Editing Herbivore (section)

==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" />