TheInfoList

A herbivore is an
animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All organisms are composed of cells ...

anatomically and physiologically adapted to eating
plant material stalk, showing vascular bundles, which include both phloem and xylem. Image:BrambleLeaf_CrossPolarisedLight_Diagram.jpg">250px, Detail of the vasculature of a tissue, formed of more than one cell type, found in vascular plant">tissue_(biology).h ...
, for example
foliage A leaf (plural leaves) is the principal lateral appendage of the vascular plant Vascular plants (from Latin ''vasculum'': duct), also known as Tracheophyta (the tracheophytes , from Greek τραχεῖα ἀρτηρία ''trācheia artē ...

or
marine algae Marine primary production is the chemical synthesis in the ocean of organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bond, ...
, for the main component of its diet. As a result of their plant diet, herbivorous animals typically have mouthparts adapted to rasping or grinding.
Horse The horse (''Equus ferus caballus'') is a domesticated Domestication is a sustained multi-generational relationship in which one group of organisms assumes a significant degree of influence over the reproduction and care of another group to ...

s and other herbivores have wide flat
teeth A tooth (plural teeth) is a hard, calcified Calcification is the accumulation of calcium salts in a Tissue (biology), body tissue. It normally occurs in the formation of bone, but calcium can be deposited abnormally in soft tissue,Miller, J. ...

grass Poaceae () or Gramineae () is a large and nearly ubiquitous family In , family (from la, familia) is a of people related either by (by recognized birth) or (by marriage or other relationship). The purpose of families is to maintain ...

,
tree bark Bark is the outermost layers of stems and root In vascular plants, the roots are the plant organ, organs of a plant that are modified to provide anchorage for the plant and take in water and nutrients into the plant body, which allows plant ...

, and other tough plant material. A large percentage of
herbivores A herbivore is an animal Animals (also called Metazoa) are multicellular A multicellular organism is an organism In biology, an organism () is any organic, life, living system that functions as an individual entity. All ...
have mutualistic
gut flora Gut or guts may refer to: Anatomy * Abdomen, the region of the body below the thorax but above the pelvic region * Beer gut, slang for an obese stomach * Gastrointestinal tract, the system of digestive organs in humans and other animals * Hu ...
that help them digest
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

matter, which is more difficult to digest than animal prey. This flora is made up of
cellulose Cellulose is an organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, properties, behavior ...

-digesting
protozoans Protozoa (also protozoan, plural protozoans) is an informal term for a group of single-celled eukaryote Eukaryotes () are organisms whose Cell (biology), cells have a cell nucleus, nucleus enclosed within a nuclear envelope. Eukaryotes bel ...
or
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

.

# Etymology

Herbivore is the anglicized form of a modern Latin coinage, ''herbivora'', cited in
Charles Lyell Sir Charles Lyell, 1st Baronet, (14 November 1797 – 22 February 1875) was a Scottish geologist who demonstrated the power of known natural causes in explaining the earth Earth is the third planet from the Sun and the only astro ...

's 1830 ''
Principles of Geology A principle is a proposition or value that is a guide for behavior or evaluation. In law, it is a Legal rule, rule that has to be or usually is to be followed. It can be desirably followed, or it can be an inevitable consequence of something, such ...
''.J.A. Simpson and E.S.C. Weiner, eds. (2000) ''The Oxford English Dictionary'', vol. 8, p. 155.
Richard Owen Sir Richard Owen (20 July 1804 – 18 December 1892) was an English biologist, comparative anatomist and paleontologist. Despite being a controversial figure, Owen is generally considered to have been an outstanding naturalist with a remark ...

employed the anglicized term in an 1854 work on fossil teeth and skeletons. ''Herbivora'' is derived from Latin ''herba'' 'small plant, herb'P.G.W. Glare, ed. (1990) ''The Oxford Latin Dictionary'', p. 791. and ''vora'', from ''vorare'' 'to eat, devour'.P.G.W. Glare, ed. (1990) ''The Oxford Latin Dictionary'', p. 2103.

