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The food web of a biological community describes the flow of energy between the species types which make up the community. Complex interactions occur between individuals and species, and these are important in defining the community as a whole. The term ‘food chain’ is also used for this purpose, but ‘web’ more usefully illustrates the intricacy of the interactions involved. Any biological community may be broken down into producers and consumers. The primary producers are green plants, which use sunlight as an energy source to construct complex, energy-rich molecules such as proteins, fats and carbohydrates: used for structure and energy storage. The consumers are those organisms which derive their energy from that trapped by producers: thus consumers include herbivores, predators and organisms such as fungi which feed upon rotting biological material. The energy transfer is inefficient, so the producers and lower consumers are much more abundant in terms of numbers and material than the higher consumers; this gives the web or chain a pyramid shape. In most communities the species diversity is such that a number of herbivores can feed on a number of plants, while being preyed upon by a variety of carnivores. The energy stored within the bodies of consumers which have few predators is trapped at this stage because the bodies of the animals, and indeed all other organisms, will ultimately be made available as a food source for other consumers. This produces the web structure of energy flow; nutrients such as carbon and nitrogen also flow within the food web in such a way as to be recycled. The pathways of energy and nutrient transfer may be mapped by the ecologist but are dynamic and fluctuate with populations and environmental conditions. RB
See also biomass; biome; competition; ecosystem. |
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