Editing Food web (section)

==Taxonomy of a food web==
[[File:FoodWebSimple.svg|thumb|upright=1.5|A simplified food web illustrating a three trophic food chain (''producers-herbivores-carnivores'') linked to decomposers. The movement of mineral nutrients is cyclic, whereas the movement of energy is unidirectional and noncyclic. Trophic species are encircled as nodes and arrows depict the links.<ref name="Kormondy84">{{cite book | last1=Kormondy | first1=E. J. | title=Concepts of ecology | edition=4th | year=1996 | page=559 | isbn=978-0-13-478116-7 | publisher=Prentice-Hall | place=New Jersey | url=https://books.google.com/books?id=pJbuAAAAMAAJ&q=kormondy+concepts+of+ecology}}</ref><ref name="Proulx05">{{cite journal|last1=Proulx |first1=S. R. |last2=Promislow |first2=D. E. L. |last3=Phillips |first3=P. C. |title=Network thinking in ecology and evolution |journal=Trends in Ecology and Evolution |volume=20 |issue=6 |pages=345–353 |year=2005 |doi=10.1016/j.tree.2005.04.004 |url=http://eeb19.biosci.arizona.edu/Faculty/Dornhaus/courses/materials/papers/Proulx%20Promislow%20Phillips%20networks%20ecol%20evol.pdf |pmid=16701391 |url-status=dead |archive-url=https://web.archive.org/web/20110815122330/http://eeb19.biosci.arizona.edu/Faculty/Dornhaus/courses/materials/papers/Proulx%20Promislow%20Phillips%20networks%20ecol%20evol.pdf |archive-date=2011-08-15 }}</ref>]]

{{quote box
| quote = Food webs are the road-maps through Darwin's famous 'entangled bank' and have a long history in ecology. Like maps of unfamiliar ground, food webs appear bewilderingly complex. They were often published to make just that point. Yet recent studies have shown that food webs from a wide range of terrestrial, freshwater, and marine communities share a remarkable list of patterns.<ref name="Pimm91" />{{Rp|669}}
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Links in food webs map the feeding connections (who eats whom) in an [[ecological community]]. ''Food cycle'' is an obsolete term that is [[synonymous]] with food web. Ecologists can broadly group all life forms into one of two trophic layers, the [[autotrophs]] and the [[heterotrophs]]. Autotrophs [[primary production|produce]] more [[biomass (ecology)|biomass]] energy, either [[chemoautotroph|chemically]] without the sun's energy or by capturing the sun's energy in [[photosynthesis]], than they use during [[metabolism|metabolic]] [[Cellular respiration|respiration]]. Heterotrophs consume rather than produce biomass energy as they metabolize, grow, and add to levels of [[secondary production]]. A food web depicts a collection of [[Oligophagy|polyphagous]] [[heterotrophs|heterotrophic]] consumers that [[ecological network|network]] and [[Recycling (ecological)|cycle]] the [[energy flow (ecology)|flow of energy]] and [[nutrients]] from a productive base of self-feeding [[autotrophs]].<ref name="Pimm91" /><ref name="Odum05" /><ref name="Benke10" />