# Definition and related terms

Herbivory is a form of
consumption Consumption may refer to: *Resource consumption *Tuberculosis, an infectious disease, historically in biology: * Consumption (ecology), receipt of energy by consuming other organisms in social sciences: * Consumption (economics), the purchasing of ...
in which an
organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological ...

principally
autotroph An autotroph or primary producer is an organism that produces complex organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bo ...
sAbraham, Martin A. A. Sustainability Science and Engineering, Volume 1. page 123. Publisher: Elsevier 2006. such as
plant Plants are predominantly photosynthetic Photosynthesis is a process used by plants and other organisms to Energy transformation, convert light energy into chemical energy that, through cellular respiration, can later be released to fuel ...

s,
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

and photosynthesizing
bacteria Bacteria (; common noun bacteria, singular bacterium) are ubiquitous, mostly free-living organisms often consisting of one Cell (biology), biological cell. They constitute a large domain (biology), domain of prokaryotic microorganisms. Typ ...

. More generally, organisms that feed on
autotroph An autotroph or primary producer is an organism that produces complex organic compound , CH4; is among the simplest organic compounds. In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen chemical bo ...
s in general are known as primary consumers. Herbivory is usually limited to animals that eat plants. Fungi, bacteria, and protists that feed on living plants are usually termed plant pathogens (plant diseases), while fungi and
microbes A microorganism, or microbe,, ''mikros'', "small") and ''organism In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical process ...
that feed on dead plants are described as
saprotroph Image:Hyphae.JPG, Mycelial cord made up of a collection of hyphae; an essential part in the process of saprotrophic nutrition, it is used for the intake of organic matter through its cell wall. The network of hyphae is referred to as a mycelium, whi ...
s. Flowering plants that obtain nutrition from other living plants are usually termed
parasitic plant A parasitic plant is a plant Plants are predominantly photosynthetic eukaryotes of the Kingdom (biology), kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; how ...

s. There is, however, no single exclusive and definitive ecological classification of consumption patterns; each textbook has its own variations on the theme.

# Evolution of herbivory

The understanding of herbivory in geological time comes from three sources: fossilized plants, which may preserve evidence of defence (such as spines), or herbivory-related damage; the observation of plant debris in fossilised ; and the construction of herbivore mouthparts. Although herbivory was long thought to be a
Mesozoic The Mesozoic Era ( ), also called the Age of Reptiles and the Age of Conifers, is the second-to-last era An era is a span of time defined for the purposes of chronology or historiography, as in the regnal eras in the history of a given monarchy ...
phenomenon, fossils have shown that plants were being consumed by arthropods within less than 20 million years after the first land plants evolved. Insects fed on the spores of early Devonian plants, and the
Rhynie chert The Rhynie chert is a Lower Devonian Sedimentary rock, sedimentary deposit exhibiting extraordinary fossil detail or completeness (a Lagerstätte). It is exposed near the village of Rhynie, Aberdeenshire, Scotland; a second unit, the Windyfield ...

also provides evidence that organisms fed on plants using a "pierce and suck" technique. During the next 75 million years, plants evolved a range of more complex organs, such as roots and seeds. There is no evidence of any organism being fed upon until the middle-late Mississippian, . There was a gap of 50 to 100 million years between the time each organ evolved and the time organisms evolved to feed upon them; this may be due to the low levels of oxygen during this period, which may have suppressed evolution. Further than their arthropod status, the identity of these early herbivores is uncertain. Hole feeding and skeletonization are recorded in the early
Permian The Permian ( ) is a geologic period The geologic time scale (GTS) is a system of chronological dating that classifies Geology, geological strata (stratigraphy) in time. It is used by geologists, paleontology, paleontologists, and other earth ...
, with surface fluid feeding evolving by the end of that period. Herbivory among four-limbed terrestrial vertebrates, the
tetrapods Tetrapods (; ) are four-limbed animals constituting the superclass Tetrapoda (). It includes extant Extant is the opposite of the word extinct Extinction is the termination of a kind of organism or of a group of kinds (taxon), usually a sp ...

, developed in the
Late Carboniferous Late may refer to: * LATE, an acronym which could stand for: ** Limbic-predominant age-related TDP-43 encephalopathy Limbic-predominant age-related TDP-43 encephalopathy (LATE) is a term that describes a prevalent condition with impaired memory ...
(307–299 million years ago). Early tetrapods were large amphibious
piscivores (''Nerodia sipedon'') eating a fish A piscivore is a carnivore, carnivorous animal that eats primarily fish. Piscivorous is equivalent to the Greek-derived word ichthyophagous. Fish were the diet of early tetrapods (amphibians); insectivory came ...
. While amphibians continued to feed on fish and insects, some reptiles began exploring two new food types, tetrapods (carnivory) and plants (herbivory). The entire
dinosaur Dinosaurs are a diverse group of reptiles of the clade Dinosauria. They first appeared during the Triassic Geological period, period, between 243 and 233.23 annum, million years ago, although the exact origin and timing of the evolution ...

