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[[Image:FoodWeb.jpg|thumb|right|280px|<center>A [[freshwater]] [[Aquatic ecosystem|aquatic]] and [[Terrestrial ecoregion|terrestrial]] food web.</center>]]
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A '''food web''' (or '''food cycle''') depicts feeding connections (what-eats-what) in an [[ecological community]] and hence is also referred to  as a [[Consumer-resource systems|consumer-resource system]]. Ecologists can broadly lump all life forms into one of two categories called [[trophic level]]s: 1) the [[autotroph]]s, and 2) the [[heterotrophs]]. To [[Maintenance of an organism|maintain]] their bodies, grow, develop, and to [[Reproduction|reproduce]], autotrophs produce [[organic matter|organic]] matter from [[inorganic]] substances, including both [[mineral]]s and [[gas]]es such as [[carbon dioxide]]. These [[chemical reaction]]s require [[energy]], which mainly comes from the [[sun]] and largely by [[photosynthesis]], although a very small amount comes from [[hydrothermal vent]]s and [[hot spring]]s.  A gradient exists between trophic levels running from complete autotrophs that obtain their sole source of carbon from the atmosphere, to [[mixotrophs]] (such as [[carnivorous plants]]) that are autotrophic organisms that partially obtain organic matter from sources other than the atmosphere, and complete [[heterotrophs]] that must feed to obtain organic matter. The linkages in a food web illustrate the feeding pathways, such as where heterotrophs obtain organic matter by feeding on autotrophs and other heterotrophs. The food web is a simplified illustration of the various methods of feeding that links an ecosystem into a unified system of exchange. There are different kinds of feeding relations that can be roughly divided into [[herbivory]], [[carnivory]], [[scavenging]] and [[parasitism]]. Some of the organic matter eaten by heterotrophs, such as [[sugar]]s, provides energy. Autotrophs and heterotrophs come in all sizes, from [[Microscopic scale|microscopic]] to many [[tonne]]s - from [[cyanobacteria]] to [[giant redwood]]s, and from [[virus]]es and [[bdellovibrio]] to [[blue whale]]s.
 
[[Charles Sutherland Elton|Charles Elton]] pioneered the concept of food cycles, food chains, and food size in his classical 1927 book "Animal Ecology"; Elton's 'food cycle' was replaced by 'food web' in a subsequent ecological text. Elton organized species into [[Functional group (ecology)|functional groups]], which was the basis for [[Raymond Lindeman]]'s classic and landmark paper in 1942 on trophic dynamics. Lindeman emphasized the important role of [[decomposer]] organisms in a [[trophic level|trophic system of classification]]. The notion of a food web has a historical foothold in the writings of [[Charles Darwin]] and his terminology, including an "entangled bank", "web of life", "web of complex relations", and in reference to the decomposition actions of earthworms he talked about "the continued movement of the particles of earth". Even earlier, in 1768 [[John Bruckner]] described nature as "one continued web of life".
 
Food webs are limited representations of real ecosystems as they necessarily aggregate many species into [[trophic species]], which are functional groups of species that have the same predators and prey in a food web. Ecologists use these simplifications in [[quantitative research|quantitative]] (or mathematical) [[Ecosystem model|models]] of trophic or [[consumer-resource systems]] dynamics. Using these models they can measure and test for generalized patterns in the structure of real food web networks. Ecologists have identified non-random properties in the [[Network topology|topographic]] structure of food webs. Published examples that are used in [[meta analysis]] are of variable quality with omissions. However, the number of empirical studies on community webs is on the rise and the mathematical treatment of food webs using [[network theory]] had identified patterns that are common to all. [[Power law|Scaling laws]], for example, predict a relationship between the topology of food web predator-prey linkages and levels of [[species richness]].
 
==Taxonomy of a food web :==
[[File:FoodWebSimple.jpg|thumb|400px|left|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=0-13-478116-3 | publisher=Prentice-Hall | place=New Jersey | url=http://books.google.ca/books?id=pJbuAAAAMAAJ&q=kormondy+concepts+of+ecology&dq=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}}</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 the antiquated term that is [[synonymous]] with food web. Ecologists can broadly lump all life forms into one of two trophic layers, the [[autotrophs]] and the [[heterotrophs]]. Autotrophs [[primary production|produce]] more [[biomass energy]], either [[chemoautotroph|chemically]] without the suns energy or by capturing the suns 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 | url=http://cas.bellarmine.edu/tietjen/Secret/PlantGenome/General%20Model%20for%20Food%20WEb%20Structure.pdf}}</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}}</ref><ref name="Uroz09">{{cite journal | last1=Uroz | first1=S. | last2=Calvarus | first2=C. | last3=Turpault | first3=M. | last4=Frey-Klett | firs4=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}}</ref>
 
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 | url=http://userwww.sfsu.edu/~parker/bio840/pdfs/WilliamsMartinez2000.pdf}}</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}}</ref>
 
===Trophic levels===
{{main|Trophic level}}
 
[[File:TrophicWeb.jpg|thumb|right|450px|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 the biomass (usually measured as total dry-weight) at each level. Plants generally have the greatest biomass. Names of trophic categories are shown to the right of the pyramid. Some ecosystems, such as many wetlands, do not organize as a strict pyramid, because aquatic plants are not as productive as 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=0-534-42066-4 | 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=}}</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 | jstor=2963482}}</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 which no other species kills directly for its food resource needs. The intermediate levels are filled with omnivores that feed on more than one trophic level and cause energy to flow through a number of 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 is equal to 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=http://books.google.ca/books?id=NOPpwVvNu44C&pg=PA51&dq=trophic+level#v=onepage&q=trophic%20level&f=false}}</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 | url=http://www.bolinfonet.org/pdf/McCann_2007_biostructure.pdf | issue=7131}}</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 | pages=612–617 | doi=10.1890/05-1454 | url=http://myweb.dal.ca/br238551/thompson_hem_star_shur_ecology07.pdf | pmid=17503589}}</ref>
 
===Trophic dynamics===
 
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}}</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">{{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 | year=2007 | pages=612–617 | url=http://myweb.dal.ca/br238551/thompson_hem_star_shur_ecology07.pdf | doi=10.1890/05-1454 | pmid=17503589}}</ref>{{rp|612}}
 