The base or basal species in a food web are those species without prey and can include autotrophs or [[saprophyte|saprophytic]] [[detritivore]]s (i.e., the community of [[decomposer]]s in [[soil]], [[biofilms]], and [[periphyton]]). Feeding connections in the web are called trophic links. The number of trophic links per consumer is a measure of food web [[Ecological network|connectance]]. [[Food chains]] are nested within the trophic links of food webs. Food chains are linear (noncyclic) feeding pathways that trace [[monophagous]] consumers from a base species up to the [[Apex predator|top consumer]], which is usually a larger predatory carnivore.<ref name="Allesina08">{{cite journal | last1=Allesina | first1=S. | last2=Alonso | first2=D. | last3=Pascual | first3=M. | title=A general model for food web structure. | journal=Science | volume=320 | issue=5876 | pages=658–661 | doi=10.1126/science.1156269 | pmid=18451301 | url=http://cas.bellarmine.edu/tietjen/Secret/PlantGenome/General%20Model%20for%20Food%20WEb%20Structure.pdf | url-status=dead | archive-url=https://web.archive.org/web/20110928122643/http://cas.bellarmine.edu/tietjen/Secret/PlantGenome/General%20Model%20for%20Food%20WEb%20Structure.pdf | archive-date=2011-09-28 | bibcode=2008Sci...320..658A | year=2008 | s2cid=11536563 | accessdate=2011-06-10 }}</ref><ref name="Azam83">{{cite journal | last1=Azam | first1=F. | last2=Fenche | first2=T. | last3=Field | first3=J. G. | last4=Gra | first4=J. S. | last5=Meyer-Reil | first5=L. A. | last6=Thingstad | first6=F. | title=The ecological role of water-column microbes in the sea | journal=Mar. Ecol. Prog. Ser. | volume=10 | pages=257–263 | year=1983 | url=http://www.soest.hawaii.edu/oceanography/courses/OCN621/Spring2011/Azam%20et%20al_loop.pdf | doi=10.3354/meps010257| bibcode=1983MEPS...10..257A | doi-access=free }}</ref><ref name="Uroz09">{{cite journal | last1=Uroz | first1=S. | last2=Calvarus | first2=C. | last3=Turpault | first3=M. | last4=Frey-Klett | first4=P. | title=Mineral weathering by bacteria: ecology, actors and mechanisms | journal=Trends in Microbiology | volume=17 | issue=8 | year=2009 | pages=378–387 | doi=10.1016/j.tim.2009.05.004 | url=http://mycor.nancy.inra.fr/GIteam/wp-content/uploads/2009/11/Uroz-TiM-2009.pdf | pmid=19660952 }}{{dead link|date=October 2017 |bot=InternetArchiveBot |fix-attempted=yes }}</ref>

{{external media | width = 210px | float = right | headerimage=  | video1 = [https://knowablemagazine.org/article/living-world/2018/why-you-should-care-about-parasites "Why you should care about parasites"], 12.14.2018, ''[[Knowable Magazine]]'' }}

Linkages connect to nodes in a food web, which are aggregates of [[taxon|biological taxa]] called [[trophic species]]. Trophic species are functional groups that have the same predators and prey in a food web. Common examples of an aggregated node in a food web might include [[parasites]], microbes, [[decomposers]], [[Saprotrophic nutrition|saprotrophs]], [[Consumer (food chain)|consumer]]s, or [[predation|predators]], each containing many species in a web that can otherwise be connected to other trophic species.<ref name="Williams00">{{cite journal | last1=Williams | first1=R. J. | last2=Martinez | first2=N. D. | title=Simple rules yield complex food webs. | journal=Nature | volume=404 | issue=6774 | pages=180–183 | year=2000 | doi=10.1038/35004572 | pmid=10724169 | bibcode=2000Natur.404..180W | s2cid=205004984 | url=http://userwww.sfsu.edu/~parker/bio840/pdfs/WilliamsMartinez2000.pdf | access-date=2011-06-13 | archive-date=2012-03-15 | archive-url=https://web.archive.org/web/20120315194008/http://userwww.sfsu.edu/~parker/bio840/pdfs/WilliamsMartinez2000.pdf | url-status=dead }}</ref><ref name="Post02">{{cite journal | last1=Post | first1=D. M. | title=The long and short of food chain length | journal=Trends in Ecology and Evolution | volume=17 | issue=6 | pages=269–277 | year=2002 | url=http://limnology.wisc.edu/courses/zoo955/Spring2005/food%20web%20seminar%20papers/post02TREE.pdf | doi=10.1016/S0169-5347(02)02455-2 | url-status=dead | archive-url=https://web.archive.org/web/20110728142821/http://limnology.wisc.edu/courses/zoo955/Spring2005/food%20web%20seminar%20papers/post02TREE.pdf | archive-date=2011-07-28 | accessdate=2011-06-10 }}</ref>

