Food chain

Overview

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 you 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 transfered.

Organisms represented in food chains

Primary producers, or autotrophs, are species capable of producing complex organic substances (essentially "food") from an energy source and inorganic materials. These organisms are typically photosynthetic plants, bacteria or algae, but in rare cases, like those organisms forming the base of deep-sea vent food webs, can be chemotrophic. 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 birds and mammals). Therefore, only a small amount of energy and biomass is incorporated into consumer's body and transferred to the next feeding level, thus showing a Pyramid of Biomass.

File:Food chain.jpg

Example of a food chain in a Swedish lake

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 — from the Greek word for nourishment, trophikos — based on how many links they are removed from the primary producers. Trophic levels may consist of 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 base of this food chain is likely phytoplankton. Phytoplankton are autotrophs, and are the base of the food chain by virtue of their ability to photosynthesize. Phytoplankton, as well as attached 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 organisms life processes or it is lost as heat to the environment. Graphic representations of the biomass or productivity at each trophic 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.

Food web

Summerhayes and Elton's 1923 food web of Bear Island

Summerhayes and Elton's 1923 food web of Bear Island

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, 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.

References

• "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 [http://www.oxfordreference.com/views/ENTRY.html?subview=Main&entry=t8.e3348