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In ecology, the trophic level is the position that an organism occupies in a food chain - what it eats, and what eats it.

Energy economy

Wildlife [[biologistssometimes eat bobcats. Since each layer of this system relates to the one below it by absorbing a fraction of the energy it consumed, each one can be understood as resting on the one below---which is called a lower trophic level.

Keep in mind that trophic relationships are rarely this simple. Very often they are more of a "web" than a "chain." For example, the mountain lion may eat the bobcat, but it also eats rabbits. The trophic categorization of the mountain lion exists on two levels, possibly more.

Every time there is an exchange of energy between one trophic level and another, there is a quite significant loss due to the fundamental laws of thermodynamics. This means so many units of grass can only support a much smaller number of units of rabbits, who can only support a smaller group of bobcats, who can only support a smaller group of mountain lions. This is why trophic levels are usually portrayed as a triangle, one that places grass on the bottom and mountain lions on top---the top is always much smaller than the bottom. Each level implies a loss of energy and efficiency and less life that can be supported by the sun.

Components of ecosystems

Ecosystems have four basic components:

The abiotic environment
Producers
Consumers
Decomposers

Producers (autotrophs) utilize energy from the sun and nutrients from the abiotic environment (carbon dioxide from the air or water, other nutrients from the soil or water) to perform photosynthesis and grow. Producers are generally green plants (those with chlorophyll). See carbon cycle for more on carbon's role.

Consumers (heterotrophs) are organisms that feed on other organisms. They depend on producers for their energy and synthesis needs. For example, herbivores are primary consumers. A carnivore that eats only herbivores is a secondary consumer. A carnivore that eats other carnivores is a tertiary consumer.

Decomposers and Detritivores utilize energy from wastes or dead organisms, and so complete the cycle by returning nutrients to the soil or water, and carbon dioxide to the air and water. See water cycle for more on water's role.

Biomass production

Primary production is generation of biomass through photosynthesis. The highest producers of biomass are

tropical rain forests, 2000 g/m²/yr of biomass
swamps and marshes, 2500 g/m²/yr of biomass
algal beds and reefs, 2000 g/m²/yr of biomass
river estuaries, 1800 g/m²/yr of biomass

Others include

Temperate forests, 1200 g/m²/yr of biomass
Cultivated lands; 600 g/m²/yr of biomass

while lowest producers are deserts and frozen areas (less than 200 g/m²/yr of biomass).

In the ocean, phytoplankton is the primary producer (the first level in the food chain or the first trophic level).Phytoplankton converts inorganic carbon into protoplasm.
Phytoplankton is consumed by microscopic animals called zooplankton (these are the second level in the food chain).
Zooplankton is consumed by Crustaceans (the third level in the food chain).
Fish that eat crustaceans could constitute the fourth trophic level, while seals consuming the fishes are the fifth.
Trophic levels are very similar on land, with plants being the first trophic level, cows eating the grass being the second, and humans eating the cows being the third.

The amount of biomass produced for a given amount of solar energy is highest at the first level. Less biomass is produced at the second level, for some energy is lost during the conversion. The more trophic levels there are, the more energy is lost through conversion.

Humans are generally primary and secondary consumers, and thus represent usually second and third trophic levels. Most humans are omnivores, which means they consume both plants and animals. Less energy is required to support vegetarian humans than omnivorous ones, for there is a significant energy loss during the conversion of grain and vegetables in animal matter.

Trophic levels and biodiversity

Each species in an ecosystem is affected by the other species in that ecosystem. There are very few single prey-single predator relationships. Most preys are consumed by more than one predator, and most predators have more than one prey. Their relationships are also influenced by other environmental factors.

Biodiversity (seen from the viewpoint of species diversity) is a major contributor to the stability of ecosystems. When an organism can exploit a wide range of resources, a decrease in biodiversity is less likely to have an impact. However, for an organism which can only exploit a limited range of resources, a decrease in biodiversity is more likely to have a strong effect. David Tilman is an ecologist who has done a lot of work establishing the theoretical basis of this phenomenon.

Reduction of habitat, hunting and fishing of some species to extinction or near extinction, and eradication of insects and pollution tend to tip the balance of biodiversity. Similarly, in-situ conservation areas need to be carefully designed to maintain a diverse and stable environnement for the extinct species to thrive.

For a systematic treatment of biodiversity within a trophic level, see unified neutral theory of biodiversity.

Multitrophic interactions

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Trophic interactions can involve more than the two players "predator" and "prey". When more than two trophic level are involved, we talk of "multitrophic interactions" or "multitrophic level interactions". The multitrophic approach has consciously entered ecological sciences with a review paper published by six authors in 1980 (Price et al., 1980, Interactions among three trophic levels: influence of plants on interactions between insect herbivores and natural enemies). The paper expands the classical plant-herbivore system to the three-tier system plant-herbivore-natural enemies of plant herbivores. The authors say: "We argue that theory on insect-plant interactions cannot progress realistically without consideration of the third trophic level." To date (December 2005) the paper is cited 794 times (Web of Science). The field of multitrophic level interactions has become important especially since the mid 1990s.

A recent book offers a synthesis of this topic: Tscharntke, T., Hawkins, B., A., (eds), 2002: Multitrophic Level Interactions, Cambridge University Press, Cambridge.

Trophic levels and evolutionary theory

A compelling sidenote to the discussion of trophic levels is the incongruence of low-level trophic efficiency and the upward trophic movement of species described in evolutionary theory.

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 == Energy economy ==
+[[Wildlife]] [[biologistssometimes eat bobcats.   Since each layer of this system relates to the one below it by absorbing a  fraction of the energy it consumed, each one can be understood as resting on the one  below---which is called a lower '''trophic level'''.
-[[Wildlife]] [[biologists]] look at a natural "[[energy economy|economy of energy]]" that ultimately rests upon solar energy.  When they look at an [[ecosystem]] there is always some [[foundation species]] that directly harvests energy from the sun, for example, grass (however in deep sea [[Hydrothermal_vent|hydrothermal vents]] [[chemosynthesis|chemosynthetic]] [[archaea]] form the base of the food chain). Next are [[herbivore]]s (primary consumers) that eat the grass, such as the rabbit. Next are [[carnivore]]s (secondary consumers) that eat the rabbit, such as a bobcat.
-There can be several intermediate links, which means that there can be another layer of predators on top, such as mountain lions,  which sometimes eat bobcats.   Since each layer of this system relates to the one below it by absorbing a  fraction of the energy it consumed, each one can be understood as resting on the one  below---which is called a lower '''trophic level'''.
 Keep in mind that trophic relationships are rarely this simple. Very often they are more of a "[[Food chain|web]]" than a "[[Food chain|chain]]." For example, the mountain lion may eat the bobcat, but it also eats rabbits. The trophic categorization of the mountain lion exists on two levels, possibly more.