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. 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 | pages=291–301 | url=http://limnology.wisc.edu/courses/zoo955/Spring2005/food%20web%20seminar%20papers/fretwell_food_chain_dynamics_oikos.pdf}}</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/Polis_1996_AmerNat.pdf}}</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}}</ref><ref name="Schmitz08">{{cite journal | last1=Schmitz | first1=O. J. | 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 | url=http://www.annualreviews.org/doi/abs/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=http://books.google.ca/books?id=_8bHeXvUc08C&printsec=frontcover&dq=Multitrophic+Level+Interactions#v=onepage&q&f=false | page=282 | isbn=0-521-79110-3}}</ref>
 
Another example of a multi-trophic 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. et al.|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}}</ref>
 
===Energy flow and 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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<div class="thumb tright" style="background:#f9f9f9; border:1px solid #ccc; margin:0.5em;">
[[File:EnergyFlowFrog.jpg|200px]][[File:EnergyFlowTransformity.jpg|200px]]
<div style="border: none; width:375px;"><div class="thumbcaption">'''Left:''' 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.<br> '''Right:''' 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. | 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}}</ref>
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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 | url=http://icb.oxfordjournals.org/content/8/1/11.short}}</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.
 
The mass (or biomass) of something is equal to its energy content. [[Mass–energy equivalence|Mass and energy are closely intertwined]]. 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}}</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}}</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= http://books.google.com/?id=Grivqd7tLuAC&pg=PA165&dq=%22is+lost+as+heat+and+wastes%22&cd=2#v=onepage&q=%22is%20lost%20as%20heat%20and%20wastes%22| 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= http://books.google.com/?id=8aw4ZWLABQkC&pg=PA511&dq=%22trophic+efficiency+of+less+than+1%25%22&cd=1#v=onepage&q=%22trophic%20efficiency%20of%20less%20than%201%25%22| 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= http://books.google.com/?id=8aw4ZWLABQkC&pg=PA510&dq=%22by+an+energy+flow+diagram%22&cd=1#v=onepage&q=%22by%20an%20energy%20flow%20diagram%22| 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}}</blockquote> 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="Odum05">{{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}}</ref><ref name="Pimm79">{{cite journal | last1=Pimm | first1=S. L. | title= The structure of food webs | journal=Theoretical population biology | volume=16 | pages=144–158 | year=1979 | url=http://www.nicholas.duke.edu/people/faculty/pimm/publications/pimmreprints/12_Pimm_TPB_1979.pdf}}</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 (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}}</ref><ref name="Paine80">{{cite journal | last1=Paine | first1=R. T. | 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}}</ref>
 
=== Ecological pyramids ===
{{See also|Ecological pyramid}}
 
<div class="thumb tleft" style="background:#f9f9f9; border:1px solid #ccc; margin:0.5em;">
[[File:Trophiclevels.jpg|170px]][[File:TrophicEnergy.jpg|192px]] <br>
[[File:EcologicalPyramids.jpg|345px]]
<div style="border: none; width:350px;"><div class="thumbcaption">'''Top Left:''' A four level trophic pyramid sitting on a layer of soil and its community of decomposers. '''Top right:''' A three layer trophic pyramid linked to the biomass and energy flow concepts. '''Bottom:''' 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" />
</div>
</div>
</div>
 
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 pyramids 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">{{cite journal|last1 = Proulx|first1=Stephen R.|last2=Promislow|first2= Daniel E.L.|last3=Phillips|first3= Patrick C.|title = Network thinking in ecology and evolution|year = 2005| journal = Trends in Ecology and Evolution|volume = 20|issue=6|pages=345–353|doi = 10.1016/j.tree.2005.04.004|pmid = 16701391}}</ref><ref name="Pimm91">{{Cite journal | format=PDF |last = Pimm |first = 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}}</ref><ref name="Raffaelli02">{{cite journal|last = Raffaelli|first = D. |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}}</ref><ref name="Groombridge02">{{Cite book |last = Groombridge |first = 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 = http://books.google.com/?id=_kHeAXV5-XwC&printsec=frontcover|isbn = 0-520-23668-8}}</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= http://books.google.com/?id=Grivqd7tLuAC&pg=PA167&dq=However,+biomass+pyramids+can+sometimes+be+inverted.&cd=1#v=onepage&q=However%2C%20biomass%20pyramids%20can%20sometimes%20be%20inverted.| 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 | url=http://people.math.gatech.edu/~weiss/pub/General_Mechanisms_Final.pdf}}</ref>
 
