Terrestrial
Ecosystems, Temperature Regime data (Esri, USGS, TNC) (Sayre et al.,
2020). Land use at the study area were extracted from information
provided within the publications, although collated into four
categories: pastoral, natural area/minimal use, protected area and
research facility. These features were used as descriptive variables to
quantitatively summarise plant trait-herbivore research in grasslands
and to examine trends in a) the effect of vertebrate and invertebrate
herbivores on plant traits and b) the response of vertebrate and
invertebrate herbivore forage selection to plant traits. In scenario a)
plant traits are the response, while in scenario b) plant traits are the
effect and herbivore forage selection is the response. As some studies
included multiple herbivory treatments or multiple sites, there were a
total of 172 records from which we extracted trait and herbivore
responses.

Where trait information had been collected across multiple sites or
species, all trait responses are recorded. Plant traits were grouped a
priori into broad categories; morphological, biochemical, physiological
and phenological. A list of all traits and the broad category they fall
into can be found in Appendix C. The response of plant traits to
herbivory or the response of herbivore forage selection to the plant
traits was extracted from each article and recorded within the ordinal
scale; negative, positive and not significant. For example, a study
which found leaf nitrogen to be lower in grazed plants compared to
ungrazed plants, would be given a \emph{negative} score. Alternatively,
if herbivore selection increased with leaf nitrogen, this would be given
a \emph{positive} score. For traits with more than 5 responses recorded
across all studies, we calculated the proportion of trait responses
which were negative, positive or not significant.

As our study is focussed on reviewing and examining coarse differences
between vertebrate and invertebrate focussed herbivore research, we did
not to collect information on effect size. We do however present the
proportion and total number of studies which found positive, negative or
non-significant results. As such this study only accounts for
statistically significant results and not overall practical
significance.

\textbf{3) Results and Discussion}

\textbf{3.1) Overall publication trends and biases}

Our final dataset comprised 139 articles from 68 journals and spanning
the year range from 1985 to 2022. Of these, 40 investigated invertebrate
herbivory and 97 investigated vertebrate herbivory. Two studies
investigated both vertebrate and invertebrate herbivores, however only
one examined the combined effect of both herbivore guilds on plant
traits. Despite diverse and abundant invertebrates' assemblages within
grasslands, relatively poor representation of invertebrates within plant
trait-herbivore research aligns with trends in ecological research more
generally (Eisenhauer et al., 2019).

Research on vertebrate herbivores focussed more on the response of plant
traits to herbivory (76\% of vertebrate studies and 51\% of invertebrate
focussed studies). In contrast research on invertebrate herbivores
focussed more on the effect of plant traits on herbivore forage
selection (24\% of vertebrate and 48\% of invertebrate studies
respectively; Table 1). These differences in focus may reflect
experimental biases, such as conducting appropriately replicated
selection experiments is easier with invertebrates or may reflect the
perceived importance of plant traits in influencing herbivore forage
selection among these two guilds of herbivores. Vertebrates are
polyphagous, which suggests that they are less affected by declines in
food quality in comparison to invertebrate herbivores (Oduor et al.,
2010). Similarly, vertebrates can generally move more easily between
food sources increasing their available food options and perhaps making
initial food selection strategies less important. Vertebrates also have
greater bite and chewing capacity than invertebrates, perhaps making
some morphological traits such as leaf hairs or leaf toughness less
relevant to their forage selection (Kotanen \& Rosenthal, 2000b).
Nevertheless, chemical cues and morphological traits are regularly
studied for understanding vertebrate forage selection in grasslands, and
this is highlighted in the 24 studies analysed in this review (Appendix
C).

\textbf{Table 1.} Summary of the 139 articles which adhered to the
criteria of this review. `Study focus' details the total number of
studies which examined trait response or effect. `Taxonomy' details the
total number of plant and herbivore orders/families/species examined
across and within all studies. `Plant traits' details the total unique
traits examined across and within all studies.

