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[[File:Far Pastures - geograph.org.uk - 52967.jpg|thumb|300px|right|'''First trophic level'''. The plants in this image, and the [[algae]] and [[phytoplankton]] in the lake, are [[primary producer]]s. They take nutrients from the soil or the water, and manufacture their own food by [[photosynthesis]], using energy from the sun.]] 

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The '''trophic level''' of an [[organism]] is the position it occupies in a [[food chain]]. The word trophic derives from the [[Ancient Greek|Greek]] τροφή (trophē) referring to food or feeding. A food chain represents a succession of organisms that eat another organism and are, in turn, eaten themselves. The number of steps an organism is from the start of the chain is a measure of its trophic level. Food chains start at trophic level 1 with [[primary producer]]s such as plants, move to [[herbivore]]s at level 2, [[predator]]s at level 3 and typically finish with [[carnivore]]s or [[apex predator]]s at level 4 or 5. The path along the chain can form either a one-way flow or a food "web". Ecological communities with higher biodiversity form more complex trophic paths.

==Overview==
[[File:ConsumerWikiPDiag.jpg|thumb|300px|[[Consumer-resource systems|Consumer categories]] based on material eaten (plant: green shades are live, brown shades are dead; animal: red shades are live, purple shades are dead; or particulate: grey shades) and feeding strategy (gatherer: lighter shade of each color; miner: darker shade of each color)]]

The three basic ways in which organisms get food are as producers, consumers and decomposers. 

* Producers ([[autotroph]]s) are typically [[plant]]s or [[algae]]. Plants and algae do not usually eat other organisms, but pull nutrients from the soil or the ocean and manufacture their own food using [[photosynthesis]]. For this reason, they are called [[primary producer]]s. In this way, it is energy from the sun that usually powers the base of the food chain.<ref>{{Cite book|url=http://books.google.com/books?id=JB4ArbvXXDEC&pg=PA537|publisher= Cengage Learning|isbn=978-0-7668-3391-3|title=Science of Earth Systems|year=2002}}</ref> An exception occurs in deep-sea [[Hydrothermal vents|hydrothermal ecosystems]], where there is no sunlight. Here primary producers manufacture food through a process called [[chemosynthesis]].<ref>{{Cite book|url=http://books.google.com/books?id=uaXuCVuYVDUC&pg=PA399|publisher= Princeton University Press|isbn= 978-0-691-04929-8|title=The Ecology of Deep-sea Hydrothermal Vents|year=2000}}</ref>

* [[Consumers (food chain)|Consumers]] ([[heterotroph]]s) are [[species]] that cannot manufacture their own food and need to consume other organisms. Animals that eat primary producers (like plants) are called [[herbivore]]s. Animals that eat other animals are called [[carnivore]]s, and animals that eat both plant and other animals are called [[omnivore]]s.

* [[Decomposer]]s ([[detritivores]]) break down dead plant and animal material and wastes and release it again as energy and nutrients into the ecosystem for recycling. Decomposers, such as [[bacteria]] and [[fungi]] (mushrooms), feed on waste and dead matter, converting it into inorganic chemicals that can be recycled as mineral nutrients for plants to use again.
Trophic levels can be represented by numbers, starting at level 1 with plants. Further trophic levels are numbered subsequently according to how far the organism is along the food chain. 

* Level 1: Plants and algae make their own food and are called primary producers.
* Level 2: Herbivores eat plants and are called primary consumers.
* Level 3: Carnivores that eat herbivores are called secondary consumers.
* Level 4: Carnivores that eat other carnivores are called tertiary consumers.
* Level 5: [[Apex predator]]s that have no predators are at the top of the food chain.

<gallery widths=180>
File:Sylvilagus floridanus.jpg|<center>'''Second trophic level'''<br />[[Rabbit]]s eat plants at the first trophic level, so they are primary consumers.</center>
File:Vulpes vulpes sitting.jpg|<center>'''Third trophic level'''<br />[[Fox]]es eat rabbits at the second trophic level, so they are secondary consumers.</center>
File:Aquila-chrysaetos-golden-eagle-0b.jpg|<center>'''Fourth trophic level'''<br />[[Golden eagle]]s eat foxes at the third trophic level, so they are tertiary consumers.</center>
File:Fungi in Borneo.jpg|<center>'''[[Decomposer]]s'''<br />The fungi on this tree feed on dead matter, converting it back to nutrients that [[primary producer]]s can use.</center>
</gallery>