==Material flux and recycling==
{{main|Nutrient cycle}}
 
Many of the Earth's [[Chemical element|element]]s and [[minerals]] (or mineral nutrients) are contained within the tissues and diets of organisms. Hence, mineral and nutrient cycles trace food web energy pathways. Ecologists employ stoichiometry to analyze the ratios of the main elements found in all organisms: carbon (C), nitrogen (N), phosphorus (P). There is a large transitional difference between many terrestrial and aquatic systems as C:P and C:N ratios are much higher in terrestrial systems while N:P ratios are equal between the two systems.<ref name="Pomeroy70">{{cite journal | last1=Pomeroy | first1=L. R. | title=The strategy of mineral cycling | journal=Annual Review of Ecology and Systematics | volume=1 | pages=171–190 | jstor=2096770}}</ref><ref name="Elser00">{{cite journal | last1=Elser | first1=J. J. | last2=Fagan | first2=W. F. | last3=Donno | first3=R. F. | last4=Dobberfuhl | first4=D. R. | last5=Folarin | first5=A. | last6=Huberty | first6=A. | last7=et al. | title=Nutritional constraints in terrestrial and freshwater food webs | year=2000 | journal=Nature | volume=408 | issue=6812 | pages=578–580 | doi=10.1038/35046058 | url=http://www.rug.nl/biologie/onderzoek/onderzoekInstituten/cees/colloquia/pdf/elser_et_al2_2000.pdf}}</ref><ref name="Koch09">{{cite book | last1=Koch | first1=P. L. | last2=Fox-Dobbs | first2=K. | last3=Newsom | first3=S. D. | title=The isotopic ecology of fossil vertebrates and conservation paleobiology. | book-title=Conservation paleobiology: Using the past to manage for the future, Paleontological Society short course | journal=The Paleontological Society Papers | volume=15 | editor1-last=Diet | first-editor1=G. P. | editor2-last=Flessa | first-editor2=K. W. | pages=95–112 | url=http://www.es.ucsc.edu/~pkoch/pdfs/Koch%20papers/2009/Koch%20et%2009%20PSP%2015.pdf}}</ref> [[Mineral nutrients]] are the material resources that organisms need for growth, development, and vitality. Food webs depict the pathways of mineral nutrient cycling as they flow through organisms.<ref name="Odum05" /><ref name="Lindeman42" /> Most of the primary production in an ecosystem is not consumed, but is recycled by detritus back into useful nutrients.<ref name="Moore04">{{cite journal | last1=Moore | first1=J. C. | last2=Berlow | first2=E. L. | last3=Coleman | first3=D. C. | last4=de Ruiter | first4=P. C. | last5=Dong | first5=Q. | last6=Hastings | first6=A. | last7=et al. | title=Detritus, trophic dynamics and biodiversity | journal=Ecology Letters | year=2004 | volume=7 | issue=7 | pages=584–600 | doi=10.1111/j.1461-0248.2004.00606.x}}</ref> Many of the Earth's microorganisms are involved in the formation of [[minerals]] in a process called [[biomineralization]].<ref name="Lowenstam81">{{cite journal | first1=Lowenstam | last1=H. A. | title=Minerals formed by organisms. | doi=10.1126/science.7008198  | journal=Science | year=1981 | volume=211 | issue=4487 | pages=1126–1131 | jstor=1685216 | pmid=7008198}}</ref><ref name="Warren03">{{cite journal | last1=Warren | first1=L. A. | last2=Kauffman | first2=M. E. | title=Microbial geoengineers | journal=Science | year=2003 | volume=299 | issue=5609 | pages=1027–1029 | doi=10.1126/science.1072076 | jstor=3833546 | pmid=12586932}}</ref><ref name="González-Muñoz10">{{cite journal | last1=González-Muñoz | first1=M. T. | last2=Rodriguez-Navarro | first2=C. | last3=Martínez-Ruiz | first3=F. | last4=Arias | first4=J. M. | last5=Merroun | first5=M. L. | last6=Rodriguez-Gallego | first6=M. | title=Bacterial biomineralization: new insights from Myxococcus-induced mineral precipitation. | journal=Geological Society, London, Special Publications | volume=336 | issue=1 | pages=31–50 | doi=10.1144/SP336.3 | url=http://sp.lyellcollection.org/content/336/1/31.abstract}}</ref> Bacteria that live in [[detritus|detrital]] [[sediments]] create and cycle nutrients and biominerals.<ref name="Gonzalez-Acosta05">{{cite journal | last1=Gonzalez-Acosta | first1=B. | last2=Bashan | first2=Y. | last3=Hernandez-Saavedra | first3=N. Y. | last4=Ascencio | first4=F. | last5=De la Cruz-Agüero | first5=G. | title=Seasonal seawater temperature as the major determinant for populations of culturable bacteria in the sediments of an intact mangrove in an arid region. | journal=FEMS Microbiology Ecology | volume=55 | issue=2 | pages=311–321 | doi=10.1111/j.1574-6941.2005.00019.x | url=http://www.bashanfoundation.org/gmaweb/pdfs/mangrovebalandra.pdf}}</ref> Food web models and nutrient cycles have traditionally been treated separately, but there is a strong functional connection between the two in terms of stability, flux, sources, sinks, and recycling of mineral nutrients.<ref name="DeAngelis89">{{cite journal | last1=DeAngelis | first1=D. L. | last2=Mulholland | first2=P. J. | last3=Palumbo | first3=A. V. | last4=Steinman | first4=A. D. | last5=Huston | first5=M. A. | last6=Elwood | first6=J. W. | title=Nutrient dynamics and food-web stability. | journal=Annual Review of Ecology and Systematics | volume=20 | year=1989 | pages=71–95 | jstor=2097085}}</ref><ref name="Twiss96">{{cite journal | last1=Twiss | first1=M. R. | last2=Campbell | first2=P. G. C. | last3=Auclair | first3=J. | title=Regeneration, recycling, and trophic transfer of trace metals by microbial food-web organisms in the pelagic surface waters of Lake Erie. | journal= Limnology and Oceanography | volume=41 | issue=7 | year=1996 | pages=1425–1437 | url=http://www.nospam.aslo.org/lo/toc/vol_41/issue_7/1425.pdf}}</ref>
 
==Kinds of food webs==
Food webs are necessarily aggregated and only illustrate a tiny portion of the complexity of real ecosystems. For example, the number of species on the planet are likely in the general order of 10<sup>7</sup>, over 95% of these species consist of [[microbes]] and [[invertebrates]], and relatively few have been named or classified by [[taxonomists]].<ref name="May88">{{cite journal | last1=May | first1=R. M. | title=How many species are there on Earth? | journal=Science | year=1988 | volume=241 | pages=1441–1449  | doi=10.1126/science.241.4872.1441 | url=http://www.cerium.ca/IMG/pdf/May_2520_281988_29_1_.pdf | issue=4872 | pmid=17790039}}</ref><ref name="Beattie10">{{cite journal | last1=Beattie  | first1=A. | last2=Ehrlich | first2=P. | title=The missing link in biodiversity conservation. | journal=Science | year=2010 | volume=328 | issue=5976 | pages=307–308 | doi=10.1126/science.328.5976.307-c | url=http://www.sciencemag.org/content/328/5976/307.3.citation}}</ref><ref name="Ehrlich08">{{cite journal | last1=Ehrlich | first1=P. R. | last2=Pringle | first2=R. M. | title=Colloquium Paper: Where does biodiversity go from here? A grim business-as-usual forecast and a hopeful portfolio of partial solutions | journal=Proceedings of the National Academy of Sciences | volume=105 | issue=S1 | pages=11579–11586 | doi=10.1073/pnas.0801911105 | pmid=18695214 | pmc=2556413}}</ref> It is explicitly understood that natural systems are 'sloppy' and that food web trophic positions simplify the complexity of real systems that sometimes overemphasize many rare interactions. Most studies focus on the larger influences where the bulk of energy transfer occurs.<ref name="Hariston93">{{cite journal | last1=Hairston | first1=N. G. | last2=Hairston | first2=N. G. | title=Cause-effect relationships in energy flow, trophic structure, and interspecific interactions. | journal=The American Naturalist | volume=142 | issue=3 | pages=379–411 | year=1993 | url=
http://limnology.wisc.edu/courses/zoo955/Spring2005/food%20web%20seminar%20papers/hairston93AmNat.pdf | doi=10.1086/285546}}</ref> "These omissions and problems are causes for concern, but on present evidence do not present insurmountable difficulties."<ref name="Pimm91" />{{rp|669}}
 
