View source for Herbivore

{{short description|Organism that eats mostly or exclusively plant material}}
{{About|anatomically and physiologically adapted diets to plants|the Japanese social phenomenon|Herbivore men}}
{{pp-vandalism|small=yes}}
[[File:White-tailed deer (Odocoileus virginianus) grazing - 20050809.jpg|thumb|250px|A [[deer]] and two fawns feeding on foliage]]
[[File:Caterpillar feeding on leaf - 20140906 121127 (cropped).jpg|thumb|250px|A [[sawfly]] larva<!--''[[Nematus ribesii]]''--> feeding on a leaf]]
[[File:Land_Snail_radula_tracks.jpg#|thumb|250px|Tracks made by terrestrial gastropods with their [[radula]]s, scraping [[green algae]] from a surface inside a greenhouse]]

A '''herbivore''' is an [[animal]] anatomically and physiologically [[evolution|evolve]]d to feed on [[plant]]s, especially upon [[vascular tissue]]s such as [[foliage]], [[fruit]]s or [[seed]]s, as the main component of its [[diet (nutrition)|diet]]. These more broadly also encompass animals that eat [[non-vascular plant|non-vascular]] [[autotroph]]s such as [[moss]]es, [[algae]] and [[lichen]]s, but do not include those feeding on [[decomposition|decompose]]d [[detritus|plant matter]]s (i.e. [[detritivore]]s) or [[macrofungi]] (i.e. [[fungivore]]s).

As a result of their plant-based diet, herbivorous animals typically have [[mouth]] structures ([[jaw]]s or [[arthropod mouthparts|mouthparts]]) well [[adaptation|adapt]]ed to [[comminution|mechanically break down]] plant materials, and their [[digestive system]]s have special [[enzyme]]s (e.g. [[amylase]] and [[cellulase]]) to digest [[polysaccharide]]s. [[Grazing (behaviour)|Grazing]] herbivores such as [[horse]]s and [[cattle]]s have wide flat-[[crown (tooth)|crown]]ed [[teeth]] that are better adapted for grinding [[grass]], [[tree bark]] and other tougher [[lignin]]-containing materials, and many of them evolved [[ruminant|rumination]] or [[cecotrope|cecotropic]] behaviors to better extract nutrients from plants. A large percentage of herbivores also have mutualistic [[gut flora]] made up of [[bacteria]] and [[protozoan]]s that help to [[biodegradation|degrade]] the [[cellulose]] in plants,<ref>"symbiosis." The Columbia Encyclopedia. New York: Columbia University Press, 2008. Credo Reference. Web. 17 September 2012.</ref> whose heavily [[cross-link]]ing [[polymer]] structure makes it far more difficult to digest than the [[protein]]- and [[fat]]-rich animal [[tissue (biology)|tissue]]s that [[carnivore]]s eat.<ref name="Moran 2006">{{cite journal |last= Moran |first= N.A. |year= 2006 |title= Symbiosis |journal=Current Biology |volume= 16 |issue= 20 |pages= 866–871 |doi= 10.1016/j.cub.2006.09.019 |pmid= 17055966|doi-access= free |bibcode= 2006CBio...16.R866M }}</ref>

==Etymology==
Herbivore is the anglicized form of a modern Latin coinage, ''herbivora'', cited in [[Charles Lyell]]'s 1830 ''[[Principles of Geology]]''.<ref name=OED>J.A. Simpson and E.S.C. Weiner, eds. (2000) ''The Oxford English Dictionary'', vol. 8, p. 155.</ref> [[Richard Owen]] employed the anglicized term in an 1854 work on fossil teeth and skeletons.<ref name="OED"/> ''Herbivora'' is derived from Latin ''herba'' 'small plant, herb'<ref name=OLD1>P.G.W. Glare, ed. (1990) ''The Oxford Latin Dictionary'', p. 791.</ref> and ''vora'', from ''vorare'' 'to eat, devour'.<ref name=OLD2>P.G.W. Glare, ed. (1990) ''The Oxford Latin Dictionary'', p. 2103.</ref>

==Definition and related terms==
Herbivory is a form of [[heterotroph|consumption]] in which an [[organism]] principally [[eating|eats]] [[autotroph]]s<ref name=MAAA>Abraham, Martin A. A. Sustainability Science and Engineering, Volume 1. page 123. Publisher: Elsevier 2006. {{ISBN|978-0444517128}}</ref> such as [[plant]]s, [[algae]] and photosynthesizing [[bacteria]]. More generally, organisms that feed on [[autotroph]]s in general are known as '''primary consumers'''.
Herbivory is usually limited to animals that eat plants. Insect herbivory can cause a variety of physical and metabolic alterations in the way the host plant interacts with itself and other surrounding biotic factors.<ref>{{Cite journal |last1=Peschiutta |first1=María Laura |last2=Scholz |first2=Fabián Gustavo |last3=Goldstein |first3=Guillermo |last4=Bucci |first4=Sandra Janet |date=2018-01-01 |title=Herbivory alters plant carbon assimilation, patterns of biomass allocation and nitrogen use efficiency |url=https://www.sciencedirect.com/science/article/pii/S1146609X17301972 |journal=Acta Oecologica |language=en |volume=86 |pages=9–16 |doi=10.1016/j.actao.2017.11.007 |bibcode=2018AcO....86....9P |issn=1146-609X|hdl=11336/86217 |hdl-access=free }}</ref><ref>{{Cite journal |last1=Cannicci |first1=Stefano |last2=Burrows |first2=Damien |last3=Fratini |first3=Sara |last4=Smith |first4=Thomas J. |last5=Offenberg |first5=Joachim |last6=Dahdouh-Guebas |first6=Farid |date=2008-08-01 |title=Faunal impact on vegetation structure and ecosystem function in mangrove forests: A review |url=https://www.sciencedirect.com/science/article/pii/S030437700800017X |journal=Aquatic Botany |series=Mangrove Ecology – Applications in Forestry and Costal Zone Management |language=en |volume=89 |issue=2 |pages=186–200 |doi=10.1016/j.aquabot.2008.01.009 |bibcode=2008AqBot..89..186C |issn=0304-3770}}</ref> Fungi, bacteria, and protists that feed on living plants are usually termed [[phytopathology|plant pathogens]] (plant diseases), while fungi and [[microbes]] that feed on dead plants are described as [[saprotroph]]s. Flowering plants that obtain nutrition from other living plants are usually termed [[parasitic plant]]s. There is, however, no single exclusive and definitive ecological classification of consumption patterns; each textbook has its own variations on the theme.<ref>Thomas, Peter & Packham, John. Ecology of Woodlands and Forests: Description, Dynamics and Diversity. Publisher: Cambridge University Press 2007. {{ISBN|978-0521834520}}</ref><ref>Sterner, Robert W.; Elser, James J.; and Vitousek, Peter. Ecological Stoichiometry: The Biology of Elements from Molecules to the Biosphere. Publisher: Princeton University Press 2002. {{ISBN|978-0691074917}}</ref><ref>Likens Gene E. Lake Ecosystem Ecology: A Global Perspective. Publisher: Academic Press 2010. {{ISBN|978-0123820020}}</ref>

