Editing Food web (section)

===Trophic dynamics and multitrophic interactions===
The trophic level concept was introduced in a historical landmark paper on trophic dynamics in 1942 by [[Raymond Lindeman|Raymond L. Lindeman]]. The basis of trophic dynamics is the transfer of energy from one part of the ecosystem to another.<ref name="Cousins85" /><ref name="Lindeman42">{{cite journal | last1=Lindeman | first1=R. L. | title=The trophic-dynamic aspect of ecology | journal=Ecology | volume=23 | issue=4 | year=1942 | pages=399–417 | url=http://www.fcnym.unlp.edu.ar/catedras/ecocomunidades/Lindman_1942.pdf | doi=10.2307/1930126 | jstor=1930126 | bibcode=1942Ecol...23..399L | access-date=2011-06-13 | archive-date=2017-03-29 | archive-url=https://web.archive.org/web/20170329165523/http://www.fcnym.unlp.edu.ar/catedras/ecocomunidades/Lindman_1942.pdf | url-status=dead }}</ref> The trophic dynamic concept has served as a useful quantitative heuristic, but it has several major limitations including the precision by which an organism can be allocated to a specific trophic level. Omnivores, for example, are not restricted to any single level. Nonetheless, recent research has found that discrete trophic levels do exist, but "above the herbivore trophic level, food webs are better characterized as a tangled web of omnivores."<ref name="Thompson07" />

A central question in the trophic dynamic literature is the nature of control and regulation over resources and production. Ecologists use simplified one trophic position food chain models (producer, carnivore, decomposer). Using these models, ecologists have tested various types of ecological control mechanisms. For example, herbivores generally have an abundance of vegetative resources, which meant that their populations were largely controlled or regulated by predators. This is known as the top-down hypothesis or [[Green world hypothesis|'green-world' hypothesis]]. Alternatively to the top-down hypothesis, not all plant material is edible and the nutritional quality or antiherbivore defenses of plants (structural and chemical) suggests a bottom-up form of regulation or control.<ref name="Hariston93" /><ref name="Fretwell87">{{cite journal | last1=Fretwell | first1=S. D. | title=Food chain dynamics: The central theory of ecology? | year=1987 | journal=Oikos | volume=50 | issue=3 | pages=291–301 | url=http://limnology.wisc.edu/courses/zoo955/Spring2005/food%20web%20seminar%20papers/fretwell_food_chain_dynamics_oikos.pdf | doi=10.2307/3565489 | url-status=dead | archive-url=https://web.archive.org/web/20110728142919/http://limnology.wisc.edu/courses/zoo955/Spring2005/food%20web%20seminar%20papers/fretwell_food_chain_dynamics_oikos.pdf | archive-date=2011-07-28 | jstor=3565489 | bibcode=1987Oikos..50..291F | accessdate=2011-06-14 }}</ref><ref name="Polis96">{{cite journal|last1=Polis|first1=G. A.|last2=Strong|first2=D. R.|title=Food web complexity and community dynamics.|year=1996|journal=[[The American Naturalist]]|volume=147|issue=5|pages=813–846|url=http://www.seaturtle.org/PDF/PolisGA_1996_AmNat.pdf |doi=10.1086/285880 |s2cid=85155900}}</ref> Recent studies have concluded that both "top-down" and "bottom-up" forces can influence community structure and the strength of the influence is environmentally context dependent.<ref name="Hoekman10">{{cite journal | last1=Hoekman | first1=D. | title=Turning up the head: Temperature influences the relative importance of top-down and bottom-up effects. | journal=Ecology | volume=91 | issue=10 | pages=2819–2825 | url=http://www.nd.edu/~underc/east/publications/documents/Hoekman2010b.pdf | doi=10.1890/10-0260.1| pmid=21058543 | year=2010 | doi-access=free | bibcode=2010Ecol...91.2819H }}</ref><ref name="Schmitz08">{{cite journal | last1=Schmitz | first1=O. J. | s2cid=86686057 | title=Herbivory from individuals to ecosystems. | journal=Annual Review of Ecology, Evolution, and Systematics | year=2008 | volume=39 | pages=133–152 | doi=10.1146/annurev.ecolsys.39.110707.173418 }}</ref> These complex multitrophic interactions involve more than two [[trophic level]]s in a food web.<ref name="Tscharntke02">{{cite book | editor1-last=Tscharntke | editor1-first=T. | editor2-last=Hawkins | editor2-first=B. A. | year=2002 | title=Multitrophic Level Interactions | publisher=Cambridge University Press | place=Cambridge | url=https://books.google.com/books?id=_8bHeXvUc08C&q=Multitrophic+Level+Interactions | page=282 | isbn=978-0-521-79110-6}}</ref> For example, such interactions have been discovered in the context of [[arbuscular mycorrhizal fungi]] and [[aphid]] herbivores that utilize the same plant [[species]].<ref>{{cite journal |last1=Babikova |first1=Zdenka |last2=Gilbert |first2=Lucy |last3=Bruce |first3=Toby |last4=Dewhirst |first4=Sarah |last5=Pickett |first5=John A. |last6=Johnson |first6=David |date= April 2014 |title= Arbuscular mycorrhizal fungi and aphids interact by changing host plant quality and volatile emission|journal=Functional Ecology|volume=28 |issue=2 |pages=375–385 |doi= 10.1111/1365-2435.12181|jstor=24033672 |doi-access=free |bibcode=2014FuEco..28..375B }}</ref> 
[[File:Euphydryas editha taylori 2.jpg|thumb|left|alt=caterpillar munching a leaf|Multitrophic interaction: ''[[Euphydryas editha taylori]]'' larvae sequester defensive compounds from specific types of plants they consume to protect themselves from bird predators]]
Another example of a multitrophic interaction is a [[trophic cascade]], in which predators help to increase plant growth and prevent [[overgrazing]] by suppressing herbivores. Links in a food-web illustrate direct trophic relations among species, but there are also indirect effects that can alter the abundance, distribution, or biomass in the trophic levels. For example, predators eating herbivores indirectly influence the control and regulation of primary production in plants. Although the predators do not eat the plants directly, they regulate the population of herbivores that are directly linked to plant trophism. The net effect of direct and indirect relations is called trophic cascades. Trophic cascades are separated into species-level cascades, where only a subset of the food-web dynamic is impacted by a change in population numbers, and community-level cascades, where a change in population numbers has a dramatic effect on the entire food-web, such as the distribution of plant biomass.<ref name="Polis00">{{cite journal|author = Polis, G.A.|title = When is a trophic cascade a trophic cascade?|year = 2000| journal = Trends in Ecology and Evolution|volume = 15|issue=11|pages=473–5|url=http://www.cof.orst.edu/leopold/class-reading/Polis%202000.pdf|doi = 10.1016/S0169-5347(00)01971-6|pmid = 11050351| bibcode=2000TEcoE..15..473P |display-authors=etal}}</ref>