[[File:Food web and trophic level of the Chengjiang and Burgess Shale - journal.pbio.0060102.g001.jpg|thumb|right|300px|Paleoecological studies can reconstruct fossil food-webs and trophic levels. Primary producers form the base (red spheres), predators at top (yellow spheres), the lines represent feeding links. Original food-webs (left) are simplified (right) by aggregating groups feeding on common prey into coarser grained trophic species.<ref name="Dunne08"/>]]
 
There are different kinds or categories of food webs:
*'''Source web''' - one or more node(s), all of their predators, all the food these predators eat, and so on.
*'''Sink web''' - one or more node(s), all of their prey, all the food that these prey eat, and so on.
*'''Community (or connectedness) web''' - a group of nodes and all the connections of who eats whom.
*'''Energy flow web''' - quantified fluxes of energy between nodes along links between a resource and a consumer.<ref name="Pimm91" /><ref name="Ricklefs96">{{cite book |title=The Economy of Nature |last= Rickleffs |first= Robert, E. |year=1996 |publisher=[[University of Chicago Press]] |isbn=0-7167-3847-3 |page=678}}</ref>
*'''[[Paleoecology|Paleoecological]] web''' - a web that reconstructs ecosystems from the fossil record.<ref name="Dunne08">{{cite journal |  last1=Dunne | first1 = J. A. | last2 = Williams | first2 = R. J. | last3 = Martinez | first3 = N. D. | last4 = Wood | first4 = R. A. | last5 = Erwin | first5 = D. H. | last6 = Dobson | first6 = Andrew P. | title = Compilation and Network Analyses of Cambrian Food Webs. | year = 2008 | journal = PLOS Biology | volume =  6 | issue=4 | doi = 10.1371/journal.pbio.0060102 | url=http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.0060102 | pages = e102}}</ref>
*'''Functional web''' - emphasizes the functional significance of certain connections having strong interaction strength and greater bearing on community organization, more so than energy flow pathways. Functional webs have compartments, which are sub-groups in the larger network where there are different densities and strengths of interaction.<ref name="Paine80">{{cite journal | last1=Paine | first1=R. T. | title=Food webs: Linkage, interaction strength and community infrastructure | journal=Journal of Animal Ecology | volume=49 | issue=3 | pages=666–685 | url=http://ib.berkeley.edu/labs/power/classes/2006fall/ib250/17.pdf}}</ref><ref name="Krause03">{{cite journal | last1=Krause | first1=A. E. | last2=Frank | first2=K. A. | last3=Mason | first3=D. M. | last4=Ulanowicz | first4=R. E. | last5=Taylor | first5=W. W. | year=2003 | title=Compartments revealed in food-web structure |doi=10.1038/nature02115| journal=Nature | volume=426 | issue=6964 | pages=282–285 | url=http://www.glerl.noaa.gov/pubs/fulltext/2003/20030014.pdf | pmid=14628050}}</ref> Functional webs emphasize that "the importance of each population in maintaining the integrity of a community is reflected in its influence on the growth rates of other populations."<ref name="Ricklefs96" />{{rp|511}}
 
Within these categories, food webs can be further organized according to the different kinds of ecosystems being investigated. For example, human food webs, agricultural food webs, detrital food webs, marine food webs, aquatic food webs, soil food webs, Arctic (or polar) food webs, terrestrial food webs, and microbial food webs. These characterizations stem from the ecosystem concept, which assumes that the phenomena under investigation (interactions and feedback loops) are sufficient to explain patterns within boundaries, such as the edge of a forest, an island, a shoreline, or some other pronounced physical characteristic.<ref name="Bormann67" /><ref name="Polis97" /><ref name="O'Neil01" />
 
=== Detrital web ===
In a detrital web, plant and animal matter is broken down by decomposers, e.g., bacteria and fungi, and moves to detritivores and then carnivores.<ref>{{cite book | last1= Gönenç| first1= I. Ethem| last2= Koutitonsky| first2= Vladimir G.| last3= Rashleigh| first3= Brenda| title= Assessment of the Fate and Effects of Toxic Agents on Water Resources| year= 2007| publisher= Springer| page= 279| url= http://books.google.com/?id=nBQYnbsUrBQC&pg=PA278&dq=Water+Resources+grazing+detrital+web&cd=1#v=onepage&q=Water%20Resources%20grazing%20detrital%20web| isbn= 978-1-4020-5527-0}}</ref> There are often relationships between the detrital web and the grazing web.  Mushrooms produced by decomposers in the detrital web become a food source for deer, squirrels, and mice in the grazing web.  [[Earthworm]]s eaten by robins are detritivores consuming decaying leaves.<ref>{{cite book | author= Gil Nonato C. Santos| coauthors= Alfonso C. Danac, Jorge P. Ocampo | title= E-Biology II| year= 2003| publisher= Rex Book Store| page= 58| url= http://books.google.com/?id=L9TwLvnIvnkC&pg=PA58&dq=grazing+web+detrital+web#v=onepage&q=grazing%20web%20detrital%20web| isbn= 978-971-23-3563-1}}</ref>
 
[[File:Soil food webUSDA.jpg|thumb|right|300px|An illustration of a soil food web.]]
 