==Evolution of herbivory{{anchor|evolution}}==
[[File:Viburnum lesquereuxii fossil.jpg|thumb|right|A [[fossil]] ''Viburnum lesquereuxii'' leaf with evidence of insect herbivory; Dakota Sandstone ([[Cretaceous]]) of Ellsworth County, Kansas. Scale bar is 10 mm.]]
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 [[coprolite|animal faeces]]; and the construction of herbivore mouthparts.<ref name=Labandeira1998>{{Cite journal
 | author = Labandeira, C.C.
 | year = 1998
 | title = Early History of Arthropod And Vascular Plant Associations 1
 | journal = Annual Review of Earth and Planetary Sciences
 | volume = 26
 | issue = 1
 | pages = 329–377
 | doi = 10.1146/annurev.earth.26.1.329
| bibcode = 1998AREPS..26..329L
 }}</ref>

Although herbivory was long thought to be a [[Mesozoic]] phenomenon, fossils have shown that plants were being consumed by arthropods within less than 20&nbsp;million years after the first land plants evolved.<ref name="Labandeira2007">{{Cite journal
 |author = Labandeira, C.|date = June 2007|title = The origin of herbivory on land: Initial patterns of plant tissue consumption by arthropods|journal = Insect Science|volume = 14|issue = 4|pages = 259–275|doi = 10.1111/j.1744-7917.2007.00141.x-i1| bibcode=2007InsSc..14..259L |s2cid = 86335068}}</ref> Insects fed on the spores of early Devonian plants, and the [[Rhynie chert]] also provides evidence that organisms fed on plants using a "pierce and suck" technique.<ref name=Labandeira1998/>

During the next 75&nbsp;million years{{Citation needed|date=August 2008}}, 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 age|Mississippian]], {{Ma|late Mississippian}}. There was a gap of 50 to 100&nbsp;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.<ref name="Labandeira2007" /> Further than their arthropod status, the identity of these early herbivores is uncertain.<ref name="Labandeira2007" /> Hole feeding and skeletonization are recorded in the early [[Permian]], with surface fluid feeding evolving by the end of that period.<ref name="Labandeira1998" />

Herbivory among four-limbed terrestrial vertebrates, the [[tetrapods]], developed in the [[Late Carboniferous]] (307–299&nbsp;million years ago).<ref name="SahneyBentonFerry2010RainforestCollapse">{{cite journal |author=Sahney |first1=Sarda |last2=Benton |first2=Michael J. |last3=Falcon-Lang |first3=Howard J. |year=2010 |title=Rainforest collapse triggered Pennsylvanian tetrapod diversification in Euramerica |url=http://geology.geoscienceworld.org/cgi/content/abstract/38/12/1079 |journal=Geology |volume=38 |issue=12 |pages=1079–1082 |bibcode=2010Geo....38.1079S |doi=10.1130/G31182.1}}</ref> The oldest known herbivorous tetrapod is [[Desmatodon]] hesperis, belonging to the clade [[Diadectidae]]. Early [[Anamniotes|anamniotic]] tetrapods were feeding on other aquatic vertebrates and invertebrates,<ref>[https://books.google.no/books?id=9f7rafLJbWUC&pg=PA148&dq=Late+Frasnian+bimodal+invertebrates+aquatic&hl=no&newbks=1&newbks_redir=0&sa=X&ved=2ahUKEwir-rjsteGLAxX2U1UIHbe5AQIQ6AF6BAgMEAM#v=onepage&q=Late%20Frasnian%20bimodal%20invertebrates%20aquatic&f=false Amniote Origins: Completing the Transition to Land]</ref> and some amphibians would also evolve into terrestrial  carnivores.<ref>[https://phys.org/news/2010-03-fossil-amphibian-earliest-widespread-evidence.html New fossil amphibian provides earliest widespread evidence of terrestrial invertebrates]</ref> The first [[amniote]]s were [[insectivore]]s due to their small size,<ref>[https://books.google.no/books?id=sENNCwAAQBAJ&pg=PA247&dq=first+amniotes+insectivorous+terrestrial&hl=no&newbks=1&newbks_redir=0&sa=X&ved=2ahUKEwi41LvQv-GLAxUHFRAIHRfIBqA4ChDoAXoECAwQAw#v=onepage&q=first%20amniotes%20insectivorous%20terrestrial&f=false The Bare Bones: An Unconventional Evolutionary History of the Skeleton]</ref> but later forms were larger and carnivious, before some eventually adated to a herbivorous lifestyle.<ref>[https://web.archive.org/web/20150421102250/https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0094518 The Oldest Caseid Synapsid from the Late Pennsylvanian of Kansas, and the Evolution of Herbivory in Terrestrial Vertebrates]</ref><ref>[https://www.nature.com/articles/s42003-024-05879-2 Predatory synapsid ecomorphology signals growing dynamism of late Palaeozoic terrestrial ecosystems]</ref> In addiction to Diadectidae, four other tetrapod clades independently evolved adaptations towards herbivory from the Late Carboniferous to early Permian; the [[Captorhinidae]], the parareptilian [[Bolosauridae]], and the two synapsid clades [[Edaphosauridae]] and [[Caseidae]]. The entire [[dinosaur]] order [[ornithischia]], which existed during Jurassic and Cretaceous, was composed of herbivorous dinosaurs.<ref name="SahneyBentonFerry2010RainforestCollapse" /><ref>[https://royalsocietypublishing.org/doi/10.1098/rsos.231566 A comprehensive phylogeny and revised taxonomy of Diadectomorpha with a discussion on the origin of tetrapod herbivory]</ref> 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.<ref name="SahneyBentonFerry2010RainforestCollapse"/>