order
ornithischia Ornithischia () is an extinct Order (biology), order of mainly Herbivore, herbivorous dinosaurs characterized by a pelvic structure superficially similar to that of birds. The name ''Ornithischia'', or "bird-hipped", reflects this similarity and is ...

was composed of herbivorous dinosaurs. Carnivory was a natural transition from insectivory for medium and large tetrapods, requiring minimal adaptation. In contrast, a complex set of adaptations was necessary for feeding on highly fibrous plant materials.
Arthropod An arthropod (, (gen. ποδός)) is an invertebrate animal having an exoskeleton, a Segmentation (biology), segmented body, and paired jointed appendages. Arthropods form the phylum Euarthropoda,Reference showing that Euarthropoda is a phylum: ...
s evolved herbivory in four phases, changing their approach to it in response to changing plant communities.Tetrapod herbivores made their first appearance in the fossil record of their jaws near the Permio-Carboniferous boundary, approximately 300 million years ago. The earliest evidence of their herbivory has been attributed to dental occlusion, the process in which teeth from the upper jaw come in contact with teeth in the lower jaw is present. The evolution of dental occlusion led to a drastic increase in plant food processing and provides evidence about feeding strategies based on tooth wear patterns. Examination of
phylogenetic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...

frameworks of tooth and jaw morphologes has revealed that dental occlusion developed independently in several lineages tetrapod herbivores. This suggests that evolution and spread occurred simultaneously within various lineages.

# Food chain

Herbivores form an important link in the food chain because they consume plants to digest the carbohydrates produced by a plant.
Carnivore A carnivore , meaning "meat Meat is animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotroph, consume orga ...
s in turn consume herbivores for the same reason, while
omnivore An omnivore () is an animal Animals (also called Metazoa) are organisms that form the Animalia. With few exceptions, animals , , are , can , and grow from a hollow sphere of , the , during . Over 1.5 million animal have been — ...
s can obtain their nutrients from either plants or animals. Due to a herbivore's ability to survive solely on tough and fibrous plant matter, they are termed the primary consumers in the food cycle (chain). Herbivory, carnivory, and omnivory can be regarded as special cases of
consumer–resource interactionsConsumer–resource interactions are the core motif of ecological food chains ''Food Chains'' is a 2014 American documentary film about agricultural labor in the United States directed by Sanjay Rawal. It was the Recipient of the 2015 James Beard F ...
.

# Feeding strategies

Two herbivore feeding strategies are
grazing In agriculture Agriculture is the science, art and practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentary Image:Family watching television 1958.jpg, Exercise trends, Increases in sede ...