"Detritus can be broadly defined as any form of non-living organic matter, including different types of plant tissue (e.g. leaf litter, dead wood, aquatic macrophytes, algae), animal tissue (carrion), dead microbes, faeces (manure, dung, faecal pellets, guano, frass), as well as products secreted, excreted or exuded from organisms (e.g. extra-cellular polymers, nectar, root exudates and leachates, dissolved organic matter, extra-cellular matrix, mucilage). The relative importance of these forms of detritus, in terms of origin, size and chemical composition, varies across ecosystems."<ref name="Moore04" />{{rp|585}}
 
==Quantitative food webs==
Ecologists collect data on trophic levels and food webs to statistically model and mathematically calculate parameters, such as those used in other kinds of network analysis (e.g., graph theory), to study emergent patterns and properties shared among ecosystems. There are different ecological dimensions that can be mapped to create more complicated food webs, including: species composition (type of species), [[species richness|richness]] (number of species), biomass (the dry weight of plants and animals), productivity (rates of conversion of energy and nutrients into growth), and stability (food webs over time). A food web diagram illustrating species composition shows how change in a single species can directly and indirectly influence many others. [[Microcosm: Model / experimental ecosystem|Microcosm studies]] are used to simplify food web research into semi-isolated units such as small springs, decaying logs, and laboratory experiments using organisms that reproduce quickly, such as [[daphnia]] feeding on [[algae]] grown under controlled environments in jars of water.<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|ref=harv}}</ref><ref name="Elser01">{{Cite journal |last = Elser |first = J. |last2 = Hayakawa |first2 = K.|last3 = Urabe |first3 = J. |title = Nutrient Limitation Reduces Food Quality for Zooplankton: Daphnia Response to Seston Phosphorus Enrichment. |journal = Ecology |volume = 82 |issue = 3 |pages=898–903 |year = 2001 |doi = 10.1890/0012-9658(2001)082[0898:NLRFQF]2.0.CO;2 |ref = harv}}</ref>
 
While the complexity of real food webs connections are difficult to decipher, ecologists have found mathematical models on networks an invaluable tool for gaining insight into the structure, stability, and laws of food web behaviours relative to observable outcomes. "Food web theory centers around the idea of connectance."<ref name="Paine88">{{cite journal | last1=Paine | first1=R. T. | title=Road maps of interactions or grist for theoretical development? | journal=Ecology | volume=69 | issue=6 | pages=1648–1654 | doi=10.2307/1941141 | url=http://limnology.wisc.edu/courses/zoo955/Spring2005/food%20web%20seminar%20papers/Paine88Ecol.pdf}}</ref>{{rp|1648}} Quantitative formulas simplify the complexity of food web structure. The number of trophic links (t<sub>L</sub>), for example, is converted into a connectance value:
 
:<math>C= \cfrac{t_L}{S(S-1)/2}</math>,
 
where, S(S-1)/2 is the maximum number of binary connections among S species.<ref name="Paine88" /> "Connectance (C) is the fraction of all possible links that are realized (L/S<sup>2</sup>) and represents a standard measure of food web complexity..."<ref name="Williams02">{{cite journal | last1=Williams | first1=R. J. | last2=Berlow | first2=E. L. | last3=Dunne | first3=J. A. | last4=Barabási | first4=A. | last5=Martinez | first5=N. D. | title= Two degrees of separation in complex food webs | journal=Proceedings of the National Academy of Sciences | year=2002 | volume=99 | issue=20 | pages=12913–12916 | doi=10.1073/pnas.192448799}}</ref>{{rp|12913}} The distance (d) between every species pair in a web is averaged to compute the mean distance between all nodes in a web (D)<ref name="Williams02" /> and multiplied by the total number of links (L) to obtain link-density (LD), which is influenced by scale dependent variables such as [[species richness]]. These formulas are the basis for comparing and investigating the nature of non-random patterns in the structure of food web networks among many different types of ecosystems.<ref name="Williams02" /><ref name="Banasek-Richter09">{{cite journal | last1=Banasek-Richter | first1=C. | last2=Bersier | first2=L. L. | last3=Cattin | first3=M. | last4=Baltensperger | first4=R. | last5=Gabriel | first5=J. | last6=Merz | first6=Y. | last7=et al. | title= Complexity in quantitative food webs | journal=Ecology | volume=90 | issue=6 | pages=1470–1477 | url=http://www.unifr.ch/biol/ecology/bersier/publications/Ecology_Banasek-Richter_2009_withAppendices.pdf}}</ref>
 
Scaling laws, complexity, choas, and patterned correlates are common features attributed to food web structure.<ref name="Riede10">{{cite book | last1=Riede | first1=J. O. | last2=Rall | first2=B. C. | last3=Banasek-Richter | first3=C. | last4=Navarrete | first4=S. A. | last5=Wieters | first5=E. A. | last6=Emmerson | first6=M. C. | last7=et al. | title=Scaling of food web properties with diversity and complexity across ecosystems. | editor1-last=Woodwoard | editor1-first=G. | booktitle=Advances in Ecological Research | volume=42 | year=2010 | pages=139–170 | publisher=Academic Press | place=Burlington | isbn=978-0-12-381363-3 | url=http://www.bio.puc.cl/caseb/pdf/prog6/Riede%20et%20al._AER%2010.pdf}}</ref><ref name="Briand87">{{cite journal | last1=Briand | first1= F. | last2= Cohen | first2=J. E. | title=Environmental correlates of food chain length. | year=1987 | journal=Science | issue=4829 | pages=956–960 | doi=10.1126/science.3672136 | url=http://spider.allegheny.edu/employee/M/mostrofs/mywebfiles/Bio330/Bio330Readings/briand_and_cohen.pdf}}</ref>
 