\emph{Final data set 139 studies, 171 records\textsuperscript{1}, 1516
trait/herbivore responses}\selectlanguage{english}
\begin{longtable}[]{@{}llll@{}}
\toprule
& \textbf{Vertebrate focussed} & \textbf{Invertebrate focussed} &
\textbf{Overall total}\tabularnewline
\textbf{Study focus} & & &\tabularnewline
Trait response to herbivory & 76 & 21 &
97\textsuperscript{2}\tabularnewline
Trait effect on herbivore selection & 20 & 20 & 40\tabularnewline
\textbf{Taxonomy} & & &\tabularnewline
\emph{Plants} & & &\tabularnewline
Total families & 23 & 11 & 28\tabularnewline
Total species & 90 & 46 & 132\tabularnewline
Total plant community studies & 58 & 11 & 69\tabularnewline
\emph{Herbivores} & & &\tabularnewline
Total orders & 7 & 8 & 15\tabularnewline
Total species & 20 & 13 & 30\tabularnewline
Total herb community studies & 6 & 8 & 14\tabularnewline
\textbf{Plant traits} & & &\tabularnewline
Total traits examined & 158 & 9 & 209\tabularnewline
\bottomrule
\end{longtable}

\textsuperscript{1} Accounting for additional sites/species within the
139 studies, 171 records made up the final data set.

\textsuperscript{2}Two studies examined plant trait response to both
vertebrate and invertebrate herbivores.

\textbf{3.2) Geographic, land use, climatic or taxonomic trends and
biases}

Research was conducted across 35 countries, with most research confined
to the Northern Hemisphere and the cool temperate ecosystems of China
and Europe (Figure 2). Tropical and sub-tropical grasslands, such as the
cerrados and savannas of South America, Africa and Australia were
significantly underrepresented. As these ecosystems make
up\textasciitilde{}50\% of grasslands across the globe (Dixon et al.,
2014), there is considerable room for improvement in our understanding
of plant trait-herbivore interactions in global grassland systems. Poor
representation of the Southern Hemisphere is a trend common within
ecological research (Carlucci et al., 2020; Weidlich et al., 2020).
Several studies have also highlighted the need for greater ecological
research within tropical ecosystems (Clarke et al., 2017; Wilson et al.,
2016).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{Figure 2.} Map showing the location of each study within the
ESRI World Terrestrial Ecosystems, Temperature Regime Map (Esri, USGS,
TNC) (Sayre et al. 2020). The focal herbivore guild (vertebrate or
invertebrate) and herbivore origin (Wild/Field, Domesticated/Laboratory)
is also shown. 11 studies are not mapped due to multiple locations or
insufficient geographic information provided.

Taxonomic trends

\emph{Herbivores}

Domestic vertebrate herbivores, namely sheep, cows and goats were by far
the most studied herbivores (Figure 3). This trend likely reflects
funding and societal priorities to support global red meat production
(Lemaire et al., 2005). Invertebrates, and other native mammalian
herbivores, however, can also have huge impacts on grassland
productivity and consequential livestock production (Risch et al., 2013;
Saunders, 2018; Umina et al., 2021). Improving our understanding on how
these herbivores impact plant defence syndromes may benefit the pastoral
industry, while also providing a better overall understanding of how
herbivores affect grassland plant communities. Many invertebrate species
examined were also commercially important or very common species such as
the cotton worm (\emph{Spodoptera littoralis} ) and cabbage moth
(\emph{Mamestra brassicae} ). Ensuring future research focuses on both
agriculturally and ecologically important herbivore species is
important.