===Trophic levels===
{{main|Trophic level}}

[[File:TrophicWeb.jpg|thumb|upright=1.8|A trophic pyramid (a) and a simplified community food web (b) illustrating ecological relations among creatures that are typical of a northern [[Boreal ecosystem|Boreal]] terrestrial ecosystem. The trophic pyramid roughly represents each level's biomass (usually measured as total dry weight). Plants generally have the greatest biomass. Names of trophic categories are shown to the right of the pyramid. Like many wetlands, some ecosystems do not organize as a strict pyramid because aquatic plants are less productive than long-lived terrestrial plants such as trees. Ecological trophic pyramids are typically one of three kinds: 1) pyramid of numbers, 2) pyramid of biomass, or 3) pyramid of energy.<ref name="Odum05">{{cite book | last1=Odum | first1=E. P. | last2=Barrett | first2=G. W. | title=Fundamentals of Ecology | edition=5th | publisher=Brooks/Cole, a part of Cengage Learning | year=2005 | isbn=978-0-534-42066-6 | url=http://www.cengage.com/aushed/instructor.do?disciplinenumber=1041&product_isbn=9780534420666&courseid=BI03&codeid=2BF6&subTab=&mainTab=About_the_Book&mailFlag=true&topicName= | url-status=dead | archive-url=https://web.archive.org/web/20110820163059/http://www.cengage.com/aushed/instructor.do?disciplinenumber=1041&product_isbn=9780534420666&courseid=BI03&codeid=2BF6&subTab=&mainTab=About_the_Book&mailFlag=true&topicName= | archive-date=2011-08-20 }}</ref>]]

Food webs have trophic levels and positions. Basal species, such as plants, form the first level and are the resource-limited species that feed on no other living creature in the web. Basal species can be autotrophs or [[detritivores]], including "decomposing organic material and its associated microorganisms which we defined as detritus, micro-inorganic material and associated microorganisms (MIP), and vascular plant material."<ref name="Tavares-Cromar96">{{cite journal | last1=Tavares-Cromar | first1=A. F. | last2=Williams | first2=D. D. | title=The importance of temporal resolution in food web analysis: Evidence from a detritus-based stream | year=1996 | journal=Ecological Monographs | volume=66 | issue=1 | pages=91–113 | doi= 10.2307/2963482 | url=https://tspace.library.utoronto.ca/bitstream/1807/768/2/Importance_of_temporal_resolution_in_food_web_analysis.pdf | jstor=2963482 | bibcode=1996EcoM...66...91T | hdl=1807/768 | hdl-access=free }}</ref>{{rp|94}} Most autotrophs capture the sun's energy in [[chlorophyll]], but some autotrophs (the [[lithotroph|chemolithotrophs]]) obtain energy by the chemical oxidation of inorganic compounds and can grow in dark environments, such as the sulfur bacterium ''[[Thiobacillus]]'', which lives in hot [[sulfur springs]]. The top level has top (or apex) predators that no other species kills directly for their food resource needs. The intermediate levels are filled with omnivores that feed on more than one trophic level and cause energy to flow through several food pathways starting from a basal species.<ref name="Pimm79" />

In the simplest scheme, the first trophic level (level 1) is plants, then herbivores (level 2), and then carnivores (level 3). The trophic level equals one more than the chain length, which is the number of links connecting to the base. The base of the food chain (primary producers or [[detritivore]]s) is set at zero.<ref name="Pimm91" /><ref name="Cousins85">{{cite journal | last1=Cousins | first1=S. | title=Ecologists build pyramids again. | journal=New Scientist | volume=1463 | pages=50–54 | url=https://books.google.com/books?id=NOPpwVvNu44C&q=trophic+level&pg=PA51 | date=1985-07-04 }}{{Dead link|date=May 2024 |bot=InternetArchiveBot |fix-attempted=yes }}</ref> Ecologists identify feeding relations and organize species into trophic species through extensive gut content analysis of different species. The technique has been improved through the use of stable isotopes to better trace energy flow through the web.<ref name="McCann07">{{cite journal|last1=McCann |first1=K. |title=Protecting biostructure |pmid=17330028 |doi=10.1038/446029a |journal=Nature |year=2007 |volume=446 |page=29 |issue=7131 |bibcode=2007Natur.446...29M |s2cid=4428058 |doi-access=free }}</ref> It was once thought that omnivory was rare, but recent evidence suggests otherwise. This realization has made trophic classifications more complex.<ref name="Thompson07">{{cite journal |last1=Thompson |first1=R. M. |last2=Hemberg |first2=M. |last3=Starzomski |first3=B. M. |last4=Shurin |first4=J. B. |title=Trophic levels and trophic tangles: The prevalence of omnivory in real food webs. |journal=Ecology |volume=88 |issue=3 |pages=612–617 |doi=10.1890/05-1454 |url=http://myweb.dal.ca/br238551/thompson_hem_star_shur_ecology07.pdf |pmid=17503589 |date=March 2007 |bibcode=2007Ecol...88..612T |url-status=dead |archive-url=https://web.archive.org/web/20110815150110/http://myweb.dal.ca/br238551/thompson_hem_star_shur_ecology07.pdf |archive-date=2011-08-15 |accessdate=2011-06-10 }}</ref>