[[Arthropod]]s evolved herbivory in four phases, changing their approach to it in response to changing plant communities.<ref name=Labandeira2006>{{Cite journal
 |author=Labandeira, C.C. 
 |year=2005 
 |title=The four phases of plant-arthropod associations in deep time 
 |journal=Geologica Acta 
 |volume=4 
 |issue=4 
 |pages=409–438 
 |url=http://www.geologica-acta.com:8080/geoacta/pdf/vol0404a01.pdf 
 |format=Free full text 
 |access-date=15 May 2008 
 |url-status=dead 
 |archive-url=https://web.archive.org/web/20080626205148/http://www.geologica-acta.com:8080/geoacta/pdf/vol0404a01.pdf 
 |archive-date=26 June 2008 
}}</ref> Tetrapod herbivores made their first appearance in the fossil record of their jaws near the Permio-Carboniferous boundary, approximately 300&nbsp;million years ago. The earliest evidence of their herbivory has been attributed to [[Occlusion (dentistry)|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]] 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.<ref>{{citation|doi=10.1002/jez.b.21115|pmid=16683226|title=Origin of dental occlusion in tetrapods: Signal for terrestrial vertebrate evolution?|journal=Journal of Experimental Zoology Part B: Molecular and Developmental Evolution|volume=306B|issue=3|pages=261–277|year=2006|last1=Reisz|first1=Robert R.|bibcode=2006JEZB..306..261R }}</ref>

==Food chain==
[[File:Leaf mining.jpg|thumb|[[Leaf miner]]s feed on leaf tissue between the epidermal layers, leaving visible trails]]
Herbivores form an important link in the food chain because they consume plants to digest the carbohydrates [[photosynthesis|photosynthetically]] produced by a plant. [[Carnivore]]s in turn consume herbivores for the same reason, while [[omnivore]]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 interactions]].<ref>{{cite journal | last1 = Getz | first1 = W | date = February 2011 | title = Biomass transformation webs provide a unified approach to consumer-resource modelling | journal = Ecology Letters | volume =  14| issue = 2| pages =  113–124| doi = 10.1111/j.1461-0248.2010.01566.x | pmid=21199247 | pmc=3032891| bibcode = 2011EcolL..14..113G }}</ref>

==Feeding strategies==

Two herbivore feeding strategies are [[grazing]] (e.g. cows) and [[browsing (herbivory)|browsing]] (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".<ref name="Janis (1990)">{{Cite book |author=Janis |first=Christine M. |title=Body Size in Mammalian Paleobiology: Estimation and Biological Implications |publisher=Cambridge University Press |year=1990 |isbn=0-521-36099-4 |editor1-last=Damuth |editor-first=John |pages=255–299 |chapter=Chapter 13: Correlation of cranial and dental variables with body size in ungulates and macropodoids |editor2-last=MacFadden |editor-first2=Bruce J.}}</ref> In their daily need to take up energy from forage, herbivores of different body mass may be selective in choosing their food.<ref name="Belovsky (1997)">{{Cite journal | author=Belovsky, G.E. | date=November 1997 | title=Optimal foraging and community structure: The allometry of herbivore food selection and competition | journal=Evolutionary Ecology | volume=11 | issue=6 | pages=641–672 | doi= 10.1023/A:1018430201230| bibcode=1997EvEco..11..641B | s2cid=23873922 }}</ref> "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.<ref name="Belovsky (1997)" /> Several theories attempt to explain and quantify the relationship between animals and their food, such as [[Kleiber's law]], 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.<ref name=":6">{{cite journal |last1=Nugent |first1=G. |last2=Challies |first2=C. N. |date=1988 |title=Diet and Food Preferences of White-Tailed Deer in Northeastern Stewart-Island |url=https://newzealandecology.org/nzje/1765 |journal=New Zealand Journal of Ecology |volume=11 |pages=61–73}}</ref> Kleiber's law states that the metabolic rate (q<sub>0</sub>) of an animal is the mass of the animal (M) raised to the 3/4 power: q<sub>0</sub>=M<sup>3/4</sup>

Therefore, the mass of the animal increases at a faster rate than the metabolic rate.<ref name=":6" />

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.

{| class="wikitable" style="margin:auto;"
|+ Types of feeding strategies
|-
! Feeding Strategy
! Diet
! Examples
|-
| [[Algivore]]s
| [[Algae]]
| [[Krill]], [[crabs]], [[sea snail]], [[sea urchin]], [[parrotfish]], [[surgeonfish]], [[flamingo]]
|-
| [[Frugivore]]s
| [[Fruit]]
| [[Ruffed lemur]]s, [[orangutan]]s
|-
| [[Folivore]]s
| [[Leaf|Leaves]]
| [[Koala]]s, [[gorilla]]s, [[red colobus]]es, many [[leaf beetle]]s
|-
| [[Nectarivore]]s
| [[Nectar]]
| [[Honey possum]]s, [[hummingbird]]s
|-
| [[Granivore]]s
| [[Seed]]s
| [[Hawaiian honeycreeper]]s, [[bean weevil]]s
|-
| [[Graminivore]]s
| [[Poaceae|Grass]]
| [[Horse]]s
|-
| [[Palynivore]]s
| [[Pollen]]
| [[Bee]]s
|-
| [[Mucivore]]s
| Plant fluids, i.e. [[sap]]
| [[Aphid]]s
|-
| [[Xylophage]]s
| [[Wood]]
| [[Termite]]s, [[Longhorn beetle|longicorn beetles]], [[ambrosia beetle]]s
|}

[[Optimal foraging theory]] 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.<ref>{{cite journal|last1=Kie|first1=John G.|title=Optimal Foraging & Risk of Predation: Effects on Behavior & Social Structure in Ungulates|date=1999|journal=Journal of Mammalogy|doi=10.2307/1383163|volume=80|issue=4|pages=1114–1129|jstor=1383163|doi-access=free}}</ref>

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.<ref>{{cite journal | last1=Pierce | first1=G. J. | last2=Ollason | first2=J. G. | date=May 1987 | title=Eight reasons why optimal foraging theory is a complete waste of time | journal=Oikos | volume=49 | issue=1| pages=111–118 | doi=10.2307/3565560| jstor=3565560 | bibcode=1987Oikos..49..111P | s2cid=87270733 }}</ref><ref>{{cite journal | last1=Stearns | first1=S. C. | last2=Schmid-Hempel | first2=P. | date=May 1987 | title=Evolutionary insights should not be wasted | journal=Oikos | volume=49 | issue=1| pages=118–125 | doi=10.2307/3565561| jstor=3565561 | bibcode=1987Oikos..49..118S }}</ref> 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.<ref>{{cite journal | last1=Lewis | first1=A. C. | date=16 May 1986 | title=Memory constraints and flower choice in Pieris rapae | journal=Science | volume=232 | issue=4752| pages=863–865 | doi=10.1126/science.232.4752.863 | pmid=17755969 | bibcode=1986Sci...232..863L | s2cid=20010229 }}</ref><ref>{{cite journal | last1=Janetos | first1=A. C. | last2=Cole | first2=B. J. | date=October 1981 | title=Imperfectly optimal animals | journal=Behav. Ecol. Sociobiol. | volume=9 | issue=3| pages=203–209 | doi=10.1007/bf00302939| bibcode=1981BEcoS...9..203J | s2cid=23501715 }}</ref>