In real world [[ecosystem]]s, there is more than one food chain for most organisms, since most organisms eat more than one kind of food or are eaten by more than one type of predator. A diagram that sets out the intricate network of intersecting and overlapping food chains for an ecosystem is called its [[food web]].<ref name=Lisowski-etal/> Decomposers are often left off food webs, but if included, they mark the end of a food chain.<ref name=Lisowski-etal /> Thus food chains start with primary producers and end with decay and decomposers. Since decomposers recycle nutrients, leaving them so they can be reused by primary producers, they are sometimes regarded as occupying their own trophic level.<ref name="AHSD"/><ref name=Pauly&Palomares />

==Biomass transfer efficiency==
[[File:Ecological pyramid.svg|thumb|260px|right|An [[energy pyramid]] illustrates how much energy is needed as it flows upward to support the next trophic level. This diagram is not to scale, because only about 10% of the energy transferred between each trophic level is converted to [[Biomass (ecology)|biomass]].]]

In general, each trophic level relates to the one below it by absorbing some of the energy it consumes, and in this way can be regarded as resting on, or supported by, the next lower trophic level. Food chains can be diagrammed to illustrate the amount of energy that moves from one feeding level to the next in a food chain. This is called an [[energy pyramid]]. The energy transferred between levels can also be thought of as approximating to a transfer in [[Biomass (ecology)|biomass]], so energy pyramids can also be viewed as biomass pyramids, picturing the amount of biomass that results at higher levels from biomass consumed at lower levels.

The efficiency with which energy or biomass is transferred from one trophic level to the next is called the [[ecological efficiency]]. Consumers at each level convert on average only about 10% of the chemical energy in their food to their own organic tissue (the [[ten percent law]]). For this reason, food chains rarely extend for more than 5 or 6 levels. At the lowest trophic level (the bottom of the food chain), plants convert about 1% of the sunlight they receive into chemical energy. It follows from this that the total energy originally present in the incident sunlight that is finally embodied in a tertiary consumer is about 0.001%<ref name="AHSD" />

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==Fractional trophic levels==
[[File:Orca porpoising.jpg|thumb|left|Killer whales ([[orca]]) are [[apex predator]]s. They hunt practically anything, including tuna, smaller sharks, and seals.]]

Food webs largely define ecosystems, and the trophic levels define the position of organisms within the webs. But these trophic levels are not always simple integers, because organisms often feed at more than one trophic level.<ref>Odum WE and Heald EJ (1975) "The detritus-based food web of an estuarine mangrove community". Pages 265–286 in L. E. Cronin, ed. ''Estuarine research''. Vol. 1. Academic Press, New York.</ref><ref>{{Cite journal | doi = 10.1038/275542a0 | last1 = Pimm | first1 = SL | last2 = Lawton | first2 = JH | year = 1978 | title = On feeding on more than one trophic level | url = http://www.nature.com/nature/journal/v275/n5680/abs/275542a0.html | journal = Nature | volume = 275 | issue = 5680| pages = 542–544 }}</ref> For example, some carnivores also eat plants, and some plants are carnivores. A large carnivore may eat both smaller carnivores and herbivores; the bobcat eats rabbits, but the mountain lion eats both bobcats and rabbits. Animals can also eat each other; the [[American bullfrog|bullfrog]] eats [[crayfish]] and crayfish eat young bullfrogs. The feeding habits of a juvenile animal, and, as a consequence, its trophic level, can change as it grows up.

The fisheries scientist [[Daniel Pauly]] sets the values of trophic levels to one in plants and detritus, two in herbivores and detritivores (primary consumers), three in secondary consumers, and so on. The definition of the trophic level, TL, for any consumer species is:<ref name=Pauly&Palomares/>

:: <math> TL_i = 1 + \sum_j (TL_j \cdot DC_{ij})\! </math>

where <math>TL_j</math> is the fractional trophic level of the prey ''j'', and <math>DC_{ij}</math> represents the fraction of ''j'' in the diet of ''i''. 

In the case of marine ecosystems, the trophic level of most fish and other marine consumers takes value between 
2.0 and 5.0. The upper value, 5.0, is unusual, even for large fish,<ref name=r1/> though it occurs in apex predators of marine mammals, such as polar bears and killer whales.<ref name=Pauly1998b/>

In addition to observational studies of animal behavior, and quantification of animal stomach contents, trophic level can be quantified through [[stable isotope]] analysis of animal tissues such as [[muscle]], [[skin]], [[hair]], bone [[collagen]]. This is because there is a consistent increase in the nitrogen isotopic composition at each trophic level caused by fractionations that occur with the synthesis of biomolecules; the magnitude of this increase in nitrogen isotopic composition is approximately 3–4‰.<ref name=r2/>{{primary source-inline|date=September 2012}}<!-- ref is a piece of primary research-->

==Mean trophic level==
[[File:Bluefin-big.jpg|thumb|right|The mean trophic level of the world fisheries catch has steadily declined because many high trophic level fish, such as this [[tuna]], have been [[overfished]]]] 

