Example of a food chain in a Swedish lake. Osprey feed on northern pike that feed on perch that eat bleak that feed on freshwater shrimp. Though unshown, the primary producers of this food chain are probably autotrophic phytoplankton.

Food chains, in ecology, are the sequence of transfers of matter and energy from organism to organism in the form of food. Food chains do not normally encompass more than five trophic levels because energy, in the form of heat, is lost at each step. Food chains combine into highly complex food webs because most organisms consume more than one type of animal or plant. (Britannica)

[edit] Organisms represented in food chains

Energy enters the food chain from the Sun. About ten percent of 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.(Ecorisk Fundamentals)

Primary producers, commonly forming autotrophs, produce complex organic substances (essentially "food") from an energy source and 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 organic substances are called heterotrophs. Heterotrophs include herbivores, which obtain their energy by consuming live plants; carnivores, which obtain energy from eating live animals. Ultimately detritivores, scavengers and decomposers may predate living or consume dead biomass.

[edit] Flow of food chains

All biological communities have a basic structure of interaction that forms a trophic pyramid. Trophic pyramids are made up of trophic levels, and food energy is passed from one level to the next along the food chain. The bottom of the pyramid is composed of autotrophs, the primary producers of the ecosystem. They do not obtain energy from eating; instead they harness solar energy by photosynthesis to make organic substances from inorganic ones. All other organisms are consumers called heterotrophs, which either directly or indirectly depend on producers for energy. (Britannica)

It is often 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. 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 is lost as heat to the environment. Graphic representations of the biomass or productivity at each tropic level are called trophic pyramids.

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.

A pyramid of numbers shows the number of consumers at each level drops significantly, so that a single top consumer (e.g. a Polar Bear) will be supported by literally millions of separate producers (e.g. Phytoplankton).

[edit] Food web

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.

A food web is a set of interconnected food chains by which energy and materials circulate within an ecosystem. The food web is divided into two broad categories: the grazing web, which typically begins with green plants, algae, or photosynthesizing plankton, and the detrital web, which begins with organic debris. These webs are made up of individual food chains. In a grazing web, materials typically pass from plants to plant eaters to flesh eaters. In a detrital web, materials pass from plant and animal matter to bacteria and fungi (decomposers), then to detritivores, and then to their predators (carnivores).

Generally, many interconnections exist within food webs. For example, the fungi that decompose matter in a detrital web may sprout mushrooms that are consumed by squirrels, mice, and deer in a grazing web. Robins are omnivores (consumers of both plants and animals) and thus are in both detrital and grazing webs. Robins typically feed on earthworms, which are detritivores that feed upon decaying leaves.

Trophic levels describe the number of energy transfers from a non-living energy source (typically the sun) to organisms within a food web. Plants generally belong to the first trophic level. Herbivores belong to the second trophic level. Carnivores, predators feeding upon the herbivores, belong to the third. Omnivores belong to both the second and third. Secondary carnivores, which are predators that feed on other predators, belong to the fourth and higher trophic levels. As the trophic levels rise, the predators generally become fewer, larger, fiercer and more agile, although parasites and pathogens are exceptions to this rule. At the second and higher levels, decomposers of the available materials function as herbivores or carnivores depending on whether their food is plant or animal material.

The feeding of one organism upon another in a sequence of food transfers is known as a food chain. Another definition is the chain of energy transfer from one organism to another.

Food webs provide a framework to help ecologists organize the complex network of interactions among species observed in nature. Perhaps the earliest graphical depiction of a food web was by Lorenzo Camerano (1880), followed independently by those of Pierce and colleagues (1912) and Victor Shelford (1913).^[1][2] Charles Elton's use of food webs in his 1927 synthesis^[3] made them a central concept in the field of ecology. Raymond Lindeman^[4] emphasized the use of the common currency of energy flow along links in a food web, initiating the extensive analysis of energy and material flows that are a core activity of ecosystem ecology. Robert Paine's experimental and descriptive study of intertidal shores^[5] increased interest in food webs by arguing that food web complexity was key to maintaining species diversity and ecological stability. This and other arguments prompted theoretical ecologists, including Sir Robert May^[6] and Stuart Pimm^[7], to examine the mathematical properties of food webs. Surprisingly, their analyses indicated that complex food webs should be highly unstable. The apparent paradox between the complexity of food webs observed in nature and the mathematical fragility of food web models is currently an area of intensive study and debate. The paradox may arise in part because of conceptual differences between persistence of a food web and equilibrial stability of a food web. Current research points to important roles of non-random structure in the connections within the food web that develop as food webs assemble over long periods of time, of patterns in the strengths of interactions among species within the food web, of variable strengths of species interactions as species abundances change, and of spatial variation in the environment that creates food webs of different structures connected by movement of individuals and materials, in the creation and persistence of complex food webs.^[8]

Summerhayes and Elton's 1923 food web of Bear Island (Arrows represent an organism being consumed by another organism).

An example of a food web is the early food web published by Victor Summerhayes and Charles Elton in 1923. Summerhayes and Elton's diagram depicted the interactions of plants, animals and bacteria on Bear Island, Norway.^[9]

[edit] Energy flow

Through these series of steps of consuming and being consumed, energy flows from one trophic level to another. Green plants, or other photosynthesizing organisms, use light energy from the sun to manufacture carbohydrates for their own needs. Most of this chemical energy is processed in metabolism and dissipated as heat in respiration. Plants convert the remaining energy to biomass, both above ground as woody and herbaceous tissue and below ground as roots. Ultimately, this material, which is stored energy, is transferred to the second trophic level, which comprises grazing herbivores, decomposers, and detrital feeders. Most of the energy assimilated at the second trophic level is again lost as heat in respiration; a fraction becomes new biomass. Organisms in each trophic level pass on as biomass much less energy than they receive. Thus, the more steps between producer and final consumer, the less energy remains available. Seldom are there more than four links, or five levels, in a food web. Eventually, all energy flowing through the trophic levels is dissipated as heat. The process whereby energy loses its capacity to do work is called entropy. The biomass of one plant will affect another and therefore the cycle will continue

[edit] See also

Ecology portal

Environment portal

Sustainable development portal

• Balance of Nature
• Biodiversity
• Ecology
• Ecosystem
• Earth Science
• Food systems
• Natural environment
• Nature
• List of feeding behaviours
• Antipredator adaptations

[edit] References

[edit] Notes

"food web." Encyclopædia Britannica. 2009. Encyclopædia Britannica Online. 05 Nov. 2009 <http://www.britannica.com/EBchecked/topic/212738/food-web>.

“Trophic Levels and Food Webs.” Ecorisk Fundamentals. 2006. Navy Guidance for Conducting Ecological Risk Assessments. 05 Nov. 2009 <http://web.ead.anl.gov/ecorisk/fundamentals/html/ch1/1.4.htm>

[edit] Bibliography

• "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 [3]

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