=== Complexity and stability ===
Food webs are complex. Complexity is a measure of an increasing number of permutations and it is also a metaphorical term that conveys the mental intractability or limits concerning unlimited algorithmic possibilities. In food web terminology, complexity is a product of the number of species and connectance.<ref name="Neutel02">{{cite journal | last1=Neutel | first1=A. | last2=Heesterbeek | first2=J. A. P. | last3=de Ruiter | first3=P. D. | title=Stability in real food webs: Weak link in long loops | year=2002 | journal=Science | volume=295 | issue=550 | pages=1120–1123 | doi=10.1126/science.1068326 | url=http://igitur-archive.library.uu.nl/vet/2006-0321-200233/heesterbeek_02_stability_webs.pdf}}</ref><ref name="Leveque03">{{cite book | editor-last=Leveque | editor-first=C. | title=Ecology: From ecosystem to biosphere | year=2003 | page=490 | publisher=Science Publishers | isbn=978-1-57808-294-0 | url=http://books.google.ca/books?id=-h3AFlmGS_kC&printsec=frontcover&dq=Ecology+from+ecosystem+to+biosphere#v=onepage&q&f=false}}</ref><ref name="Proctor05">{{cite journal | last1=Proctor | first1=J. D. | last2=Larson | first2=B. M. H. | title=Ecology, complexity, and metaphor | journal=BioScience | volume=55 | issue=12 | pages=1065–1068 | year=2005 | doi=10.1641/0006-3568(2005)055[1065:ECAM]2.0.CO;2 | url=http://media.eurekalert.org/release_graphics/pdf/EcologyComplexity112205.pdf}}</ref> Connectance is "the fraction of all possible links that are realized in a network".<ref name="Dunne02">{{cite journal | last1=Dunne | first1=J. A. | last2=Williams | first2=R. J. | last3=Martinez | first3=N. D. | title=Food-web structure and network theory: The role of connectance and size | journal=Proceedings of the National Academy of Sciences | volume=99 | issue=20 | pages=12917–12922 | year=2002 | doi=10.1073/pnas.192407699}}</ref>{{rp|12917}} These concepts were derived and stimulated through the suggestion that complexity leads to stability in food webs, such as increasing the number of trophic levels in more species rich ecosystems. This hypothesis was challenged through mathematical models suggesting otherwise, but subsequent studies have shown that the premise holds in real systems.<ref name="Neutel02" /><ref name="Banašek-Richter09">{{cite journal | last1=Banašek-Richter | first1=C. | last2=Bersier | first2=L. | last3=Cattin | first3=M. | last4=Baltensperger | first4=R. | last5=Gabriel | first5=J. | last6=Merz | first6=J. | last7=et al. | title=Complexity in quantitative food webs | year=2009 | journal=Ecology | volume=90 | pages=1470–1477 | doi=10.1890/08-2207.1 | url=http://doc.rero.ch/lm.php?url=1000,43,4,20090727160545-QX/Banasek-Richter_Carolin_-_Complexity_in_quantitative_food_webs_20090727.pdf}}</ref>
 
At different levels in the hierarchy of life, such as the stability of a food web, "the same overall structure is maintained in spite of an ongoing flow and change of components."<ref name="Capra07">{{cite journal | last1=Capra | first1=F. | title=Complexity and life | journal=Syst. Res. | volume=24 | pages=475–479 | year=2007 | doi=10.1002/sres.848}}</ref>{{rp|476}} The farther a living system (e.g., ecosystem) sways from equilibrium, the greater its complexity.<ref name="Capra07" /> Complexity has multiple meanings in the life sciences and in the public sphere that confuse its application as a precise term for analytical purposes in science.<ref name="Proctor05" /><ref name="Peters88">{{cite journal | last1=Peters | first1=R. H. | title=Some general problems for ecology illustrated by food web theory | journal=Ecology | volume=69 | issue=6 | pages=1673–1676 | year=1988 | jstor=1941145}}</ref> Complexity in the life sciences (or [[biocomplexity]]) is defined by the "properties emerging from the interplay of behavioral, biological, physical, and social interactions that affect, sustain, or are modified by living organisms, including humans".<ref name="Michener01">{{cite journal | last1=Michener | first1=W. K. | last2=Baerwald | first2=T. J. | last3=Firth | first3=P. | last4=Palmer | first4=M. A. | last5=Rosenberger | first5=J. L. | last6=Sandlin | first6=E. A. | last7=Zimmerman | first7=H. | year=2001 | title=Defining and unraveling biocomplexity | journal=BioScience | volume=51 | pages=1018–1023 | url=http://holisticbiology.stanford.edu/biocomplexity.pdf}}</ref>{{rp|1018}}
 
Several concepts have emerged from the study of complexity in food webs. Complexity explains many principals pertaining to self-organization, non-linearity, interaction, cybernetic feedback, discontinuity, emergence, and stability in food webs. Nestedness, for example, is defined as "a pattern of interaction in which specialists interact with species that form perfect subsets of the species with which generalists interact"<ref name="Bascompte07">{{cite journal | last1=Bascompte | first1=J. | last2=Jordan | first2=P. | title=Plant-animal mutualistic networks: The architecture of biodiversity. | year=2007 | journal=Annu. Rev. Ecol. Evol. Syst. | volume=38 | pages=567–569 | url=http://ieg.ebd.csic.es/JordiBascompte/Publications/AREES-07.pdf}}</ref>{{rp|575}}, "—that is, the diet of the most specialized species is a subset of the diet of the next more generalized species, and its diet a subset of the next more generalized, and so on."<ref name="Montoya06">{{cite journal | last1=Montoya | first1=J. M. | last2=Pimm | first2=S. L. | last3=Solé | first3=R. V. | title=Ecological networks and their fragility | journal=Nature | volume=442 | issue=7100 | pages=259–264 | year=2006 | doi=10.1038/nature04927 | url=http://eeb19.biosci.arizona.edu/Faculty/Dornhaus/courses/materials/papers/Montoya%20Pimm%20Sole%20networks%20ecol.pdf}}</ref> Until recently, it was thought that food webs had little nested structure, but empirical evidence shows that many published webs have nested subwebs in their assembly.<ref name="Michio10">{{cite journal | last1=Michio | first1=K. | last2=Kato | first2=S. | last3=Sakato | first3=Y. | year=2010 | title=Food webs are built up with nested subwebs | journal=Ecology | volume=91 | pages=3123–3130 | doi=10.1890/09-2219.1 | url=http://www.esajournals.org/doi/pdf/10.1890/09-2219.1}}</ref>
 