In fisheries, the mean trophic level for the fisheries catch across an entire area or ecosystem is calculated for year y as: 

:: <math> TL_y = \frac{\sum_i (TL_i \cdot Y_{iy})}{\sum_i Y_{iy}} </math>

where <math>Y_{iy}</math> is the catch of the species or group i in year y, and <math>TL_i</math> is the trophic level for species i as defined above.<ref name=Pauly&Palomares /> 

It was once believed that fish at higher trophic levels usually have a higher economic value; resulting in [[overfishing]] at the higher trophic levels. Earlier reports found precipitous declines in mean trophic level of [[fisheries]] catch, in a process known as [[fishing down the food web]].<ref name=r3/> However, more recent work finds no relation between economic value and trophic level;<ref name=r4/> and that mean trophic levels in catches, surveys and stock assessments have not in fact declined, suggesting that fishing down the food web is not a global phenomenon.<ref name=r5/> However Pauly et al. note that trophic levels peaked at 3.4 in 1970 in the northwest and west-central Atlantic, followed by a subsequent decline to 2.9 in 1994. They report a shift away from long-lived, piscivorous, high-trophic-level bottom fishes, such as cod and haddock, to short-lived, planktivorous, low-trophic-level invertebrates (e.g., shrimps) and small, pelagic fish (e.g., herrings). This shift from high-trophic-level fishes to low-trophic-level invertebrates and fishes is a response to changes in the relative abundance of the preferred catch. They argue this is part of the global fishery collapse.<ref name=Pauly1998b/><ref name=Pauly1998a/>

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==FiB index==
Since biomass [[Ecological efficiency|transfer efficiencies]] are only about 10%, it follows that the rate of biological production is much greater at lower trophic levels than it is at higher levels. Fisheries catches, at least to begin with, will tend to increase as the trophic level declines. At this point the fisheries will target species lower in the food web.<ref name=Pauly1998a/> In 2000, this led Pauly and others to construct a "Fisheries in Balance" index, usually called the FiB index.<ref name=Pauly2000/> The FiB index is defined, for any year y, by<ref name=Pauly&Palomares /> 

:: <math> FiB_y = log((Y_y/(TE)^{TL_y})/(Y_0/(TE)^{TL_0})) </math>

where <math>Y_y</math> is the catch at year y, <math>{TL}_y</math> is the mean trophic level of the catch at year y, <math>Y_0</math> is the catch, <math>{TL}_0</math> the mean trophic level of the catch at the start of the series being analyzed, and <math>TE</math> is the transfer efficiency of biomass or energy between trophic levels. 

The FiB index is stable (zero) over periods of time when changes in trophic levels are matched by appropriate changes in the catch in the opposite direction. The index increases if catches increase for any reason, e.g. higher fish biomass, or geographic expansion.<ref name=Pauly&Palomares /> Such decreases explain the “backward-bending” plots of trophic level versus catch originally observed by Pauly and others in 1998.<ref name=Pauly1998a />

==Tritropic and other interactions==

One aspect of trophic levels is called tritrophic interaction. Ecologists often restrict their research to two tropic levels as a way of simplify the analysis; however, this can be misleading if tritrophic interactions (such as plant–herbivore–predator) are not easily understood by simply adding pairwise interactions (plant–herbivore plus herbivore–predator, for example). Significant interactions can occur between the first trophic level (plant) and the third trophic level (a predator) in determining herbivore population growth, for example. Simple genetic changes may yield morphological variants in plants that then differ in their resistance to herbivores because of the effects of the plant architecture on enemies of the herbivore.<ref name=Kareiva&Sahakain/>

==See also==
* [[Cascade effect (ecology)|Cascade effect]]
* [[Energy flow (ecology)]]
* [[Trophic cascade]]
* [[Trophic state index]] – applied to lakes
* [[Trophic dynamics]] - ''Food web''

==References==
{{reflist|35em|refs=
<ref name="AHSD">American Heritage Science Dictionary, 2005. Houghton Mifflin Company.</ref>

<ref name=Lisowski-etal>Lisowski M, Miaoulis I, Cyr M, Jones LC, Padilla MJ, Wellnitz TR (2004) ''Prentice Hall Science Explorer: Environmental Science'', Pearson Prentice Hall. ISBN 978-0-13-115090-4</ref>

<ref name=Pauly1998a>{{Cite journal | doi = 10.1126/science.279.5352.860 | last1 = Pauly | first1 = D | last2 = Christensen v | first2 = V | last3 = Dalsgaard | first3 = J | last4 = Froese | first4 = R | last5 = Torres Jr | first5 = FC Jr | year = 1998 | title = Fishing down marine food webs | journal = Science | volume = 279 | issue = 5352| pages = 860–863 | pmid = 9452385 }}</ref> 