Most invertebrate studies were not focussed on particular invertebrate
species but examined the response and effect of invertebrate communities
to/on plant traits, usually by measuring total leaf damage (Effah et
al., 2020) or by excluding invertebrate with the use of insecticide
and/or exclosures (Carson \& Root, 1999; Firn et al., 2017). These
studies are useful for examining the overall influence of invertebrate
herbivore on plants and can be easier than species focussed studies to
conduct in the field.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{Figure 3.} Number of studies per studied herbivore taxon, a)
vertebrates and b) invertebrates. For vertebrates, dark orange circles
denote domestic animals and light orange circles wild herbivores. For
invertebrates, dark blue circles denote laboratory-based studies and
light blue circles denote field-based studies.

\emph{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 (\emph{Taraxacum
officinale} ) and common milk weed (\emph{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.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{Figure 4} Number of studies per studied plant family for
vertebrate (orange) and invertebrate (blue) focussed studies.

\textbf{3.3) What plant traits are measured in vertebrate and
invertebrate focussed studies?}

Across all studies, 209 unique plant traits were measured. Of these 209,
115 were unique to vertebrate focussed studies, 50 unique to
invertebrate focussed studies. Of these traits, 44 were measured
commonly across vertebrate and invertebrate studies (Appendix C). Leaf
traits were the most frequently measured (115), but stem (34), whole
plant (24), root (16), flower/fruit (13) and seed traits (7) were also
measured.

Key areas of difference were between the proportion of biochemical and
morphological traits examined, with the former a greater focus in
invertebrate, and the later, in vertebrate studies (Figure 5). This
difference in traits studied between vertebrate and invertebrate studies
may be due to the scale at which these two herbivore guilds operate,
explained largely by differences in body size (Kotanen \& Rosenthal,
2000a). Due to their greater capacity for defoliation at larger scales,
vertebrates can influence plant morphology readily (e.g., plant height
or biomass) and thus plants are potentially more likely to change their
morphology in response to herbivory. In contrast, invertebrate herbivory
usually occurs over relatively longer time periods (e.g., a caterpillar
feeds on one leaf longer than a cow), at smaller scales and targeted to
specific tissues (Hulme, 1996; Oduor et al., 2010). This may allow the
plant more time to distribute re-mobilise nutrients and induce a
biochemical defence against herbivores, for example using VOCs (Ameye et
al., 2018) or other secondary metabolites. Nevertheless, there are some
instances where insects can have huge impacts on plant morphological
traits, such as biomass. One example are migratory insects (e.g.,
grasshoppers) that can reach high densities and in regard to extent of
consumption, act like vertebrate herbivores (Tscharntke \& Greiler,
1995). Many vertebrate studies were also conducted over several years
(Appendix A) and are therefore more likely to capture changes to plant
morphology than invertebrate focussed studies, which were mostly
conducted within a month or less.

Physiological traits, such as photosynthetic capacity and water use
efficiency were only examined in relation to vertebrate herbivory
(Figure 5). This is perhaps surprising as invertebrate impact on plant
physiology has been relatively well-studied for crops (Nabity et al.,
2008; Peterson et al., 1998; Thomson et al., 2003; Velikova et al.,
2010), and findings have shown invertebrate herbivory to be both
negatively and positively associated with photosynthetic rate.
Similarly, across all studies only one phenological trait, flowering
onset, was examined and only in four vertebrate focussed studies. This
may reflect constraints within research budgets and priorities as
phenological research generally requires experiments to run over several
years, for which funding is generally limited (Hughes et al., 2017;
Lindenmayer et al., 2012). The lack of focus on plant phenological
responses to herbivory in grasslands is concerning as phenological
patterns can influence regeneration capacity (Rawal et al., 2015),
community composition (Lavorel \& Garnier, 2002), and adaptation across
trophic levels (Bagella et al., 2013; Wray \& Elle, 2015). For example,
research in the alpine rocky ecosystems in Colorado found herbivory by
mule deer delayed flowering phenology in a perennial herb which
consequentially reduced invertebrate seed predation and overall
increased plant fitness (Freeman et al., 2003). Understanding how plant
phenology and herbivory interact with climate is also an important area
of future research, particularly under uncertain climatic conditions
(Hamann et al., 2021; Lemoine et al., 2017). With advances in remote
sensing technologies, studies on plant phenological studies are now
easier and cheaper to perform and we see these technologies already
starting to be used to answer other phenological questions (Dronova \&
Taddeo, 2022).