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.<ref>{{Cite book |last=Stephens |first=David W. |url=https://www.worldcat.org/oclc/13902831 |title=Foraging theory |date=1986 |others=J. R. Krebs |isbn=0-691-08441-6 |location=Princeton, New Jersey |oclc=13902831}}</ref>{{page needed|date=August 2015}} 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))<br /><math>R=Ef/(Ts + Th) </math>

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.<ref>{{Cite journal |last=Charnov |first=Eric L. |date=1976-04-01 |title=Optimal foraging, the marginal value theorem |url=https://digitalrepository.unm.edu/cgi/viewcontent.cgi?article=1008&context=biol_fsp |journal=Theoretical Population Biology |language=en |volume=9 |issue=2 |pages=129–136 |doi=10.1016/0040-5809(76)90040-X |pmid=1273796 |bibcode=1976TPBio...9..129C |issn=0040-5809}}</ref> 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.<ref>{{Cite journal |last1=Brown |first1=Joel S. |last2=Kotler |first2=Burt P. |last3=Mitchell |first3=William A. |date=1997-11-01 |title=Competition between birds and mammals: A comparison of giving-up densities between crested larks and gerbils |url=https://doi.org/10.1023/A:1018442503955 |journal=Evolutionary Ecology |language=en |volume=11 |issue=6 |pages=757–771 |doi=10.1023/A:1018442503955 |bibcode=1997EvEco..11..757B |s2cid=25400875 |issn=1573-8477}}</ref> The Giving Up Time (GUT) is used when an animal continuously assesses the patch quality.<ref>{{Cite journal |last1=Breed |first1=Michael D. |last2=Bowden |first2=Rachel M. |last3=Garry |first3=Melissa F. |last4=Weicker |first4=Aric L. |date=1996-09-01 |title=Giving-up time variation in response to differences in nectar volume and concentration in the giant tropical ant,Paraponera clavata (Hymenoptera: Formicidae) |url=https://doi.org/10.1007/BF02213547 |journal=Journal of Insect Behavior |language=en |volume=9 |issue=5 |pages=659–672 |doi=10.1007/BF02213547 |bibcode=1996JIBeh...9..659B |s2cid=42555491 |issn=1572-8889}}</ref>

==Plant-herbivore interactions==
Interactions between plants and herbivores can play a prevalent role in [[ecosystem]] dynamics such [[Community (ecology)|community]] structure and functional processes.<ref name=":1">{{Cite journal|last1=Sandsten|first1=Håkan|last2=Klaassen|first2=Marcel|date=2008-03-12|title=Swan foraging shapes spatial distribution of two submerged plants, favouring the preferred prey species|journal=Oecologia|volume=156|issue=3|pages=569–576|doi=10.1007/s00442-008-1010-5|pmid=18335250|pmc=2373415|bibcode=2008Oecol.156..569S|issn=0029-8549|doi-access=free}}</ref><ref name=":2">{{Cite journal|last1=Descombes|first1=Patrice|last2=Marchon|first2=Jérémy|last3=Pradervand|first3=Jean-Nicolas|last4=Bilat|first4=Julia|last5=Guisan|first5=Antoine|last6=Rasmann|first6=Sergio|last7=Pellissier|first7=Loïc|date=2016-10-17|title=Community-level plant palatability increases with elevation as insect herbivore abundance declines|journal=Journal of Ecology|volume=105|issue=1|pages=142–151|doi=10.1111/1365-2745.12664|s2cid=88658391 |issn=0022-0477|doi-access=free}}</ref> Plant [[Species diversity|diversity]] and [[Species distribution|distribution]] is often driven by herbivory, and it is likely that trade-offs between plant [[Competition (biology)|competitiveness]] and [[Plant defense against herbivory|defensiveness]], and between colonization and mortality allow for coexistence between species in the presence of herbivores.<ref>{{Cite journal|last=Lubchenco|first=Jane|date=January 1978|title=Plant Species Diversity in a Marine Intertidal Community: Importance of Herbivore Food Preference and Algal Competitive Abilities|url=http://dx.doi.org/10.1086/283250|journal=The American Naturalist|volume=112|issue=983|pages=23–39|doi=10.1086/283250|bibcode=1978ANat..112...23L |s2cid=84801707|issn=0003-0147}}</ref><ref>{{Cite journal|last1=Gleeson|first1=Scott K.|last2=Wilson|first2=David Sloan|date=April 1986|title=Equilibrium Diet: Optimal Foraging and Prey Coexistence|url=http://dx.doi.org/10.2307/3565460|journal=Oikos|volume=46|issue=2|pages=139|doi=10.2307/3565460|jstor=3565460|bibcode=1986Oikos..46..139G |issn=0030-1299}}</ref><ref>{{Cite journal|last1=Olff|first1=Han|last2=Ritchie|first2=Mark E.|date=July 1998|title=Effects of herbivores on grassland plant diversity|url=http://dx.doi.org/10.1016/s0169-5347(98)01364-0|journal=Trends in Ecology & Evolution|volume=13|issue=7|pages=261–265|doi=10.1016/s0169-5347(98)01364-0|pmid=21238294|bibcode=1998TEcoE..13..261O |issn=0169-5347|hdl=11370/3e3ec5d4-fa03-4490-94e3-66534b3fe62f|hdl-access=free}}</ref><ref name=":3">{{Cite journal|last1=Hidding|first1=Bert|last2=Nolet|first2=Bart A.|last3=de Boer|first3=Thijs|last4=de Vries|first4=Peter P.|last5=Klaassen|first5=Marcel|date=2009-09-10|title=Above- and below-ground vertebrate herbivory may each favour a different subordinate species in an aquatic plant community|journal=Oecologia|volume=162|issue=1|pages=199–208|doi=10.1007/s00442-009-1450-6|pmid=19756762|pmc=2776151|issn=0029-8549|doi-access=free}}</ref> 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]],<ref>{{Cite journal|last1=Arcese|first1=P.|last2=Schuster|first2=R.|last3=Campbell|first3=L.|last4=Barber|first4=A.|last5=Martin|first5=T. G.|date=2014-09-11|title=Deer density and plant palatability predict shrub cover, richness, diversity and aboriginal food value in a North American archipelago|journal=Diversity and Distributions|volume=20|issue=12|pages=1368–1378|doi=10.1111/ddi.12241|s2cid=86779418 |issn=1366-9516|doi-access=free|bibcode=2014DivDi..20.1368A }}</ref> while other large mammalian herbivores like bison control dominant species which allows other species to flourish.<ref>{{Cite journal|last=Collins|first=S. L.|date=1998-05-01|title=Modulation of Diversity by Grazing and Mowing in Native Tallgrass Prairie|url=http://dx.doi.org/10.1126/science.280.5364.745|journal=Science|volume=280|issue=5364|pages=745–747|doi=10.1126/science.280.5364.745|pmid=9563952|bibcode=1998Sci...280..745C|issn=0036-8075}}</ref> Plant-herbivore interactions can also operate so that plant communities mediate herbivore communities.<ref name=":4">{{Cite journal |last1=Pellissier |first1=Loïc |last2=Ndiribe |first2=Charlotte |last3=Dubuis |first3=Anne |last4=Pradervand |first4=Jean-Nicolas |last5=Salamin |first5=Nicolas |last6=Guisan |first6=Antoine |last7=Rasmann |first7=Sergio |date=2013 |title=Turnover of plant lineages shapes herbivore phylogenetic beta diversity along ecological gradients |journal=Ecology Letters |volume=16 |issue=5 |pages=600–608 |doi=10.1111/ele.12083 |pmid=23448096|bibcode=2013EcolL..16..600P }}</ref> Plant communities that are more diverse typically sustain greater herbivore richness by providing a greater and more diverse set of resources.<ref>{{Cite journal|last=Tilman|first=D.|date=1997-08-29|title=The Influence of Functional Diversity and Composition on Ecosystem Processes|url=http://dx.doi.org/10.1126/science.277.5330.1300|journal=Science|volume=277|issue=5330|pages=1300–1302|doi=10.1126/science.277.5330.1300|issn=0036-8075}}</ref>