===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|url-access=subscription }}</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>

===Energy flow and biomass===
[[File:EnergyFlowFrog.jpg|thumb|upright=1.25|Energy flow diagram of a frog. The frog represents a node in an extended food web. The energy ingested is utilized for metabolic processes and transformed into biomass. The energy flow continues on its path if the frog is ingested by predators, parasites, or as a decaying [[Carrion|carcass]] in soil. This energy flow diagram illustrates how energy is lost as it fuels the metabolic process that transform the energy and nutrients into biomass.]]
{{main|Energy flow (ecology)}}
{{See also|Ecological efficiency}}
{{quote box
| quote = The Law of Conservation of Mass dates from Antoine Lavoisier's 1789 discovery that mass is neither created nor destroyed in chemical reactions. In other words, the mass of any one element at the beginning of a reaction will equal the mass of that element at the end of the reaction.<ref name="Sterner11">{{cite journal | last1=Sterner | first1=R. W. | last2=Small | first2=G. E. | last3=Hood | first3=J. M. | title= The conservation of mass | journal=Nature Education Knowledge | volume=2 | issue=1 | page=11 | url=http://www.nature.com/scitable/knowledge/library/the-conservation-of-mass-17395478}}</ref>{{Rp|11}}
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[[File:EnergyFlowTransformity.jpg|thumb|upright=1.25|An expanded three link energy food chain (1. plants, 2. herbivores, 3. carnivores) illustrating the relationship between food flow diagrams and energy transformity. The transformity of energy becomes degraded, dispersed, and diminished from higher quality to lesser quantity as the energy within a food chain flows from one trophic species into another. Abbreviations: I=input, A=assimilation, R=respiration, NU=not utilized, P=production, B=biomass.<ref name="Odum88">{{cite journal | last1=Odum | first1=H. T. | s2cid=27517361 | year=1988 | title=Self-organization, transformity, and information |doi=10.1126/science.242.4882.1132 | journal=Science | volume=242 | issue=4882 | pages=1132–1139 | jstor=1702630 | pmid=17799729| bibcode=1988Sci...242.1132O | hdl=11323/5713 | hdl-access=free }}</ref>]]

Food webs depict energy flow via trophic linkages. Energy flow is directional, which contrasts against the cyclic flows of material through the food web systems.<ref name="Odum68">{{cite journal | last1=Odum | first1=E. P. | title=Energy flow in ecosystems: A historical review | journal=American Zoologist | year=1968 | volume=8 | issue=1 | pages=11–18 | doi=10.1093/icb/8.1.11 | doi-access=free }}</ref> Energy flow "typically includes production, consumption, assimilation, non-assimilation losses (feces), and respiration (maintenance costs)."<ref name="Benke10">{{cite journal | last1=Benke | first1=A. C. | title=Secondary production | journal=Nature Education Knowledge | volume=1 | issue=8 | page=5 | year=2010 | url=http://www.nature.com/scitable/knowledge/library/secondary-production-13234142}}</ref>{{rp|5}} In a very general sense, energy flow (E) can be defined as the sum of [[metabolism|metabolic]] production (P) and respiration (R), such that E=P+R.