At the individual trait level and ignoring additional responses from
multiple species or sites examined within studies, the five most common
traits assessed in vertebrate focussed studies were specific leaf area
(SLA), plant height, leaf nitrogen, LDMC and leaf area (descending order
of use; Figure 4; Table 2). For invertebrate focused studies the most
common traits were VOCs, SLA, aboveground plant biomass, LDMC and leaf
nitrogen (Figure 4, Table 2). Specific leaf area, LDMC and leaf nitrogen
were common focal traits across both vertebrate and invertebrate
studies. These traits are often referred to as `soft' traits as they are
relatively easy to measure and have been found to correlate with traits
which are harder to measure such as relative growth rate (Hodgson et
al., 1999; Perez-Harguindeguy et al., 2016). These `soft' traits also
represent important components of the leaf economic spectrum (Hodgson et
al., 1999; Wright et al., 2004) and inform us about the plant's
individual response to abiotic and biotic factors, and in the context of
herbivory can inform us about the species ability to tolerate or avoid
herbivory.

The study of VOCs was a key point of difference between vertebrate and
invertebrate focussed studies, with eight studies examining VOCs in
relation to invertebrate herbivory and only one in relation to
vertebrate herbivory (Zhang et al., 2014). Analysis of VOCs is usually
done via dynamic headspace sampling (Chen et al., 2003) under
laboratory-controlled conditions, which may limit its capacity for use
on vertebrate herbivores. Zhang et al. 2014 however used this system to
first identify and isolate the VOCs released from grass
species\emph{Artesmia fridgida} and then apply these VOCs to control
plants during a selection experiment with domestic sheep. Some studies
have also successfully employed the head-space sampling system in a
field setting to examine VOC production from grass leaves with high and
low levels of invertebrate herbivory (see Kigathi et.al. 2009). It may
be possible to use a similar strategy to examine VOC production in
response to vertebrate herbivory. In response to invertebrate herbivory,
VOC production can act as a signal to attract vertebrate and
invertebrate predators (Kessler \& Baldwin, 2001; M\selectlanguage{ngerman}äntylä et al., 2008)
and to communicate the potential for herbivore attack to neighbouring
plants (Baldwin et al., 2002). In other studies, not reviewed here,
vertebrates have been shown to use plant VOCs to find food and increase
feeding efficacy (Bedoya-Pérez et al., 2014; Stutz et al., 2016).
Overall, the influence of VOCs were mostly studied in relation to
invertebrates and expanding this research to further examine their
response to, and effect on vertebrate herbivory would be an interesting
avenue for future exploration.

The huge diversity and high proportion (61\%) of traits examined from a
single study was in part due to traits being generally characteristic to
particular plant families or functional groups, such as rhizome length
(Amiaud et al., 2007), woody density of shrubs and woody forbs within
grasslands (Whitworth-Hulse et al., 2016), thorns (Woodward \& Coppock,
1995) or latex (Rasmann et al., 2009). Some studies researched unique
aspects of plant-herbivore interactions. For example, Ribeiro et al.
(2017) examined the influence of metals and micronutrients on
invertebrate herbivore selection and this study accounted for 31 (15 \%)
of the single study traits. They found most metals examined did not
affect invertebrate selection, although aluminium, iron, magnesium,
manganese, and total leaf metals were found to have a negative effect on
herbivore selection. Other studies examined specific morphological
attributes, such as leaf symmetry or grass blade width, or specific
anatomical aspects of a plant, such as spine angle. For example, Santos
et al. (2013) found herbivory by gall midge was negatively correlated
with leaf symmetry in the plant \emph{Bauhinia brevipes} . They also
found less symmetric leaves had lower leaf nitrogen, which is thought to
be favourable for gall development. These results reveal interesting
relationships between morphological and biochemical traits and their
interaction with herbivores.