[[Coevolution]] and [[Phylogenetics|phylogenetic]] 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.<ref name=":2" /><ref name=":4" /><ref>{{Cite journal|last1=Mitter|first1=Charles|last2=Farrell|first2=Brian|last3=Futuyma|first3=Douglas J.|date=September 1991|title=Phylogenetic studies of insect-plant interactions: Insights into the genesis of diversity|url=http://dx.doi.org/10.1016/0169-5347(91)90007-k|journal=Trends in Ecology & Evolution|volume=6|issue=9|pages=290–293|doi=10.1016/0169-5347(91)90007-k|pmid=21232484|bibcode=1991TEcoE...6..290M |issn=0169-5347}}</ref> This is apparent in the [[adaptation]]s plants develop to [[Plant tolerance to herbivory|tolerate]] and/or [[Plant defense against herbivory|defend]] from insect herbivory and the [[Herbivore adaptations to plant defense|responses of herbivores]] to overcome these adaptations. The evolution of antagonistic and mutualistic plant-herbivore interactions are not mutually exclusive and may co-occur.<ref name=":5">{{Cite journal|last1=de Mazancourt|first1=Claire|last2=Loreau|first2=Michel|last3=Dieckmann|first3=Ulf|date=August 2001|title=Can the Evolution of Plant Defense Lead to Plant-Herbivore Mutualism?|url=http://dx.doi.org/10.1086/321306|journal=The American Naturalist|volume=158|issue=2|pages=109–123|doi=10.1086/321306|pmid=18707340|bibcode=2001ANat..158..109D |s2cid=15722747|issn=0003-0147}}</ref> 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]] and phylogenetic [[beta diversity]] of insect [[clade]]s such as [[Butterfly|butterflies]].<ref name=":4" /> These types of eco-evolutionary feedbacks between plants and herbivores are likely the main driving force behind plant and herbivore diversity.<ref name=":4" /><ref>{{Cite journal|last1=Farrell|first1=Brian D.|last2=Dussourd|first2=David E.|last3=Mitter|first3=Charles|date=October 1991|title=Escalation of Plant Defense: Do Latex and Resin Canals Spur Plant Diversification?|url=http://dx.doi.org/10.1086/285258|journal=The American Naturalist|volume=138|issue=4|pages=881–900|doi=10.1086/285258|bibcode=1991ANat..138..881F |s2cid=86725259|issn=0003-0147}}</ref>

Abiotic factors such as [[climate]] and [[Biogeography|biogeographical features]] 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.<ref name=":1" /><ref name=":3" /> These seasonal herbivore communities differ in both their assemblage and functions within the [[Wetland|wetland ecosystem]].<ref name=":3" /> Such differences in herbivore modalities can potentially lead to trade-offs that influence species traits and may lead to additive effects on [[Community (ecology)|community]] composition and ecosystem functioning.<ref name=":1" /><ref name=":3" /> Seasonal changes and environmental gradients such as elevation and [[Latitudinal gradients in species diversity|latitude]] often affect the [[palatability]] of plants which in turn influences herbivore community assemblages and vice versa.<ref name=":2" /><ref>{{Cite journal|last1=Pennings|first1=Steven C.|last2=Silliman|first2=Brian R.|date=September 2005|title=LINKING BIOGEOGRAPHY AND COMMUNITY ECOLOGY: LATITUDINAL VARIATION IN PLANT–HERBIVORE INTERACTION STRENGTH|url=http://dx.doi.org/10.1890/04-1022|journal=Ecology|volume=86|issue=9|pages=2310–2319|doi=10.1890/04-1022|bibcode=2005Ecol...86.2310P |issn=0012-9658}}</ref> Examples include a decrease in abundance of leaf-chewing larvae in the fall when hardwood leaf palatability decreases due to increased [[tannin]] levels which results in a decline of arthropod [[species richness]],<ref>{{Cite journal|last1=Futuyma|first1=Douglas J.|last2=Gould|first2=Fred|date=March 1979|title=Associations of Plants and Insects in Deciduous Forest|url=http://dx.doi.org/10.2307/1942571|journal=Ecological Monographs|volume=49|issue=1|pages=33–50|doi=10.2307/1942571|jstor=1942571|bibcode=1979EcoM...49...33F |issn=0012-9615}}</ref> and increased palatability of plant communities at higher elevations where [[grasshopper]]s abundances are lower.<ref name=":2" /> Climatic stressors such as [[ocean acidification]] can lead to responses in plant-herbivore interactions in relation to palatability as well.<ref>{{Cite journal|last1=Poore|first1=Alistair G. B.|last2=Graba-Landry|first2=Alexia|last3=Favret|first3=Margaux|last4=Sheppard Brennand|first4=Hannah|last5=Byrne|first5=Maria|last6=Dworjanyn|first6=Symon A.|date=2013-05-15|title=Direct and indirect effects of ocean acidification and warming on a marine plant–herbivore interaction|url=http://dx.doi.org/10.1007/s00442-013-2683-y|journal=Oecologia|volume=173|issue=3|pages=1113–1124|doi=10.1007/s00442-013-2683-y|pmid=23673470|bibcode=2013Oecol.173.1113P|s2cid=10990079|issn=0029-8549}}</ref>