Food webs are complex [[ecological network|networks]]. As networks, they exhibit similar structural properties and mathematical laws that have been used to describe other complex systems, such as [[small-world network|small world]] and [[scale free network|scale free properties]]. The small world attribute refers to the many loosely connected nodes, non-random dense clustering of a few nodes (i.e., trophic or [[keystone species]] in ecology), and small path length compared to a regular lattice.<ref name="Dunne02">{{cite journal | last1=Dunne | first1=J. A. | last2=Williams | first2=R. J. | last3=Martinez | first3=N. D. | year=2002 | title=Food-web structure and network theory: The role of connectance and size | journal=Proceedings of the National Academy of Sciences | volume=99 | issue=20 | pages=12917–12922 | doi=10.1073/pnas.192407699}}</ref><ref name="Montoya02">{{cite journal | last1=Montoya | first1=J. M. | last2=Solé | first2=R. V. | title=Small world patterns in food webs | journal=Journal of Theoretical Biology | volume=214 | issue=3 | pages=405–412 | year=2002 | doi=10.1006/jtbi.2001.2460 | url=http://complex.upf.es/papers/Small_World_Patterns_in_Food_Webs.pdf}}</ref> "Ecological networks, especially mutualistic networks, are generally very heterogeneous, consisting of areas with sparse links among species and distinct areas of tightly linked species. These regions of high link density are often referred to as cliques, hubs, compartments, cohesive sub-groups, or modules...Within food webs, especially in aquatic systems, nestedness appears to be related to body size because the diets of smaller predators tend to be nested subsets of those of larger predators (Woodward & Warren 2007; YvonDurocher et al. 2008), and phylogenetic constraints, whereby related taxa are nested based on their common evolutionary history, are also evident (Cattin et al. 2004)."<ref name="Montoya09">{{cite journal | last1=Montoya | first1=J. M. | last2=Blüthgen | first2=N | last3=Brown | first3=L. | last4=Dormann | first4=C. F. | last5=Edwards | first5=F. | last6=Figueroa | first6=D. | last7=et al. | title=Ecological networks: beyond food webs | year=2009 | journal=Journal of Animal Ecology | volume=78 | pages=253–269 | doi=10.1111/j.1365-2656.2008.01460.x | url=http://www.higrade.ufz.de/data/Ings2008JAnimEcol942012634.pdf}}</ref>{{rp|257}} "Compartments in food webs are subgroups of taxa in which many strong interactions occur within the subgroups and few weak interactions occur between the subgroups. Theoretically, compartments increase the stability in networks, such as food webs."<ref name="Krause03" />
 
Food webs are also complex in the way that they change in scale, seasonally, and geographically. The components of food webs, including organisms and mineral nutrients, cross the thresholds of ecosystem boundaries. This has led to the concept or area of study known as [[cross-boundary subsidy]].<ref name="Bormann67">{{cite journal | last1=Bormann | first1=F. H. | last2=Likens | first2=G. E. | year=1967 | title=Nutrient cycling | journal=Science | volume=155 | issue=3761 | pages=424–429 | doi=10.1126/science.155.3761.424 |url=http://www.biology.duke.edu/upe302/pdf%20files/Emily_BormannLikens1967.pdf}}</ref><ref name="Polis97">{{cite journal | last1=Polis | first1=G. A. | last2=Anderson | first2=W. B. | last3=Hold | first3=R. D. | title=Toward an integration of landscape and food web ecology: The dynamics of spatially subsidized food webs | journal=Annual Review of Ecology and Systematics | volume=28 | year=1997 | pages=289–316 | url=http://limnology.wisc.edu/courses/zoo955/Fall2006/Papers/Polis_1997_Toward.pdf}}</ref> "This leads to anomalies, such as food web calculations determining that an ecosystem can support one half of a top carnivore, without specifying which end."<ref name="O'Neil01">{{cite journal | last1=O'Neil | first1=R. V. | title=Is it time to bury the ecosystem concept? (With full military honors, of course!) | year=2001 | journal=Ecology | volume=82 | issue=12 | pages=3275–3284 | url=http://ss1.webkreator.com.mx/4_2/000/000/00c/04a/Concepto%20de%20Ecosistema.pdf | doi=10.1890/0012-9658(2001)082[3275:IITTBT]2.0.CO;2}}</ref> Nonetheless, real differences in structure and function have been identified when comparing different kinds of ecological food webs, such as terrestrial vs. aquatic food webs.<ref name="Shurin06">{{cite journal | last1=Shurin | first1=J. B. | last2=Gruner | first2=D. S. | last3=Hillebrand | first3=H. | title=All wet or dried up? Real differences between aquatic and terrestrial food webs | journal=Proc. R. Soc. B | year=2006 | volume=273 | pages=1–9 | doi=10.1098/rspb.2005.3377 | url=http://rspb.royalsocietypublishing.org/content/273/1582/1.full.pdf+html | issue=1582 | pmid=16519227 | pmc=1560001}}</ref>
 
== History of food webs ==
[[Image:EltonFW.jpg|left|thumb|275px|Victor Summerhayes and [[Charles Sutherland Elton|Charles Elton]]'s 1923 food web of Bear Island (''Arrows point to an organism being consumed by another organism'').]]
 