Using a common list of traits across research groups exploring
plant-trait herbivore interactions would help to standardise trait
measurements and improve comparability across studies and this has been
flagged in previous reviews on the topic (Diaz et al., 2007) but at the
same time, researchers should remain open to new emerging plant traits
as technologies advance and our understandings improve. Shortlists of
traits that should be favoured or disfavoured by herbivory have been
identified (Coley et al., 1985; Weiher et al., 1999). Although these are
mostly focussed on vertebrate herbivory, used alongside results from
this review, a list of potential focal plant traits which are relevant
to both vertebrate and invertebrate herbivores can be determined.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{Figure 5} Proportion of biochemical, morphological,
physiological and phenological traits examined in vertebrate and
invertebrate focussed studies. Biochemical, morphological, phenological
(only 1 trait; flowering onset) and physiological traits were examined
in vertebrate focussed studies. Only biochemical and morphological
traits were examined in invertebrate focussed studies.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{Figure 6} Response of plant traits to vertebrate (left) and
invertebrate (right) herbivory, measured as a total proportion of
studies which found a positive, negative, and non-significant result.
Only traits with more than 5 responses recorded overall are included.
Traits are grouped into broad categories biochemical, morphological, and
physiological. There were less than five responses recorded for
phenological traits. Numbers in black show the total number of studies.
Where there was the same total number of studies recording a positive
and non-significant result for a particular trait, only one number is
shown.

\textbf{3.4) Plant trait response and effect}

Accounting for the multiple traits and site and/or species records
within studies, we had a total of 1516 recorded responses (plant trait
response to herbivory or plant trait effect on herbivore selection).
Despite some similarities in studied plant traits between vertebrate and
invertebrate focused studies overall, once responses were separated into
either trait response to herbivory or trait effect on herbivore
selection there was little overlap, and thus our capacity to compare
responses between vertebrate and invertebrate herbivores is limited.
Subsequently, interpretations below predominantly discuss trait response
and effect for these two herbivore guilds separately. We also focus our
discussion predominantly on the top-five traits of the most measured
responses (Table 2). All trait information can be found in Appendix B
and C.

\emph{Traits response to herbivory}

Overall, there was a high proportion of studies which found
non-significant trait responses to herbivory (Figure 6). At the study
level, plant traits might not change with herbivory because the traits
measured may not be indicators of specific plant-herbivore interactions
studied in their focal system. It may also be that some traits which
confer grazing tolerance/avoidance are also helpful, or of neutral
advantage under ungrazed conditions. This is particularly relevant for
vertebrate focussed studies which often compared plant traits values
between grazed areas and exclusion plots, which usually had grazers
excluded for between 5-10 years. The dominant plants that exist in these
exclosures likely possess traits which confer grazing
avoidance/tolerance, but these traits may not be a fitness disadvantage
to the plant in ungrazed conditions and remain similarly expressed.

Trait response to vertebrate herbivory was most frequently recorded for
SLA, LDMC, leaf nitrogen, plant height and leaf area; wherefor
invertebrate herbivory it volatile organic compounds (VOCs), leaf
nitrogen, aboveground biomass, total phenols and total tannins were most
frequent. Vertebrate herbivory was mostly positively correlated with
leaf nitrogen and SLA, and negatively with LDMC (Figure 6). As SLA and
leaf nitrogen are positively correlated with high relative growth and
LDMC positively correlated with leaf carbon content (Perez-Harguindeguy
et al., 2016), these results suggest that many of the plant species and
communities examined in these grassland studies, invest more in growth
to tolerate herbivory, and less in carbon based defensive structures to
avoid herbivory. This also aligns with the response of photosynthetic
capacity and leaf phosphorous content, which mostly responded positively
to vertebrate herbivory (Figure 6). Indirect influences from grazing
such as nitrogen deposition through urine and faeces, may also increase
the availability of nutrients in grazed environments and facilitate
growth (de Mazancourt et al., 1998). Tolerating herbivory through rapid
growth, is a common strategy for species from the Poaceae family, which
dominate grasslands. This is particularly true for grass species growing
in productive environments (Briske, 1996; Briske, 1999; Diaz et al.,
2007; D\selectlanguage{ngerman}íaz et al., 2001), where their dominance is often mediated via
herbivory (Lunt et al., 2007). When grazers are removed in these
environments, dominant grasses can sometimes reach a point of maximum
growth, where restrictions space and/or nutrients limits further growth,
and their growth rates may decline. In this scenario, grasses may invest
more carbon in their stems and less in their leaves as structural
support to grow tall under competitive light conditions (Irving, 2015).