(e.g. cows) and
browsing Browsing is a kind of orienting strategy. It is supposed to identify something of relevance for the browsing organism. When used about human beings it is a metaphor A metaphor is a figure of speech that, for rhetorical effect, directly refers ...
(e.g. moose). For a terrestrial mammal to be called a grazer, at least 90% of the forage has to be grass, and for a browser at least 90% tree leaves and twigs. An intermediate feeding strategy is called "mixed-feeding". In their daily need to take up energy from forage, herbivores of different body mass may be selective in choosing their food. "Selective" means that herbivores may choose their forage source depending on, e.g., season or food availability, but also that they may choose high quality (and consequently highly nutritious) forage before lower quality. The latter especially is determined by the body mass of the herbivore, with small herbivores selecting for high-quality forage, and with increasing body mass animals are less selective. Several theories attempt to explain and quantify the relationship between animals and their food, such as
Kleiber's lawKleiber's law, named after Max Kleiber for his biology work in the early 1930s, is the observation that, for the vast majority of animals, an animal's metabolic rate Metabolism (, from el, μεταβολή ''metabolē'', "change") is the s ...
, Holling's disk equation and the marginal value theorem (see below). Kleiber's law describes the relationship between an animal's size and its feeding strategy, saying that larger animals need to eat less food per unit weight than smaller animals. Kleiber's law states that the metabolic rate (q0) of an animal is the mass of the animal (M) raised to the 3/4 power: q0=M3/4 Therefore, the mass of the animal increases at a faster rate than the metabolic rate. Herbivores employ numerous types of feeding strategies. Many herbivores do not fall into one specific feeding strategy, but employ several strategies and eat a variety of plant parts.
Optimal Foraging Theory Optimal foraging theory (OFT) is a behavioral ecology model that helps predict how an animal behaves when searching for food. Although obtaining food provides the animal with energy, searching for and capturing the food require both energy and ti ...
is a model for predicting animal behavior while looking for food or other resources, such as shelter or water. This model assesses both individual movement, such as animal behavior while looking for food, and distribution within a habitat, such as dynamics at the population and community level. For example, the model would be used to look at the browsing behavior of a deer while looking for food, as well as that deer's specific location and movement within the forested habitat and its interaction with other deer while in that habitat. This model has been criticized as circular and untestable. Critics have pointed out that its proponents use examples that fit the theory, but do not use the model when it does not fit the reality. Other critics point out that animals do not have the ability to assess and maximize their potential gains, therefore the optimal foraging theory is irrelevant and derived to explain trends that do not exist in nature. Holling's disk equation models the efficiency at which predators consume prey. The model predicts that as the number of prey increases, the amount of time predators spend handling prey also increases, and therefore the efficiency of the predator decreases. In 1959, S. Holling proposed an equation to model the rate of return for an optimal diet: Rate (R )=Energy gained in foraging (Ef)/(time searching (Ts) + time handling (Th))
$R=Ef/\left(Ts + Th\right)$
Where s=cost of search per unit time f=rate of encounter with items, h=handling time, e=energy gained per encounter
In effect, this would indicate that a herbivore in a dense forest would spend more time handling (eating) the vegetation because there was so much vegetation around than a herbivore in a sparse forest, who could easily browse through the forest vegetation. According to the Holling's disk equation, a herbivore in the sparse forest would be more efficient at eating than the herbivore in the dense forest. The marginal value theorem describes the balance between eating all the food in a patch for immediate energy, or moving to a new patch and leaving the plants in the first patch to regenerate for future use. The theory predicts that absent complicating factors, an animal should leave a resource patch when the rate of payoff (amount of food) falls below the average rate of payoff for the entire area. According to this theory, an animal should move to a new patch of food when the patch they are currently feeding on requires more energy to obtain food than an average patch. Within this theory, two subsequent parameters emerge, the Giving Up Density (GUD) and the Giving Up Time (GUT). The Giving Up Density (GUD) quantifies the amount of food that remains in a patch when a forager moves to a new patch. The Giving Up Time (GUT) is used when an animal continuously assesses the patch quality.

# Plant-herbivore interactions

Interactions between plants and herbivores can play a prevalent role in
ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient cycles and energy flows. Energy enters the syst ...

dynamics such
community A community is a social unit The term "level of analysis" is used in the social sciences to point to the location, size, or scale of a research target. "Level of analysis" is distinct from the term "unit of observation" in that the former refer ...
structure and functional processes. Plant
diversity Diversity, diversify, or diverse may refer to: Business *Diversity (business) The "business case for diversity" stems from the progression of the models of diversity within the workplace since the 1960s. The original model for diversity was situ ...
and
distributionDistribution may refer to: Mathematics *Distribution (mathematics) Distributions, also known as Schwartz distributions or generalized functions, are objects that generalize the classical notion of functions in mathematical analysis. Distr ...
is often driven by herbivory, and it is likely that trade-offs between plant
competitiveness In economics Economics () is the social science that studies how people interact with value; in particular, the Production (economics), production, distribution (economics), distribution, and Consumption (economics), consumption of goods ...
and
defensiveness In psychoanalytic theory, a defence mechanism (American English: defense mechanism), is an Unconscious mind, unconscious psychological mechanism that reduces anxiety arising from unacceptable or potentially harmful stimuli. Defence mechanisms ...
, and between colonization and mortality allow for coexistence between species in the presence of herbivores. However, the effects of herbivory on plant diversity and richness is variable. For example, increased abundance of herbivores such as deer decrease plant diversity and
species richness File:Global Amphibian Richness Grids, 2015 Release, All Amphibians (28794889801).jpg, 300px, Global amphibian richness (2015) Species richness is the number of different species represented in an community (ecology), ecological community, landscape ...
, while other large mammalian herbivores like bison control dominant species which allows other species to flourish.  Plant-herbivore interactions can also operate so that plant communities mediate herbivore communities. Plant communities that are more diverse typically sustain greater herbivore richness by providing a greater and more diverse set of resources.
Coevolution In biology, coevolution occurs when two or more species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined ...
and
phylogenetic In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structure, Biochemistry, chemical processes, Molecular biology, molecular interactions, Physiology, physiological mechanism ...