Biomass represents stored energy. However, concentration and quality of nutrients and energy is variable. Many plant fibers, for example, are indigestible to many herbivores leaving grazer community food webs more nutrient limited than detrital food webs where bacteria are able to access and release the nutrient and energy stores.<ref name="Mann88">{{cite journal | last1=Mann | first1=K. H. | year=1988 | title=Production and use of detritus in various freshwater, estuarine, and coastal marine ecosystems | journal=Limnol. Oceanogr. | volume=33 | issue=2 | pages=910–930 | url=http://nospam.aslo.org/lo/toc/vol_33/issue_4_part_2/0910.pdf | doi=10.4319/lo.1988.33.4_part_2.0910 | url-status=dead | archive-url=https://web.archive.org/web/20120425235224/http://nospam.aslo.org/lo/toc/vol_33/issue_4_part_2/0910.pdf | archive-date=2012-04-25 | accessdate=2011-06-28 }}</ref><ref name="Kooijman04">{{cite journal | last1=Koijman | first1=S. A. L. M. | last2=Andersen | first2=T. | last3=Koo | first3=B. W. | title=Dynamic energy budget representations of stoichiometric constraints on population dynamics | journal=Ecology | volume=85 | issue=5 | pages=1230–1243 | year=2004 | url=http://www.bio.vu.nl/thb/research/bib/KooyAnde2004.pdf | doi=10.1890/02-0250| bibcode=2004Ecol...85.1230K }}</ref> "Organisms usually extract energy in the form of carbohydrates, lipids, and proteins. These polymers have a dual role as supplies of energy as well as building blocks; the part that functions as energy supply results in the production of nutrients (and carbon dioxide, water, and heat). Excretion of nutrients is, therefore, basic to metabolism."<ref name="Kooijman04" />{{rp|1230–1231}} The units in energy flow webs are typically a measure mass or energy per m<sup>2</sup> per unit time. Different consumers are going to have different metabolic assimilation efficiencies in their diets. Each trophic level transforms energy into biomass. Energy flow diagrams illustrate the rates and efficiency of transfer from one trophic level into another and up through the hierarchy.<ref name="Andersen09">{{cite journal | last1=Anderson | first1=K. H. | last2=Beyer | first2=J. E. | last3=Lundberg | first3=P. | title=Trophic and individual efficiencies of size-structured communities | journal=Proc Biol Sci | year=2009 | volume=276 | issue=1654 | pages=109–114 | doi=10.1098/rspb.2008.0951 | pmc=2614255 | pmid=18782750}}</ref><ref name="Benke11">{{cite journal | last1=Benke | first1=A. C. | year=2011 | title=Secondary production, quantitative food webs, and trophic position | journal=Nature Education Knowledge | volume=2 | issue=2 | page=2 | url=http://www.nature.com/scitable/knowledge/library/secondary-production-quantitative-food-webs-and-trophic-17653963}}</ref>

It is the case that the [[biomass]] of each [[trophic level]] decreases from the base of the chain to the top. This is because energy is lost to the environment with each transfer as [[entropy]] increases. About eighty to ninety percent of the energy is expended for the organism's life processes or is lost as heat or waste. Only about ten to twenty percent of the organism's energy is generally passed to the next organism.<ref name="entropy">{{cite book | last= Spellman| first= Frank R.| title= The Science of Water: Concepts and Applications| year= 2008| publisher= CRC Press| page= 165| url= https://books.google.com/books?id=Grivqd7tLuAC&q=%22is+lost+as+heat+and+wastes%22&pg=PA165| isbn= 978-1-4200-5544-3}}</ref> The amount can be less than one percent in [[animals]] consuming less digestible plants, and it can be as high as forty percent in [[zooplankton]] consuming [[phytoplankton]].<ref>{{cite book | last= Kent| first= Michael| title= Advanced Biology| year= 2000| publisher= Oxford University Press US| page= 511| url= https://books.google.com/books?id=8aw4ZWLABQkC&q=%22trophic+efficiency+of+less+than+1%25%22&pg=PA511| isbn= 978-0-19-914195-1}}</ref> Graphic representations of the biomass or productivity at each tropic level are called [[ecological pyramid]]s or trophic pyramids. The transfer of energy from primary producers to top consumers can also be characterized by energy flow diagrams.<ref>{{cite book | last= Kent| first= Michael| title= Advanced Biology| year= 2000| publisher= Oxford University Press US| page= 510| url= https://books.google.com/books?id=8aw4ZWLABQkC&q=%22by+an+energy+flow+diagram%22&pg=PA510| isbn= 978-0-19-914195-1}}</ref>