<ref name=Pauly2000>{{Cite journal | doi = 10.1006/jmsc.2000.0726 | last1 = Pauly | first1 = D | last2 = Christensen | first2 = V | last3 = Walters | first3 = C | year = 2000 | title = Ecopath, Ecosim and Ecospace as tools for evaluating ecosystem impact of fisheries | journal = ICES J. Mar. Sci. | volume = 57 | issue = 3| pages = 697–706 }}</ref>

<ref name=Pauly1998b>{{Cite journal | doi = 10.1006/jmsc.1997.0280 | last1 = Pauly | first1 = D | last2 = Trites | first2 = A | last3 = Capuli | first3 = E | last4 = Christensen | first4 = V | year = 1998 | title = Diet composition and trophic levels of marine mammals | journal = ICES J. Mar. Sci. | volume = 55 | issue = 3| pages = 467–481 }}</ref>

<ref name=Pauly&Palomares>{{Cite journal | last1 = Pauly | first1 = D. | last2 = Palomares | first2 = M.L. | year = 2005 | title = Fishing down marine food webs: it is far more pervasive than we thought | url = http://www.seaaroundus.org/Journal/2005/FishingDownMarineFoodWebItisfarMorePervasive.pdf | journal = Bulletin of Marine Science | volume = 76 | issue = 2| pages = 197–211 }}</ref> 

<ref name=r1>{{Cite journal | doi = 10.1006/jmsc.1999.0489 | last1 = Cortés | first1 = E | year = 1999 | title = Standardized diet compositions and trophic levels of sharks | journal = ICES J. Mar. Sci. | volume = 56 | issue = 5| pages = 707–717 }}</ref>

<ref name=r2>{{Cite journal |last=Szpak |first=Paul |last2=Orchard |first2=Trevor J. |last3=McKechnie |first3=Iain |last4=Gröcke |first4=Darren R. |title=Historical Ecology of Late Holocene Sea Otters (Enhydra lutris) from Northern British Columbia: Isotopic and Zooarchaeological Perspectives | url=http://uwo.academia.edu/PaulSzpak/Papers/1216885/Historical_Ecology_of_Late_Holocene_Sea_Otters_Enhydra_lutris_from_Northern_British_Columbia_Isotopic_and_Zooarchaeological_Perspectives |journal=[[Journal of Archaeological Science]] |year=2012 |volume=39 |issue=5 |pages=1553–1571 |doi=10.1016/j.jas.2011.12.006 }}</ref>

<ref name=r3>[[Millennium Ecosystem Assessment]] (2005) [http://www.millenniumassessment.org/documents/document.356.aspx.pdf ''Ecosystems and Human Well-being: Synthesis''] Island Press. pp. 32–33.</ref>

<ref name=r4>{{Cite journal | last1 = Sethi | first1 = SA | last2 = Branch | first2 = TA | last3 = Watson | first3 = R | year = 2010 | title = Global fishery development patterns are driven by profit but not trophic level| journal = Proceedings of the National Academy of Sciences | volume = 107 | pages = 12163–12167 | doi = 10.1073/pnas.1003236107 | issue = 27 }}</ref>

<ref name=r5>{{Cite journal| last1 = Branch | first1 = TA | year = 2010 | title = Trophic fingerprint of marine fisheries | url = http://dev.fmap.dal.ca/ramweb/papers-total/Branch%20et%20al_Nature_2010.pdf | journal = Nature | volume = 468 | pages = 431–435 | doi=10.1038/nature09528 | last2 = Watson | first2 = Reg | last3 = Fulton | first3 = Elizabeth A. | last4 = Jennings | first4 = Simon | last5 = McGilliard | first5 = Carey R. | last6 = Pablico | first6 = Grace T. | last7 = Ricard | first7 = Daniel | last8 = Tracey | first8 = Sean R. | issue = 7322 | pmid = 21085178 }}</ref>

<ref name=Kareiva&Sahakain>{{Cite journal | doi = 10.1038/345433a0 | last1 = Kareiva | first1 =  Peter | last2 = Sahakian | first2 = Robert | year = 1990 | title = Letters to Nature:Tritrophic effects of a simple architectural mutation in pea plants | journal = Nature | volume = 35 | pages = 433–434 | issue = 6274 }}</ref>

}}

==External links==
* [http://www.bbc.co.uk/nature/blueplanet/infobursts/trophic_levels_bg.shtml Trophic levels] BBC. Last updated March 2004.

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{{aquatic ecosystem topics|expanded=none}}

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[[Category:Fisheries]]
[[Category:Ecology]]