In contrast, some studies found negative relationships between
vertebrate herbivory and leaf area and plant height (Figure 6), which
are typical mechanisms of grazing avoidance, documented in many other
studies (Landsberg et al., 1999; McNaughton \& Sabuni, 1988; Noy-Meir et
al., 1989; Sala et al., 1986). A negative relationship between
herbivory, leaf area and plant height may also be due to increased light
availability in grazed areas, potentially reducing the need for plants
to grow tall to access the light (Borer et al., 2014).

VOCs were mostly examined in relation to invertebrate herbivory and
responded mostly positively, with only a handful of studies reporting a
negative relationship with herbivory (Figure 6). This suggests that VOCs
are mostly expressed or up regulated in response to herbivory. VOCs may
also increase in response to a variety of other stimuli (Vivaldo et al.,
2017) and play an important role in plant-plant and cross-trophic
signalling (Baldwin et al., 2002; Mäntylä et al., 2008). Kigathi et al.
(2009) who found the expression of some VOCs to decrease with
invertebrate herbivory in the field, suggest that under field conditions
the plant is responding to multiple stimuli at once and this may also
affect the type and abundance of VOC emissions. They also highlight that
reduced VOC production in response to herbivory may be due to the
allocation of carbon and nitrogen being prioritised for growth or for
production of others defensive compounds over VOCs.

Where a significant response was found, leaf nitrogen was usually
positively correlated with invertebrate herbivory, while leaf carbon to
nitrogen ratio was usually negatively correlated (Figure 6). Similar to
the response to vertebrate herbivory this result suggests that for these
plant species, herbivory potentially results in a greater investment in
growth over carbon-based defence as indicated by higher leaf nitrogen
for every part carbon. Because many invertebrate focussed studies
compared leaf traits from leaves with and without visual evidence of
herbivory, greater nitrogen in grazed leaves may reflect herbivore
preference for leaves with higher nitrogen, rather than the leaf
increasing nitrogen in response to herbivory (Loranger et al., 2012).

Aboveground biomass (measured for individual species) responded mostly
negatively, but also showed little evidence of a change with
invertebrate herbivory. The non-significant responses mostly came from
one paper however (Ladygina et al., 2010) which looked at the effect of
belowground herbivores (nematodes and wire worms) on aboveground
biomass. Because aboveground biomass was not being directly removed via
herbivory the effect of herbivory may be more nuanced. Aboveground
biomass was also mostly negatively associated with vertebrate herbivory.
One study was the exception, finding perennial forb biomass was higher
in deer grazed pastures in comparison to ungrazed exclosures, likely due
to reduced competition from grass species (Paige, 1992). In fact, in
some grasslands the abundance and richness of forbs is thought to
dependent on niche construction by large herbivores, due to the legacy
of large and mostly extinct vertebrates, such as mammoths (Bråthen et
al., 2021). This points to an important distinction between direct
herbivore effects and indirect herbivore effects on plant traits.