===Food chain===
{{Main|food chain}}

A common metric used to quantify food web trophic structure is food chain length. Food chain length is another way of describing food webs as a measure of the number of species encountered as energy or nutrients move from the plants to top predators.<ref name="Post93">{{cite journal|last=Post|first=D. M.|title= The long and short of food-chain length|year=1993|journal = Trends in Ecology and Evolution|volume=17|issue=6| pages=269–277|doi=10.1016/S0169-5347(02)02455-2}}</ref>{{Rp|269}} There are different ways of calculating food chain length depending on what parameters of the food web dynamic are being considered: connectance, energy, or interaction.<ref name="Post93" /> In its simplest form, the length of a chain is the number of links between a trophic consumer and the base of the web. The mean chain length of an entire web is the arithmetic average of the lengths of all chains in a food web.<ref name="Odum">{{cite book | last1=Odum | first1=E. P. | last2=Barrett | first2=G. W. | title=Fundamentals of ecology | publisher=Brooks Cole <!--| isbn= 0-534-42066-4--> | isbn=978-0-534-42066-6 | year=2005 | page=598 | url=http://www.cengage.com/search/totalsearchresults.do?N=16&image.x=0&image.y=0&keyword_all=fundamentals+of+ecology }}{{dead link|date=January 2018 |bot=InternetArchiveBot |fix-attempted=yes }}</ref><ref name="Pimm79">{{cite journal |last1=Pimm |first1=S. L. |title=The structure of food webs |journal=Theoretical Population Biology |volume=16 |issue=2 |pages=144–158 |year=1979 |url=http://www.nicholas.duke.edu/people/faculty/pimm/publications/pimmreprints/12_Pimm_TPB_1979.pdf |doi=10.1016/0040-5809(79)90010-8 |pmid=538731 |bibcode=1979TPBio..16..144P |url-status=dead |archive-url=https://web.archive.org/web/20110927230926/http://www.nicholas.duke.edu/people/faculty/pimm/publications/pimmreprints/12_Pimm_TPB_1979.pdf |archive-date=2011-09-27 |accessdate=2011-06-13 }}</ref>

In a simple predator-prey example, a deer is one step removed from the plants it eats (chain length = 1) and a wolf that eats the deer is two steps removed from the plants (chain length = 2). The relative amount or strength of influence that these parameters have on the food web address questions about:
* the identity or existence of a few dominant species (called strong interactors or keystone species)
* the total number of species and food-chain length (including many weak interactors) and
* how community structure, function and stability is determined.<ref name="Worm03">{{cite journal|last1=Worm|first1=B.|last2=Duffy|first2=J.E.|title= Biodiversity, productivity and stability in real food webs|year=2003|journal = Trends in Ecology and Evolution|volume=18|issue=12| pages=628–632|doi=10.1016/j.tree.2003.09.003|bibcode=2003TEcoE..18..628W }}</ref><ref name="Paine80">{{cite journal | last1=Paine | first1=R. T. | s2cid=55981512 | title=Food webs: Linkage, interaction strength and community infrastructure. | journal=Journal of Animal Ecology | volume=49 | issue=3 | year=1980 | pages=666–685 | jstor=4220 | doi=10.2307/4220| bibcode=1980JAnEc..49..666P }}</ref>

=== Ecological pyramids ===
[[File:EcologicalPyramids.jpg|thumb|upright=1.5|Illustration of a range of ecological pyramids, including '''top''' pyramid of numbers, '''middle''' pyramid of biomass, and '''bottom''' pyramid of energy. The terrestrial forest (summer) and the [[English Channel]] ecosystems exhibit inverted pyramids.''Note:'' trophic levels are not drawn to scale and the pyramid of numbers excludes microorganisms and soil animals. ''Abbreviations:'' P=Producers, C1=Primary consumers, C2=Secondary consumers, C3=Tertiary consumers, S=Saprotrophs.<ref name="Odum05" />]]
[[File:Trophiclevels.jpg|thumb|A four level trophic pyramid sitting on a layer of soil and its community of decomposers.]]
[[File:TrophicEnergy.jpg|thumb|A three layer trophic pyramid linked to the biomass and energy flow concepts.]]

In a pyramid of numbers, the number of consumers at each level decreases significantly, so that a single [[top consumer]], (e.g., a [[polar bear]] or a [[human]]), will be supported by a much larger number of separate producers. There is usually a maximum of four or five links in a food chain, although food chains in [[aquatic ecosystems]] are more often longer than those on land. Eventually, all the energy in a food chain is dispersed as heat.<ref name="Odum05"/>