The field of [[chemical ecology]] has elucidated multitrophic interactions that entail the transfer of defensive compounds across multiple trophic levels.<ref>{{cite book |last1=Tscharntke |first1=Teja |last2=Hawkins |first2=Bradford A. |date=2002 |title=Multitrophic Level Interactions |pages=10, 72 |publisher= Cambridge University Press |location= Cambridge |isbn=978-0-511-06719-8}}</ref> For example, certain plant species in the ''[[Castilleja]]'' and ''[[Plantago]]'' genera have been found to produce defensive compounds called [[Glycoside#Iridoid glycosides|iridoid glycosides]] that are sequestered in the tissues of the [[Euphydryas editha taylori|Taylor's checkerspot butterfly]] [[larva]]e that have developed a tolerance for these compounds and are able to consume the foliage of these plants.<ref name="HBB">{{cite journal |last1=Haan |first1=Nate L. |last2=Bakker |first2=Jonathan D. |last3=Bowers |first3=M. Deane |date= 14 January 2021 |title= Preference, performance, and chemical defense in an endangered butterfly using novel and ancestral host plants |journal=Scientific Reports |volume=11 |issue=992 |page=992 |doi=10.1038/s41598-020-80413-y |pmid=33446768 |pmc=7809109 |bibcode=2021NatSR..11..992H }}</ref><ref name="Haan">{{cite journal |last1=Haan |first1=Nate L. |last2=Bakker |first2=Jonathan D. |last3=Bowers |first3=M. Deane |date= May 2018 |title= Hemiparasites can transmit indirect effects from their host plants to herbivores |url=https://www.jstor.org/stable/26624251 |journal=Ecology |volume=99 |issue=2 |pages=399–410 |doi= 10.1002/ecy.2087|jstor=26624251 |pmid=29131311 |bibcode=2018Ecol...99..399H |access-date=2022-05-02}}</ref> These sequestered iridoid glycosides then confer chemical protection against bird predators to the butterfly larvae.<ref name="HBB"/><ref name="Haan"/> Another example of this sort of [[Tritrophic interactions in plant defense|multitrophic interaction in plants]] is the transfer of defensive [[alkaloid]]s produced by [[endophyte]]s living within a grass host to a hemiparasitic plant that is also using the grass as a host.<ref>{{cite journal |last1=Lehtonen |first1=Päivi  |last2=Helander |first2=Marjo |last3=Wink |first3=Michael |last4=Sporer |first4=Frank |last5=Saikkonen |first5=Kari |date= 12 October 2005 |title= Transfer of endophyte-origin defensive alkaloids from a grass to a hemiparasitic plant |journal=Ecology Letters|volume=8 |issue=12 |pages=1256–1263 |doi=10.1111/j.1461-0248.2005.00834.x |doi-access=free |bibcode=2005EcolL...8.1256L }}</ref>