===Herbivore offense===
[[File:Aphid-sap.jpg|right|thumb|[[Aphid]]s are fluid feeders on [[plant sap]].]]
{{Main|Herbivore adaptations to plant defense}}

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.<ref>{{cite journal | last1 = Dearing | first1 = M.D. | last2 = Mangione | first2 = A.M. | last3 = Karasov | first3 = W.H. | date = May 2000 | title = Diet breadth of mammalian herbivores: nutrient versus detoxification constraints | journal = Oecologia | volume = 123 | issue = 3| pages = 397–405 | doi = 10.1007/s004420051027 | pmid = 28308595 | bibcode = 2000Oecol.123..397D | s2cid = 914899 }}</ref>

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 metabolite]]s,<ref name="Karban, R 2002">{{cite journal | last1 = Karban | first1 = R. | last2 = Agrawal | first2 = A.A. | date = November 2002 | title = Herbivore Offense | journal = Annual Review of Ecology and Systematics | volume = 33 | issue = 1 | pages = 641–664 | doi=10.1146/annurev.ecolsys.33.010802.150443| bibcode = 2002AnRES..33..641K }}</ref> sequestering toxins unaltered,<ref>{{cite journal | last1 = Nishida | first1 = R. | date = January 2002 | title = Sequestration of Defensive Substances from Plants by Lepidoptera | journal = Annual Review of Entomology | volume = 47 | pages = 57–92 | pmid = 11729069 | doi = 10.1146/annurev.ento.47.091201.145121 }}</ref> 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 acid]]s lacking in their sap diet.<ref>{{cite journal | last1 = Douglas | first1 = A.E. | date = January 1998 | title = Nutritional Interactions in Insect–Microbial Symbioses: Aphids and Their Symbiotic Bacteria Buchnera | journal = Annual Review of Entomology | volume = 43 | pages = 17–37 | doi = 10.1146/annurev.ento.43.1.17 | pmid = 15012383 }}</ref>

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.<ref>{{cite journal | last1 = Sagers | first1 = C.L. | year = 1992 | title = Manipulation of host plant quality: herbivores keep leaves in the dark | journal = Functional Ecology | volume = 6 | issue = 6| pages = 741–743 | doi=10.2307/2389971| jstor = 2389971 | bibcode = 1992FuEco...6..741S }}</ref>

===Plant defense===
{{Main|Plant defense against herbivory}}
{{see also|Plant tolerance to herbivory}}

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.<ref name=":0">{{Cite journal|last1=Call|first1=Anson|last2=St Clair|first2=Samuel B|date=2018-10-01|editor-last=Ryan|editor-first=Michael|title=Timing and mode of simulated ungulate herbivory alter aspen defense strategies|url=https://academic.oup.com/treephys/article/38/10/1476/5048677|journal=Tree Physiology|language=en|volume=38|issue=10|pages=1476–1485|doi=10.1093/treephys/tpy071|pmid=29982736|issn=1758-4469|doi-access=free}}</ref> This can occur by diverting herbivory to non-essential plant parts, resource allocation, compensatory growth, or by rapid regrowth and recovery from herbivory.<ref>{{Cite journal |last1=Hawkes |first1=Christine V. |last2=Sullivan |first2=Jon J. |title=The Impact of Herbivory on Plants in Different Resource Conditions: A Meta-Analysis |date=2001 |journal=Ecology |volume=82 |issue=7 |pages=2045–2058 |doi=10.1890/0012-9658(2001)082[2045:TIOHOP]2.0.CO;2 |doi-access=free }}</ref> Resistance refers to the ability of a plant to reduce the amount of damage it receives from herbivores.<ref name=":0" /> This can occur via avoidance in space or time,<ref>{{cite journal | last1 = Milchunas | first1 = D.G. | last2 = Noy-Meir | first2 = I. | date = October 2002 | title = Grazing refuges, external avoidance of herbivory and plant diversity | journal = Oikos | volume = 99 | issue = 1| pages = 113–130 | doi=10.1034/j.1600-0706.2002.990112.x| bibcode = 2002Oikos..99..113M }}</ref> 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.<ref>{{cite journal | last1 = Edwards | first1 = P.J. | last2 = Wratten | first2 = S.D. | date = March 1985 | title = Induced plant defences against insect grazing: fact or artefact? | journal = Oikos | volume = 44 | issue = 1| pages = 70–74 | doi=10.2307/3544045| jstor = 3544045 | bibcode = 1985Oikos..44...70E }}</ref>

Physical, or mechanical, defenses are barriers or structures designed to deter herbivores or reduce intake rates, lowering overall herbivory. [[Thorns, spines, and prickles|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]] may cover leaves or stems and are especially effective against invertebrate herbivores.<ref>{{cite journal | last1 = Pillemer | first1 = E.A. | last2 = Tingey | first2 = W.M. | date = 6 August 1976 | title = Hooked Trichomes: A Physical Plant Barrier to a Major Agricultural Pest | journal = Science | volume = 193 | issue = 4252| pages = 482–484 | doi = 10.1126/science.193.4252.482 | pmid = 17841820 | bibcode = 1976Sci...193..482P | s2cid = 26751736 }}</ref> In addition, some plants have [[waxes]] or [[resins]] 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.<ref>{{Cite journal |last=Epstein |first=E |date=1994-01-04 |title=The anomaly of silicon in plant biology. |journal=Proceedings of the National Academy of Sciences |language=en |volume=91 |issue=1 |pages=11–17 |doi=10.1073/pnas.91.1.11 |issn=0027-8424 |pmc=42876 |pmid=11607449|bibcode=1994PNAS...91...11E |doi-access=free }}</ref>

Chemical defenses are [[secondary metabolites]] 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.<ref>{{Cite web |date=2022-07-21 |title=16.3: Herbivory |url=https://bio.libretexts.org/Workbench/General_Ecology_Ecology/Chapter_16%3A_Antagonistic_Interactions/16.3%3A_Herbivory |access-date=2024-04-10 |website=Biology LibreTexts |language=en}}</ref>