correlation between herbivores and plants are important aspects of the influence of herbivore and plant interactions on communities and ecosystem functioning, especially in regard to herbivorous insects. This is apparent in the
adaptation In , adaptation has three related meanings. Firstly, it is the dynamic evolutionary process that fits s to their environment, enhancing their . Secondly, it is a state reached by the population during that process. Thirdly, it is a or adapti ...

s plants develop to
tolerate Toleration is the allowing, permitting, or acceptance Acceptance in human Humans (''Homo sapiens'') are the most populous and widespread species of primates, characterized by bipedality, opposable thumbs, hairlessness, and intelligence a ...
and/or defend from insect herbivory and the responses of herbivores to overcome these adaptations. The evolution of antagonistic and mutualistic plant-herbivore interactions are not mutually exclusive and may co-occur. Plant phylogeny has been found to facilitate the colonization and community assembly of herbivores, and there is evidence of phylogenetic linkage between plant
beta diversity In ecology, beta diversity (β-diversity or true beta diversity) is the ratio between regional and local species diversity. The term was introduced by Robert Whittaker (ecologist), R. H. Whittaker together with the terms alpha diversity (α-diversit ...
and phylogenetic
beta diversity In ecology, beta diversity (β-diversity or true beta diversity) is the ratio between regional and local species diversity. The term was introduced by Robert Whittaker (ecologist), R. H. Whittaker together with the terms alpha diversity (α-diversit ...
of insect
clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyly, monophyletic – that is, composed of a common ancestor and all its lineage (evolution), lineal descendants - on a phylogenetic tree. R ...

s such as
butterflies Butterflies are insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''communicare'', m ...

. These types of eco-evolutionary feedbacks between plants and herbivores are likely the main driving force behind plant and herbivore diversity. Abiotic factors such as
climate Climate is the long-term pattern of weather Weather is the state of the atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a la ...

and also impact plant-herbivore communities and interactions. For example, in temperate freshwater wetlands herbivorous waterfowl communities change according to season, with species that eat above-ground vegetation being abundant during summer, and species that forage below-ground being present in winter months. These seasonal herbivore communities differ in both their assemblage and functions within the . Such differences in herbivore modalities can potentially lead to trade-offs that influence species traits and may lead to additive effects on
community A community is a social unit The term "level of analysis" is used in the social sciences to point to the location, size, or scale of a research target. "Level of analysis" is distinct from the term "unit of observation" in that the former refer ...
composition and ecosystem functioning. Seasonal changes and environmental gradients such as elevation and
latitude In geography Geography (from Greek: , ''geographia'', literally "earth description") is a field of science devoted to the study of the lands, features, inhabitants, and phenomena of the Earth and planets. The first person to use the ...
often affect the
palatability Palatability is the hedonic reward (i.e., pleasure Pleasure refers to experience that feels good, that involves the enjoyment of something. It contrasts with pain Pain is a distressing feeling often caused by intense or damaging stimuli. ...
of plants which in turn influences herbivore community assemblages and vice versa. Examples include a decrease in abundance of leaf-chewing larvae in the fall when hardwood leaf palatability decreases due to increased
tannin Tannins (or tannoids) are a class of , ic s that bind to and s and various other organic compounds including s and s. The term ''tannin'' (from ''tanner'', from ''tannāre'', from ''tannum'', ) refers to the use of oak and other bark ...
levels which results in a decline of arthropod
species richness File:Global Amphibian Richness Grids, 2015 Release, All Amphibians (28794889801).jpg, 300px, Global amphibian richness (2015) Species richness is the number of different species represented in an community (ecology), ecological community, landscape ...
, and increased palatability of plant communities at higher elevations where
grasshopper Grasshoppers are a group of insect Insects (from Latin Latin (, or , ) is a classical language A classical language is a language A language is a structured system of communication Communication (from Latin ''comm ...

s abundances are lower. Climatic stressors such as
ocean acidification Ocean acidification is the ongoing decrease in the pH value of the Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of cont ...
can lead to responses in plant-herbivore interactions in relation to palatability as well.