Phenols are often attributed to herbivore defence and are usually
hypothesised to increase in response to herbivory (Salminen \& Karonen,
2011). Phenols are an extremely diverse group of compounds, however, and
play several other roles in plant metabolism (Salminen \& Karonen,
2011). The production of phenols is also reliant on adequate
photosynthetic capacity to allow for the accumulation of carbon
molecules (Frier et al., 2012). Total phenols responded mostly
positively to vertebrate and invertebrate herbivory, although one study
found phenols to decline in response to herbivory across multiple sites
(Knappová et al., 2018) (Figure 6). Reported high rates of folivory in
this study may have compromised the photosynthetic capacity of plants
and limited phenol production (Knappová et al., 2018).

Leaf silica was relatively frequently measured (31 measures) in response
to both vertebrate and invertebrate herbivory and where a significant
effect was found was mostly positively associated with herbivory (Figure
6). Most studies examining the response of silica were on grass species,
which are known for their relatively high silica content in comparison
to forbs and their ability to accumulate silica in response to
herbivory, but also in response to increases in soil silica content
(Hall et al., 2019). Within the studies from this review, silica
responded positively to herbivory in eight different grass species
(Appendix B). One study from the tundra grasslands in Norway, found that
for some grass species, silica content declined with summer reindeer
herbivory (Petit Bon et al., 2022). Here they suggest that silica
accumulates with age in these grass species, and perhaps herbivory is
keeping the phenological age of the leaves young and thus maintaining
lower leaf silica levels than un-grazed plants (Bañuelos \& Obeso,
2000). Understanding how plant defence traits change with stages in
plant phenology is an interesting area of future research and may help
to untangle variable responses of plant traits to herbivory.

\emph{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.

\textbf{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.\selectlanguage{english}
\begin{longtable}[]{@{}llllllll@{}}
\toprule
\textbf{Study level} & \textbf{Study level} & \textbf{Response level} &
\textbf{Response level} & \textbf{Response level} & \textbf{Response
level} & \textbf{Response level} & \textbf{Response
level}\tabularnewline
\midrule
\endhead
Most common traits & Most common traits & Most frequently measured
traits overall & Most frequently measured traits overall & Most
frequently measured traits examining \textbf{trait response to
herbivory.} & Most frequently measured traits examining \textbf{trait
response to herbivory.} & Most frequently measured traits examining
\textbf{trait effect on herbivory.} & Most frequently measured traits
examining \textbf{trait effect on herbivory.}\tabularnewline
Vertebrate & Invertebrate & Vertebrate & Invertebrate & Vertebrate &
Invertebrate & Vertebrate & Invertebrate\tabularnewline
& & & & & & &\tabularnewline
SLA & VOCs & SLA & VOCs & SLA & VOCs & Leaf nitrogen & Leaf
hairs\tabularnewline
Plant height & SLA & Plant Height & Leaf nitrogen & LDMC & Leaf nitrogen
& Leaf fibre & LDMC\tabularnewline
Leaf nitrogen & Above-ground biomass & Leaf nitrogen & Above-ground
biomass & Leaf nitrogen & Above-ground biomass & Total phenols &
SLA\tabularnewline
LDMC & LDMC & LDMC & Leaf C: N & Plant height & Total phenols & Total
tannins & Leaf nitrogen\tabularnewline
Leaf area & Leaf nitrogen & Leaf area & LDMC & Leaf area & Total tannins
& SLA & Leaf lignin\tabularnewline
\bottomrule
\end{longtable}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image9/image9}

\end{center}
\end{figure}

\textbf{Figure 7} Effect of plant traits on vertebrate (left) and
invertebrate (right) herbivore selection, measured as a total proportion
of studies which found a positive, negative, and non-significant result.
Only traits with more than 5 responses recorded overall are included.
Traits are grouped into broad categories biochemical and morphological.
There were less than five responses recorded for phenological and
physiological traits. Numbers in black show the total number of studies.
Where there was the same total number of studies recording a positive
and non-significant result for a particular trait, only one number is
shown.

\textbf{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.

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