[[Ecological pyramid]]s place the primary producers at the base. They can depict different numerical properties of ecosystems, including numbers of individuals per unit of area, biomass (g/m<sup>2</sup>), and energy (k cal m<sup>−2</sup> yr<sup>−1</sup>). The emergent pyramidal arrangement of trophic levels with amounts of energy transfer decreasing as species become further removed from the source of production is one of several patterns that is repeated amongst the planets ecosystems.<ref name="Proulx05" /><ref name="Pimm91">{{Cite journal |last1=Pimm |first1=S. L. |last2=Lawton |first2=J. H. |last3=Cohen |first3=J. E. |title=Food web patterns and their consequences |journal=Nature |volume=350 |issue=6320 |pages=669–674 |year=1991 |url=http://www.nicholas.duke.edu/people/faculty/pimm/publications/pimmreprints/71_Pimm_Lawton_Cohen_Nature.pdf |doi=10.1038/350669a0 |url-status=dead |archive-url=https://web.archive.org/web/20100610135513/http://nicholas.duke.edu/people/faculty/pimm/publications/pimmreprints/71_Pimm_Lawton_Cohen_Nature.pdf |archive-date=2010-06-10 |bibcode=1991Natur.350..669P |s2cid=4267587 |accessdate=2011-06-13 }}</ref><ref name="Raffaelli02">{{cite journal|last = Raffaelli|first = D. |s2cid = 177263265 |title =From Elton to mathematics and back again|year = 2002| journal = Science|volume = 296|issue=5570|pages=1035–1037|doi=10.1126/science.1072080|pmid = 12004106}}</ref> The size of each level in the pyramid generally represents biomass, which can be measured as the dry weight of an organism.<ref name="Ricklefs96"/> Autotrophs may have the highest global proportion of biomass, but they are closely rivaled or surpassed by microbes.<ref name="Whitman98">{{Cite journal |last1 = Whitman |first1 = W. B. |last2 = Coleman |first2 = D. C. |last3 = Wieb |first3 = W. J. |title = Prokaryotes: The unseen majority |journal = Proc. Natl. Acad. Sci. USA |volume = 95 |pages=6578–83 |year = 1998 |doi = 10.1073/pnas.95.12.6578 |pmid = 9618454 |issue = 12 |pmc = 33863|bibcode = 1998PNAS...95.6578W |doi-access = free }}</ref><ref name="Groombridge02">{{Cite book |last1 = Groombridge |first1 = B. |last2 = Jenkins |first2 = M. |title = World Atlas of Biodiversity: Earth's Living Resources in the 21st Century |publisher = World Conservation Monitoring Centre, United Nations Environment Programme |year = 2002 |url = https://books.google.com/books?id=_kHeAXV5-XwC|isbn = 978-0-520-23668-4}}</ref>

Pyramid structure can vary across ecosystems and across time. In some instances biomass pyramids can be inverted. This pattern is often identified in aquatic and coral reef ecosystems. The pattern of biomass inversion is attributed to different sizes of producers. Aquatic communities are often dominated by producers that are smaller than the consumers that have high growth rates. Aquatic producers, such as planktonic algae or aquatic plants, lack the large accumulation of [[secondary growth]] as exists in the woody trees of terrestrial ecosystems. However, they are able to reproduce quickly enough to support a larger biomass of grazers. This inverts the pyramid. Primary consumers have longer lifespans and slower growth rates that accumulates more biomass than the producers they consume. Phytoplankton live just a few days, whereas the zooplankton eating the phytoplankton live for several weeks and the fish eating the zooplankton live for several consecutive years.<ref>{{cite book | last= Spellman| first= Frank R.| title= The Science of Water: Concepts and Applications| year= 2008| publisher= CRC Press| page= 167| url= https://books.google.com/books?id=Grivqd7tLuAC&q=However,+biomass+pyramids+can+sometimes+be+inverted.&pg=PA167| isbn= 978-1-4200-5544-3}}</ref> Aquatic predators also tend to have a lower death rate than the smaller consumers, which contributes to the inverted pyramidal pattern. Population structure, migration rates, and environmental refuge for prey are other possible causes for pyramids with biomass inverted. Energy pyramids, however, will always have an upright pyramid shape if all sources of food energy are included and this is dictated by the [[second law of thermodynamics]].<ref name="Odum05" /><ref name="Wang09">{{cite journal | last1=Wang | first1=H. | last2=Morrison | first2=W. | last3=Singh | first3=A. | last4=Weiss | first4=H. | title=Modeling inverted biomass pyramids and refuges in ecosystems | journal=Ecological Modelling | volume=220 | issue=11 | pages=1376–1382 | doi=10.1016/j.ecolmodel.2009.03.005 | year=2009 | bibcode=2009EcMod.220.1376W | url=http://people.math.gatech.edu/~weiss/pub/General_Mechanisms_Final.pdf | url-status=dead | archive-url=https://web.archive.org/web/20111007214333/http://people.math.gatech.edu/~weiss/pub/General_Mechanisms_Final.pdf | archive-date=2011-10-07 | accessdate=2011-07-05 }}</ref>