In ecology, trophic dynamics is the system of trophic levels (Greek τροφή, trophē, food or feeding), which describes the position that an organism occupies in a food chain — what an organism eats, and what eats the organism.

Contents 1 Energy economy 1.1 Components of ecosystems 1.2 Biomass production 2 Multitrophic interactions 3 Food chain 3.1 Organisms represented in food chains 3.2 Food web 4 Notes 5 See also

[edit] Energy economy

Ecologists study the energy economies of natural systems. Foundation species (also known as primary producers) harvest an energy source such as sunlight and turn it into biomass. This biomass is consumed by other organisms (primary consumers), which are in turn consumed by others. Each link in this chain of consumption is termed a trophic level. Because only a fraction of the energy used by a level is converted to biomass, less energy is available at higher levels.

Most ecosystems ultimately rely upon the Sun for energy and upon photosynthetic organisms to harness that energy. There are only a few exceptions to this, such as chemosynthetic archaea, which derive energy from the break down of sulfur rich compounds around deep sea hydrothermal vents and acid mine drainage.

In terrestrial ecosystems, plants such as grass are the primary producers and form the first trophic level. Next are herbivores (primary consumers) that eat the grass, such as rabbits. Next are carnivores (secondary consumers) that eat the rabbits, such as a bobcat.

Every time there is an exchange of energy between one trophic level and another, there is quite a 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 pyramid, 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.

There is no in-principle limit to the number of levels in a trophic system, but as only a fraction of the energy of each level can be processed by the next (10%), trophic systems with more than five levels of consumption are exceptional^{[citation needed]}.

[edit] 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.

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.

[edit] Biomass production

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

• swamps and marshes, 2500 g/m²/yr of biomass
• tropical rain forests, 2000 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).^{[citation needed]}

In the ocean, phytoplankton is usually 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, and include larval animals (such as young fish, squid and crab/lobster) — as well as adult crustaceans called copepods, and many other types)).

Zooplankton is consumed both by other, larger predatory zooplankters and by fish. (the third level in the food chain). Fish that eat zooplankton could constitute the fourth trophic level, while seals consuming the fish are the fifth. Alternatively, for example, whales may consume zooplankton directly — leading to an environment with one less trophic level.

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.

(These examples are simplified, but intended only as a brief description of an area hard to scientifically quantify.)

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.

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. When referring to omnivore from an ecological standpoint it means to consume from different trophic levels. 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. This concept is generally represented using trophic pyramids.

Each species in an ecosystem is affected by the other species in that ecosystem. There are very few single prey-single predator relationships. Most prey are consumed by more than one predator, and most predators have more than one prey. Their relationships are also influenced by other environmental factors. In most cases, if one species is removed from an ecosystem, other species will most likely be affected, in ways such as extinction.

Biodiversity can contribute to the stability of ecosystems, due to the diversity of functional responses of community members to perturbation.^[1] From the point of view of an individual organism this can vary with different life history characteristics. For example, when an organism can exploit a wide range of resources (i.e., is a generalist), a decrease in biodiversity is often less likely to have an impact on that organism. However, for an organism that can only exploit a limited range of resources (a specialist), a decrease in biodiversity is more likely to have a strong effect.

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 environment for the threatened species to thrive.

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

[edit] Multitrophic interactions

Multitrophic interactions are those that involve more than two trophic levels in a food web.^[2][3] The term is most often applied to interactions among plants, herbivores and predators. One example of a multi-trophic interaction is a trophic cascade, in which predators benefit plants by suppressing herbivores. A simple way to show more than two trophic levels can be a pyramid, which shows the flow of energy throughout an ecosystem.

Simple names for plants in an ecosystem would be "Producer" as plants create their own food and glucose. Producers contain the most energy in an ecosystem. Every level in a food pyramid loses 90% of the energy consumed to create heat.

The next level would be "Primary Consumers," which is the second trophic level. This level includes herbivores, such as mice and rabbits. The third level in an ecosystem is called "Secondary Consumers". These species can consume the two levels before it. The fourth level is the "Tertiary Consumers". Tertiary Consumers can eat all three levels below them. All of these levels are consumed by detritovores and decomposers, which return nutrients to the soil.

[edit] Food chain

Food chains, also called food webs, food networks and/or trophic networks, describe the feeding relationships between species within an ecosystem. Organisms are connected to the organisms they consume by arrows representing the direction of biomass transfer. It also shows how the energy from the producer is given to the consumer.Typically a food chain or food web refers to a graph where only connections are recorded, and a food network or ecosystem network refers to a network where the connections are given weights representing the quantity of nutrients or energy being transferred.