#Carbon-based defenses include [[terpenes]] and [[polyphenol|phenolics]]. 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 [[Adenosine triphosphate|adenosine triphosphate (ATP)]] formation, molting [[hormones]], or the nervous system.<ref>{{cite journal | last1 = Langenheim | first1 = J.H. | date = June 1994 | title = Higher plant terpenoids: a phytocentric overview of their ecological roles | journal = Journal of Chemical Ecology | volume = 20 | issue = 6| pages = 1223–1280 | doi = 10.1007/BF02059809 | pmid = 24242340 | bibcode = 1994JCEco..20.1223L | s2cid = 25360410 }}</ref> 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]], 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]] and cyanogens. Alkaloids include commonly recognized substances such as [[caffeine]], [[nicotine]], and [[morphine]]. 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]] stored within their tissues. This is released when the plant is damaged and inhibits cellular respiration and electron transport.{{citation needed|date=March 2015}}

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]]s that reduce herbivory.<ref>{{cite journal | last1 = Heil | first1 = M. | last2 = Koch | first2 = T. | last3 = Hilpert | first3 = A. | last4 = Fiala | first4 = B. | last5 = Boland | first5 = W. | last6 = Linsenmair | first6 = K. Eduard | date = 30 January 2001 | title = Extrafloral nectar production of the ant-associated plant, Macaranga tanarius, is an induced, indirect, defensive response elicited by jasmonic acid | journal = Proceedings of the National Academy of Sciences | volume = 98 | issue = 3| pages = 1083–1088 | doi = 10.1073/pnas.031563398 | pmid=11158598 | pmc=14712| bibcode = 2001PNAS...98.1083H | doi-access = free }}</ref> A given plant species often has many types of defensive mechanisms, mechanical or chemical, constitutive or induced, which allow it to escape from herbivores.<ref>{{Cite web |title=Plants, Herbivores, and Predators |url=https://www2.nau.edu/~gaud/bio301/content/genfdbk.htm |access-date=2024-04-10 |website=www2.nau.edu}}</ref>

===Predator–prey theory===
According to the theory of [[predator]]–prey interactions, the relationship between herbivores and plants is cyclic.<ref name=":7">{{Cite book |last=Gotelli |first=Nicholas J. |url=https://www.worldcat.org/oclc/31816245 |title=A primer of ecology |date=1995 |publisher=Sinauer Associates |isbn=0-87893-270-4 |location=Sunderland, Mass. |oclc=31816245}}</ref> When prey (plants) are numerous their predators (herbivores) increase in numbers, reducing the prey population, which in turn causes predator number to decline.<ref name=":7" /> 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.<ref name=":8">{{Cite book |last=Smith |first=Robert Leo |url=https://www.worldcat.org/oclc/44391592 |title=Ecology & field biology |date=2001 |publisher=Benjamin Cummings |others=T. M. Smith |isbn=0-321-04290-5 |edition=6th |location=San Francisco |oclc=44391592}}</ref> 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.<ref name=":8" /> Alternating between two or more plant types provides population stability for the herbivore, while the populations of the plants oscillate.<ref name=":7" /> 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.<ref name=":8" /> When an invasive herbivore or plant enters the system, the balance is thrown off and the diversity can collapse to a monotaxon system.<ref name=":8" />

The back and forth relationship of plant defense and herbivore offense drives [[coevolution]] between plants and herbivores, resulting in a "coevolutionary arms race".<ref name="Karban, R 2002"/><ref>{{cite journal | last1=Mead | first1=R.J. | last2=Oliver | first2=A.J. | last3=King | first3=D.R. | last4=Hubach | first4=P.H. | date=March 1985 | title=The Co-Evolutionary Role of Fluoroacetate in Plant–Animal Interactions in Australia | journal=Oikos | volume=44 | issue=1| pages=55–60 | doi=10.2307/3544043| jstor=3544043 | bibcode=1985Oikos..44...55M }}</ref> 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]].<ref name= Ehrlich>{{cite journal | last1=Ehrlich | first1=P. R. | last2=Raven | first2=P. H. | date=December 1964 | title=Butterflies and plants: a study of coevolution | journal=Evolution | volume=18 | issue=4| pages=586–608 | doi=10.2307/2406212 | jstor=2406212 }}</ref><ref name= Thompson>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.</ref>

=== 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 [[Mutualism (biology)|mutualism]]s in which both partners benefit in some way from the interaction. [[Seed dispersal]] by herbivores and [[pollination]] are two forms of mutualistic herbivory in which the herbivore receives a food resource and the plant is aided in reproduction.<ref>{{cite journal|last1=Herrera|first1=C.M.|date=March 1985|title=Determinants of Plant-Animal Coevolution: The Case of Mutualistic Dispersal of Seeds by Vertebrates|journal=Oikos|volume=44|issue=1|pages=132–141|doi=10.2307/3544054|jstor=3544054|bibcode=1985Oikos..44..132H }}</ref> Plants can also be indirectly affected by herbivores through [[Nutrient cycle|nutrient recycling]], with plants benefiting from herbivores when nutrients are recycled very efficiently.<ref name=":5" /> Another form of plant-herbivore mutualism is physical changes to the environment and/or plant community structure by herbivores which serve as [[ecosystem engineer]]s, such as [[wallowing]] by bison.<ref>{{Cite journal|last1=Nickell|first1=Zachary|last2=Varriano|first2=Sofia|last3=Plemmons|first3=Eric|last4=Moran|first4=Matthew D.|date=September 2018|title=Ecosystem engineering by bison (Bison bison ) wallowing increases arthropod community heterogeneity in space and time|journal=Ecosphere|volume=9|issue=9|pages=e02436|doi=10.1002/ecs2.2436|issn=2150-8925|doi-access=free|bibcode=2018Ecosp...9E2436N }}</ref> Swans form a mutual relationship with the plant species that they [[forage]] by digging and disturbing the sediment which removes competing plants and subsequently allows colonization of other plant species.<ref name=":1" /><ref name=":3" />

==Impacts==
[[File:Mixed feeding shoal herbivorous fish.jpg|thumb|Mixed feeding shoal of [[herbivorous fish]] on a [[coral reef]]]]