## Herbivore offense

The myriad defenses displayed by plants means that their herbivores need a variety of skills to overcome these defenses and obtain food. These allow herbivores to increase their feeding and use of a host plant. Herbivores have three primary strategies for dealing with plant defenses: choice, herbivore modification, and plant modification. Feeding choice involves which plants a herbivore chooses to consume. It has been suggested that many herbivores feed on a variety of plants to balance their nutrient uptake and to avoid consuming too much of any one type of defensive chemical. This involves a tradeoff however, between foraging on many plant species to avoid toxins or specializing on one type of plant that can be detoxified. Herbivore modification is when various adaptations to body or digestive systems of the herbivore allow them to overcome plant defenses. This might include detoxifying secondary metabolites, sequestering toxins unaltered, or avoiding toxins, such as through the production of large amounts of saliva to reduce effectiveness of defenses. Herbivores may also utilize symbionts to evade plant defenses. For example, some aphids use bacteria in their gut to provide essential amino acids lacking in their sap diet. Plant modification occurs when herbivores manipulate their plant prey to increase feeding. For example, some caterpillars roll leaves to reduce the effectiveness of plant defenses activated by sunlight.

## Plant defense

A plant defense is a trait that increases plant fitness when faced with herbivory. This is measured relative to another plant that lacks the defensive trait. Plant defenses increase survival and/or reproduction (fitness) of plants under pressure of predation from herbivores. Defense can be divided into two main categories, tolerance and resistance. Tolerance is the ability of a plant to withstand damage without a reduction in fitness. This can occur by diverting herbivory to non-essential plant parts, resource allocation, compensatory growth, or by rapid regrowth and recovery from herbivory. Resistance refers to the ability of a plant to reduce the amount of damage it receives from herbivores. This can occur via avoidance in space or time, physical defenses, or chemical defenses. Defenses can either be constitutive, always present in the plant, or induced, produced or translocated by the plant following damage or stress. Physical, or mechanical, defenses are barriers or structures designed to deter herbivores or reduce intake rates, lowering overall herbivory. Thorns such as those found on roses or acacia trees are one example, as are the spines on a cactus. Smaller hairs known as
trichomes Trichomes ( or ), from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its populatio ...

may cover leaves or stems and are especially effective against invertebrate herbivores. In addition, some plants have
waxes Waxes are a diverse class of organic compound In chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, prope ...
or
resins In polymer chemistry Polymer chemistry is a sub-discipline of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, stru ...
that alter their texture, making them difficult to eat. Also the incorporation of silica into cell walls is analogous to that of the role of lignin in that it is a compression-resistant structural component of cell walls; so that plants with their cell walls impregnated with silica are thereby afforded a measure of protection against herbivory. Chemical defenses are
secondary metabolites Secondary metabolites, also called specialised metabolite In biochemistry Biochemistry or biological chemistry, is the study of chemical process In a scientific Science (from the Latin Latin (, or , ) is a classical language bel ...
produced by the plant that deter herbivory. There are a wide variety of these in nature and a single plant can have hundreds of different chemical defenses. Chemical defenses can be divided into two main groups, carbon-based defenses and nitrogen-based defenses. #Carbon-based defenses include
terpenes Terpenes () are a class of natural product A natural product is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than ...
and . Terpenes are derived from 5-carbon isoprene units and comprise essential oils, carotenoids, resins, and latex. They can have several functions that disrupt herbivores such as inhibiting formation, molting
hormones A hormone (from the Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is appr ...
, or the nervous system. Phenolics combine an aromatic carbon ring with a hydroxyl group. There are several different phenolics such as lignins, which are found in cell walls and are very indigestible except for specialized microorganisms;
tannins Tannins (or tannoids) are a class of astringent 200px, A crystal of the astringent alum An astringent (sometimes called adstringent) is a chemical that shrinks or constricts body tissue In biology Biology is the natural scie ...
, which have a bitter taste and bind to proteins making them indigestible; and furanocumerins, which produce free radicals disrupting DNA, protein, and lipids, and can cause skin irritation. #Nitrogen-based defenses are synthesized from amino acids and primarily come in the form of
alkaloids Alkaloids are a class of basic BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of General-purpose programming language, general-purpose, high-level programming languages whose design philosophy emphasizes ease of use. The ...

and cyanogens. Alkaloids include commonly recognized substances such as
caffeine Caffeine is a central nervous system The central nervous system (CNS) is the part of the nervous system In biology Biology is the natural science that studies life and living organisms, including their anatomy, physical structu ...