[edit] Organisms represented in food chains

Primary producers, commonly called autotrophs, produce complex organic substances (essentially "food") from an energy source and inorganic materials. These organisms are typically photosynthetic plants, which use sunlight as their energy source. A few, such as those organisms forming the base of deep-sea vent food webs, are chemotrophic, using chemical energy instead. Organisms that get their energy by consuming organic substances are called heterotrophs. Heterotrophs include herbivores, which obtain their energy by consuming live plants; carnivores, which obtain energy from eating live animals; as well as detritivores, scavengers and decomposers, which all consume dead biomass. Energy enters the food chain from the sun. Some energy and/or biomass is lost at each stage of the food chain as; feces (solid waste), movement energy and heat energy (especially by warm-blooded creatures). Therefore, only a small amount of energy and biomass is incorporated into the consumer's body and transferred to the next feeding level, thus showing a Pyramid of Biomass.

A food chain is the flow of energy from one organism to the next and to the next and to the next. Organisms in a food chain are grouped into trophic levels, based on how many links they are removed from the primary producers. Trophic levels may contain either a single species or a group of species that are presumed to share both predators and prey. They usually start with a primary producer and end with a carnivore. The diagram at right is a food chain from a Swedish lake. It can be described as follows: osprey feed on northern pike that feed on perch that eat bleak that feed on freshwater shrimp. Although they are not shown in this diagram, the primary producers of this food chain are probably autotrophic phytoplankton. Phytoplankton and algae form the base of most freshwater food chains. It is often the case that 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. On average, only 10% of the organism's energy is passed on to its predator. The other 90% is used for the organism's life processes or it is lost as heat to the environment. Graphic representations of the biomass or productivity at each tropic level are called trophic pyramids. In this food chain for example, the biomass of osprey is smaller than the biomass of pike, which is smaller than the biomass of perch. Some producers, especially phytoplankton, are so productive and have such a high turnover rate that they can actually support a larger biomass of grazers. This is called an inverted pyramid, and can occur when consumers live longer and grow more slowly than the organisms they consume. In this food chain, the productivity of phytoplankton is much greater than that of the zooplankton consuming them. The biomass of the phytoplankton, however, may actually be less than that of the copepods. Directly linked to this are pyramids of numbers, which show that as the chain is travelled along, the number of consumers at each level drops very significantly, so that a single top consumer (e.g. a Polar Bear) will be supported by literally millions of separate producers (e.g. Phytoplankton). Food chains are overly simplistic as representatives of what typically happens in nature. The food chain shows only one pathway of energy and material transfer. Most consumers feed on multiple species and are, in turn, fed upon by multiple other species. The relations of detritivores and parasites are seldom adequately characterized in such chains as well. The food chain has a producer, consumer, herbivore, carnivore, omnivore, decomposer Arrows in a food web represent an organism getting eaten by another organism.

[edit] Food web

A food web extends the food chain concept from a simple linear pathway to a complex network of interactions. The earliest food webs were published by Victor Summerhayes and Charles Elton in 1923 and Hardy in 1924. Summerhayes and Elton's (right) depicted the interactions of plants, animals and bacteria on Bear Island, Norway,^[4] while Hardy's food web showed the interactions of herring and plankton in the North Sea.

The direct steps as shown in the food chain example above seldom reflect reality. This web makes it possible to show much bigger animals (like a seal) eating very small organisms (like plankton). Food sources of most species in an ecosystem are much more diverse, resulting in a complex web of relationships as shown in the figure on the right. In this figure, the grouping of Algae → Protozoa → Oligochaeta → Northern Eider → Arctic Fox is a chain; the whole complex network is a food web.

[edit] Notes

1. ^ Tillman, D, and J.A. Downing. 1994. Biodiversity and stability in grasslands. Nature 367: 363-365.
2. ^ Price et al (1980) Interactions among three trophic levels: influence of plants on interactions between insect herbivores and natural enemies
3. ^ Tscharntke, T., Hawkins, B., A., (eds) (2002) Multitrophic Level Interactions, Cambridge University Press, Cambridge
4. ^ Summerhayes VS, Elton CS (1923) Contributions to the Ecology of Spitsbergen and Bear Island. Journal of Ecology 11:214–286

• "Food chain" A Dictionary of Zoology. Ed. Michael Allaby. Oxford University Press, 1999. Oxford Reference Online. Oxford University Press. University of Utah. 22 November 2007 [1]

[edit] See also

• Efficiency of conversion of ingested food to body substance
• List of ecology topics
• Primary nutritional groups

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