=== Trophic cascades and environmental degradation ===
When herbivores are affected by [[trophic cascade]]s, plant communities can be indirectly affected.<ref>{{Cite journal|last1=van Veen|first1=F. J. F.|last2=Sanders|first2=D.|date=May 2013|title=Herbivore identity mediates the strength of trophic cascades on individual plants|journal=Ecosphere|volume=4|issue=5|pages=art64|doi=10.1890/es13-00067.1|issn=2150-8925|doi-access=free}}</ref> 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.<ref>{{Cite journal|last=Ripple|first=William J.|date=2011-04-20|title=Wolves, Elk, Bison, and Secondary Trophic Cascades in Yellowstone National Park|journal=The Open Ecology Journal|volume=3|issue=3|pages=31–37|doi= 10.2174/1874213001003040031 |doi-broken-date=22 November 2024 |doi-access=free|issn=1874-2130}}</ref> With the size of herbivores having an effect on the amount of energy intake that is needed, larger herbivores need to forage on higher quality or more plants to gain the optimal amount of nutrients and energy compared to smaller herbivores.<ref>{{Cite journal |last1=Forbes |first1=Elizabeth S. |last2=Cushman |first2=J. Hall |last3=Burkepile |first3=Deron E. |last4=Young |first4=Truman P. |last5=Klope |first5=Maggie |last6=Young |first6=Hillary S. |date=2019-06-17 |title=Synthesizing the effects of large, wild herbivore exclusion on ecosystem function |journal=Functional Ecology |volume=33 |issue=9 |pages=1597–1610 |doi=10.1111/1365-2435.13376 |s2cid=182023675 |issn=0269-8463|doi-access=free |bibcode=2019FuEco..33.1597F }}</ref> [[Environmental degradation]] from [[white-tailed deer]] (''Odocoileus virginianus'') in the US alone has the potential to both change vegetative communities<ref>{{Cite journal|last1=Seager|first1=S Trent|last2=Eisenberg|first2=Cristina|last3=St. Clair|first3=Samuel B.|date=July 2013|title=Patterns and consequences of ungulate herbivory on aspen in western North America|journal=Forest Ecology and Management|language=en|volume=299|pages=81–90|doi=10.1016/j.foreco.2013.02.017|bibcode=2013ForEM.299...81S }}</ref> through over-browsing and cost [[forest restoration]] projects upwards of $750&nbsp;million annually. Another example of a trophic cascade involved plant-herbivore interactions are [[coral reef]] ecosystems. [[Herbivorous fish]] and marine animals are important [[algae]] and seaweed grazers, and in the absence of plant-eating fish, corals are outcompeted and seaweeds deprive corals of sunlight.<ref>{{cite web |date=n.d. |others=Photos by Richard Ling and Rian Tan |title=Plant-eating fish, Information sheets for fishing communities No 29 |url=https://spccfpstore1.blob.core.windows.net/digitallibrary-docs/files/89/89be346cf7b9cc83be17ef32d0614a5a.pdf?sv=2015-12-11&sr=b&sig=ytjzqw6EadlW%2FKW9XJ9ntZguXsJ2%2ByvIIsPojnaDheI%3D&se=2023-01-20T10%3A43%3A18Z&sp=r&rscc=public%2C+max-age%3D864000%2C+max-stale%3D86400&rsct=application%2Fpdf&rscd=inline%3B+filename%3D%22Anon_13_ISFC_29_Plant_eating_fish.pdf%22 |url-status=live |archive-url=https://web.archive.org/web/20220724104433/https://spccfpstore1.blob.core.windows.net/digitallibrary-docs/files/89/89be346cf7b9cc83be17ef32d0614a5a.pdf?sv=2015-12-11&sr=b&sig=ytjzqw6EadlW%2FKW9XJ9ntZguXsJ2%2ByvIIsPojnaDheI%3D&se=2023-01-20T10%3A43%3A18Z&sp=r&rscc=public%2C+max-age%3D864000%2C+max-stale%3D86400&rsct=application%2Fpdf&rscd=inline%3B+filename%3D%22Anon_13_ISFC_29_Plant_eating_fish.pdf%22 |archive-date=24 July 2022 |access-date=24 July 2022 |publisher=SPC (www.spc.int) in collaboration with the LMMA Network (www.lmmanetwork.org)}}</ref>

=== Economic impacts ===
Agricultural crop damage by the same species totals approximately $100&nbsp;million every year. Insect crop damages also contribute largely to annual crop losses in the U.S.<ref>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</ref> 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.{{citation needed|date=August 2015}} [[Ecotourism]] 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.{{citation needed|date=August 2015}}

==See also==
{{Commons category|Herbivores}}
{{col div|colwidth=30em}}
* [[Carnivore]] and [[omnivore]]
* [[Consumer-resource systems]]
* [[List of feeding behaviours]]
* [[List of herbivorous animals]]
* [[Plant-based diet]]
* [[Productivity (ecology)]]
* [[Seed predation]]
* [[Tritrophic interactions in plant defense]]
* [[Veganism]]
* [[Vegetarianism]]
{{colend}}

==References==
{{Reflist}}

==Further reading==
* Bob Strauss, 2008, [http://dinosaurs.about.com/od/herbivorousdinosaurs/Herbivorous_Dinosaurs.htm Herbivorous Dinosaurs] {{Webarchive|url=https://web.archive.org/web/20121118190049/http://dinosaurs.about.com/od/herbivorousdinosaurs/Herbivorous_Dinosaurs.htm |date=18 November 2012 }}, [[The New York Times]]
*Danell, K., R. Bergström, P. Duncan, J. Pastor (Editors)(2006) ''Large herbivore ecology, ecosystem dynamics and conservation'' Cambridge, UK : Cambridge University Press. 506 p.&nbsp;{{ISBN|0-521-83005-2}}
*Crawley, M. J. (1983) ''Herbivory : the dynamics of animal-plant interactions'' Oxford : Blackwell Scientific. 437 p.&nbsp;{{ISBN|0-632-00808-3}}
*Olff, H., V.K. Brown, R.H. Drent (editors) (1999) ''Herbivores : between plants and predators'' Oxford; Malden, Ma. : Blackwell Science. 639 p.&nbsp;{{ISBN|0-632-05155-8}}

==External links==
*[http://www.herbivores.com/ Herbivore information resource website]
*[http://www.johnsankey.ca/senecio.html The herbivore defenses of ''Senecio viscusus'']
*[https://web.archive.org/web/20060908202106/http://www.muhlenberg.edu/depts/biology/nsf/ Herbivore defense in ''Lindera benzoin'']
*[http://www.herbivory.com/ website of the herbivory lab at Cornell University]

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[[Category:Herbivory|*]]
[[Category:Herbivorous animals| ]]
[[Category:Ecology terminology]]
[[Category:Animals by eating behaviors]]