,
nicotine Nicotine is a naturally produced alkaloid Alkaloids are a class of basic BASIC (Beginners' All-purpose Symbolic Instruction Code) is a family of general-purpose, high-level programming language In computer science Computer sci ...

, and
morphine Morphine is a of the family that is found naturally in a dark brown, resinous form, from the poppy plant ('). It can be taken orally or injected. It acts directly on the (CNS) to induce analgesia and alter perception and emotional respons ...

. These compounds are often bitter and can inhibit DNA or RNA synthesis or block nervous system signal transmission. Cyanogens get their name from the
cyanide A cyanide is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical element, element held together by che ...

stored within their tissues. This is released when the plant is damaged and inhibits cellular respiration and electron transport. Plants have also changed features that enhance the probability of attracting natural enemies to herbivores. Some emit semiochemicals, odors that attract natural enemies, while others provide food and housing to maintain the natural enemies' presence, e.g.
ant Ants are eusocial Eusociality (from Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population i ...

s that reduce herbivory. A given plant species often has many types of defensive mechanisms, mechanical or chemical, constitutive or induced, which allow it to escape from herbivores.

## Predator–Prey Theory

According to the theory of
predator Predation is a biological interaction In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical en ...

–prey interactions, the relationship between herbivores and plants is cyclic. When prey (plants) are numerous their predators (herbivores) increase in numbers, reducing the prey population, which in turn causes predator number to decline. The prey population eventually recovers, starting a new cycle. This suggests that the population of the herbivore fluctuates around the carrying capacity of the food source, in this case, the plant. Several factors play into these fluctuating populations and help stabilize predator-prey dynamics. For example, spatial heterogeneity is maintained, which means there will always be pockets of plants not found by herbivores. This stabilizing dynamic plays an especially important role for specialist herbivores that feed on one species of plant and prevents these specialists from wiping out their food source. Prey defenses also help stabilize predator-prey dynamics, and for more information on these relationships see the section on Plant Defenses. Eating a second prey type helps herbivores' populations stabilize.Smith and Smith, 2001 Alternating between two or more plant types provides population stability for the herbivore, while the populations of the plants oscillate. This plays an important role for generalist herbivores that eat a variety of plants. Keystone herbivores keep vegetation populations in check and allow for a greater diversity of both herbivores and plants. When an invasive herbivore or plant enters the system, the balance is thrown off and the diversity can collapse to a monotaxon system. The back and forth relationship of plant defense and herbivore offense drives
coevolution In biology, coevolution occurs when two or more species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined ...
between plants and herbivores, resulting in a "coevolutionary arms race". The escape and radiation mechanisms for coevolution, presents the idea that adaptations in herbivores and their host plants, has been the driving force behind
speciation Speciation is the evolutionary process by which populations evolve to become distinct species. The biologist Orator F. Cook coined the term in 1906 for cladogenesis, the splitting of lineages, as opposed to anagenesis, phyletic evolution within ...

.Thompson, J. 1999. What we know and do not know about coevolution: insect herbivores and plants as a test case. Pages 7–30 in H. Olff, V. K. Brown, R. H. Drent, and British Ecological Society Symposium 1997 (Corporate Author), editors. Herbivores: between plants and predators. Blackwell Science, London, UK.

## Mutualism

While much of the interaction of herbivory and plant defense is negative, with one individual reducing the fitness of the other, some is beneficial. This beneficial herbivory takes the form of mutualisms in which both partners benefit in some way from the interaction.
Seed dispersal Seed dispersal is the movement, spread or transport of seed A seed is an embryonic ''Embryonic'' is the twelfth studio album by experimental rock band the Flaming Lips released on October 13, 2009, on Warner Bros. Records, Warner Bros. The ban ...
by herbivores and
pollination Pollination is the transfer of pollen Pollen is a powdery substance consisting of pollen grains which are Sporophyte, microsporophytes of spermatophyta, seed plants, which produce male gametes (sperm cells). Pollen grains have a hard coat ...

are two forms of mutualistic herbivory in which the herbivore receives a food resource and the plant is aided in reproduction. Plants can also be indirectly affected by herbivores through , with plants benefiting from herbivores when nutrients are recycled very efficiently. Another form of plant-herbivore mutualism is physical changes to the environment and/or plant community structure by herbivores which serve as
ecosystem engineer An ecosystem engineer is any species In biology, a species is the basic unit of biological classification, classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest gr ...
s, such as wallowing by bison. Swans form a mutual relationship with the plant species that they
forage Forage is a plant material (mainly plant leaves and stems) eaten by grazing In agriculture Agriculture is the science, art and practice of cultivating plants and livestock. Agriculture was the key development in the rise of sedentism, se ...

by digging and disturbing the sediment which removes competing plants and subsequently allows colonization of other plant species.

