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4. Interdependence
Food webs and food chains represent trophic (feeding) relationships, and can include primary producers, consumers and decomposers
Food webs show which organisms are consumed by other organisms. They are particularly useful for illustrating the complex interactions that characterise most ecological communities, and provide an indication of the pathways that nutrients and energy take within the community. Food chains are simply 'vertical' subsets of food webs. Every community contains organisms that fulfill three basic trophic functions, obtaining their nutrients in different ways: primary producers, consumers and decomposers.
Primary producers (autotrophic organisms) form the base of all food webs. They are the only members of the community that can form organic compounds from simple inorganic ones, and thus actually produce food for the community. Green, photosynthesising plants are the main primary producers in almost all communities. The rare exceptions include the communities that are established around undersea volcanic vents. Here, chemosynthetic bacteria form the base of food webs that are completely independent of energy from the sun. Communities of bacteria deep underground have been discovered recently that are also independent of solar energy.
The second group of species within a community are the consumers (heterotrophic organisms) - organisms that feed upon and kill primary producers, or other consumers. The former are usually referred to as herbivores, the latter carnivores.
The third group of species are less familiar, yet up to 80% of the energy fixed by land plants may be utilised by them. These are the decomposers (saprotrophic organisms) that feed on the dead and decaying remains of other plants and animals. Fungi and earthworms are examples of this form of feeding. Decomposers that are animals are often referred to as detritivores.
What are primary producers, consumers and decomposers?