Food webs serve as a framework to help ecologists organize the complex network of interactions among species observed in nature and around the world. One of the earliest descriptions of a food chain was described by a [[medieval]] [[Afro-Arab]] scholar named [[Al-Jahiz]]: "All animals, in short, cannot exist without food, neither can the hunting animal escape being hunted in his turn."<ref name="Egerton02">{{cite journal | last1=Egerton | first1=F. N. | title=A history of the ecological sciences, part 6: Arabic language science: Origins and zoological writings. | journal=Bulletin of the Ecological Society of America | volume=83 | issue=2 | pages=142–146 | url=http://www.esapubs.org/bulletin/current/history_list/history_part6.pdf}}</ref>{{rp|143}} The earliest graphical depiction of a food web was by [[Lorenzo Camerano]] in 1880, followed independently by those of Pierce and colleagues in 1912 and [[Victor Ernest Shelford|Victor Shelford]] in 1913.<ref>{{cite journal | last1 = Egerton | first1 = FN | year = 2007 | title = Understanding food chains and food webs, 1700-1970 | doi =10.1890/0012-9623(2007)88[50:UFCAFW]2.0.CO;2 | journal = Bulletin of the Ecological Society of America | volume = 88 | issue = | pages = 50–69 }}</ref><ref>Shelford, V (1913) [http://books.google.com/books?id=99UGAAAAYAAJ&dq=victor+shelford+an  imal+communities&printsec=frontcover&source=bl&ots=XT6Rz02AEZ&sig=hOm3G1CE5r4PYmq2kuR0VNKrxjU&hl=en&ei=mRvzSrCjG4aSMc_-kOgF&sa=X&oi=book_result&ct=result&resnum=2&ved=0CAoQ6AEwAQ#v=onepage&q=&f=false Animal Communities in Temperate America as Illustrated in the Chicago Region]. University of Chicago Press.</ref> Two food webs about [[herring]] were produced by Victor Summerhayes and [[Charles Sutherland Elton|Charles Elton]]<ref>{{cite journal | last1 = Summerhayes | first1 = VS | last2 = Elton | first2 = CS | year = 1923 | title = Contributions to the Ecology of Spitsbergen and Bear Island | url = | journal = Journal of Ecology | volume = 11 | issue = | pages = 214–286 }}</ref> and [[Alister Hardy]]<ref>{{cite journal | last1 = Hardy | first1 = AC | year = 1924 | title = The herring in relation to its animate environment. Part 1. The food and feeding habits of the herring with special reference to the east coast of England | url = | journal = Fisheries Investigation London Series II | volume = 7 | issue = 3| pages = 1–53 }}</ref> in 1923 and 1924. [[Charles Sutherland Elton|Charles Elton]] subsequently pioneered the concept of food cycles, food chains, and food size in his classical 1927 book "Animal Ecology"; Elton's 'food cycle' was replaced by 'food web' in a subsequent ecological text.<ref name="Elton27">{{cite book|last=Elton|first=C. S.|title=Animal Ecology|publisher=Sidgwick and Jackson|place=London, UK.|year=1927|isbn=0-226-20639-4}}</ref> After Charles Elton's use of food webs in his 1927 synthesis,<ref>Elton CS (1927) Animal Ecology. Republished 2001. University of Chicago Press.</ref> they became a central concept in the field of [[ecology]]. Elton<ref name="Elton27" /> organized species into [[Functional group (ecology)|functional groups]], which formed the basis for the [[trophic level|trophic system of classification]] in [[Raymond Lindeman]]'s classic and landmark paper in 1942 on trophic dynamics.<ref name="Lindeman42" /><ref name="Paine80">{{cite journal | last1=Paine | first1=R. T. | title=Food webs: Linkage, interaction strength and community infrastructure. | journal= Journal of Animal Ecology | volume=49 | issue=3 | year=1980 | pages=666–685 | url=http://cmbc.ucsd.edu/content/1/docs/paine1980.pdf}}</ref><ref name="Allee32">{{cite book|last=Allee|first=W. C.|title= Animal life and social growth|publisher=The Williams & Wilkins Company and Associates|place=Baltimore|year=1932}}</ref> The notion of a food web has a historical foothold in the writings of [[Charles Darwin]] and his terminology, including an "entangled bank", "web of life", "web of complex relations", and in reference to the decomposition actions of earthworms he talked about "the continued movement of the particles of earth". Even earlier, in 1768 John Bruckner described nature as "one continued web of life".<ref name="Pimm91" /><ref name="Stauffer60">{{cite journal | last1=Stauffer | first1=R. C. | title=Ecology in the long manuscript version of Darwin's "Origin of Species" and Linnaeus' "Oeconomy of Nature" | journal=[[Proceedings of the American Philosophical Society]] | volume=104 | issue=2 | year=1960 | pages=235–241 | jstor=985662}}</ref><ref name="Darwin81">{{cite journal | last1=Darwin | first1=C. R. | title= The formation of vegetable mould, through the action of worms, with observations on their habits. | year=1881 | place=London | publisher=John Murray | url=http://darwin-online.org.uk/content/frameset?viewtype=text&itemID=F1357&pageseq=1}}</ref><ref name="Worster94">{{cite book | last1=Worster | first1=D. | title=Nature's economy: A history of ecological ideas | publisher=Cambridge University Press | edition=2nd | year=1994 | page=423 | isbn=978-0-521-46834-3 |url=http://books.google.ca/books?id=2Ng-5B5H2wcC&pg=PR9&dq=food+web+history+ecology+linnaeus+economy+of+nature#v=onepage&q&f=false}}</ref>
 
Interest in food webs increased after Robert Paine's experimental and descriptive study of intertidal shores<ref>{{cite journal | last1 = Paine | first1 = RT | year = 1966 | title = Food web complexity and species diversity | url = | journal = The American Naturalist | volume = 100 | issue = | pages = 65–75 | doi=10.1086/282400}}</ref> suggesting that food web complexity was key to maintaining species diversity and ecological stability. Many [[theoretical ecologist]]s, including [[Robert May, Baron May of Oxford|Sir Robert May]]<ref>May RM (1973) Stability and Complexity in Model Ecosystems. [[Princeton University Press]].</ref> and Stuart Pimm,<ref>Pimm SL (1982) Food Webs, [[Chapman & Hall]].</ref> were prompted by this discovery and others to examine the mathematical properties of food webs.
 
== See also ==
{{Portal|Environment|Ecology|Earth sciences|Biology|Sustainable development}}
{{Refbegin|2}}
 
* [[Antipredator adaptation]]s
* [[Apex predator]]
* [[Aquatic-terrestrial subsidies]]
* [[Balance of Nature]]
* [[Biodiversity]]
* [[Biogeochemical cycle]]
* [[Consumer-resource systems]]
* [[Ecological network]]
* [[Food systems]]
* [[Ecology of the San Francisco Estuary#Food web|Food web of the San Francisco Estuary]]
* [[Microbial food web]]
* [[Natural environment]]
* [[List of feeding behaviours]]
* [[Soil food web]]
* [[Kelp forest#Trophic ecology|Trophic ecology of kelp forests]]
* [[Lentic ecosystem#Trophic relationships|Trophic relationships in lakes]]
* [[Lotic ecosystem#Trophic relationships|Trophic relationships in rivers]]
{{Refend}}
{{br}}
 
==References==
{{Reflist|2}}
 
==Further reading==
* {{cite book|year=1978|last=Cohen|first=Joel E.|authorlink=Joel E. Cohen|title=Food webs and niche space|series=Monographs in Population Biology|volume=11|location=Princeton, NJ|publisher=[http://press.princeton.edu/titles/324.html  Princeton University Press]|pages=xv+1–190|ref=harv|isbn=978-0-691-08202-8}}
*{{cite web|title=Aquatic Food Webs|url=http://www.education.noaa.gov/Marine_Life/Aquatic_Food_Webs.html|work=NOAA Education Resources|publisher=National Oceanic and Atmospheric Administration}}
 
{{feeding}}
{{modelling ecosystems|state=expanded}}
 
{{DEFAULTSORT:Food Chain}}
[[Category:Trophic ecology]]

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