# Impacts

When herbivores are affected by
trophic cascadeTrophic cascades are powerful indirect interactions that can control entire ecosystems, occurring when a trophic level in a food web is suppressed. For example, a top-down cascade will occur if predators are effective enough in predation to reduce th ...

s, plant communities can be indirectly affected. Often these effects are felt when predator populations decline and herbivore populations are no longer limited, which leads to intense herbivore foraging which can suppress plant communities. Environmental degradation from
white-tailed deer The white-tailed deer (''Odocoileus virginianus''), also known as the whitetail or Virginia deer, is a medium-sized deer Deer or true deer are ed s forming the Cervidae. The two main groups of deer are the , including the , the (wapit ...

(''Odocoileus virginianus'') in the US alone has the potential to both change vegetative communities through over-browsing and cost
forest restoration Forest restoration is defined as “actions to re-instate ecological processes, which accelerate recovery of forest structure, ecological functioning and biodiversity levels towards those typical of climax forest In ecology, scientific ecology, ...
projects upwards of \$750 million annually. Another example of a trophic cascade involved plant-herbivore interactions are
coral reef A coral reef is an underwater ecosystem An ecosystem (or ecological system) consists of all the organisms and the physical environment with which they interact. These biotic and abiotic components are linked together through nutrient c ...

ecosystems. Herbivorous fish and marine animals are important
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic Photosynthesis is a process used by plants and other organisms to convert Conversion or convert may refer to: Arts, entertainment, and media * Co ...

and seaweed grazers, and in the absence of plant-eating fish, corals are outcompeted and seaweeds deprive corals of sunlight.

## Economic Impacts

Agricultural crop damage by the same species totals approximately \$100 million every year. Insect crop damages also contribute largely to annual crop losses in the U.S.An Integrated Approach To Deer Damage Control Publication No. 809 West Virginia Division of Natural Resources Cooperative Extension Service, Wildlife Resources Section West Virginia University, Law Enforcement Section Center for Extension and Continuing Education, March 1999 Herbivores also affect economics through the revenue generated by hunting and ecotourism. For example, the hunting of herbivorous game species such as white-tailed deer, cottontail rabbits, antelope, and elk in the U.S. contributes greatly to the billion-dollar annually, hunting industry.
Ecotourism Ecotourism is a form of tourism at the archaeological site of Chichén Itza. in Vienna. Tourism is travel for pleasure or business; also the theory and practice of touring (disambiguation), touring, the business of attracting, accommodating ...
is a major source of revenue, particularly in Africa, where many large mammalian herbivores such as elephants, zebras, and giraffes help to bring in the equivalent of millions of US dollars to various nations annually.

* Consumer-resource systems *
List of feeding behaviours Feeding is the process by which organisms, typically animal Animals (also called Metazoa) are multicellular eukaryotic organisms that form the Kingdom (biology), biological kingdom Animalia. With few exceptions, animals Heterotro ...
* List of herbivorous animals *
Plant-based diet A plant-based diet or a plant-rich diet is a diet consisting mostly or entirely of plant-based foods. Plant-based foods are foods derived from plants (including vegetables, grains, nuts, seeds, legumes, and fruits) with no animal-source foods or ...
*
Productivity (ecology) In ecology Ecology (from el, οἶκος, "house" and el, -λογία, label=none, "study of") is the study of the relationships between living organisms, including humans, and their physical environment. Ecology considers organisms ...
*
Seed predation Image:Aardbei muizenschade.jpg, A strawberry aggregate fruit, aggregate accessory fruit damaged by a mouse eating the seeds (achenes). Seed predation, often referred to as granivory, is a type of plant-animal interaction in which granivores (seed ...
*
Tritrophic interactions in plant defense Tritrophic interactions in plant defense against herbivory describe the ecological impacts of three trophic levels on each other: the plant, the herbivore, and its natural enemies. They may also be called multitrophic interactions when further tro ...
* Herbivore men