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The fungi on this tree are decomposers.

Decomposers (or saprotrophs) are organisms that consume dead organisms, and, in doing so, carry out the natural process of decomposition. Like herbivores and predators, decomposers are heterotrophic, meaning that they use organic substrates to get their energy, carbon and nutrients for growth and development. Decomposers use deceased organisms and non-living organic compounds as their food source. The primary decomposers are bacteria and fungi.

When a plant or animal dies, it leaves behind nutrients and energy in the organic material that comprised its body. Scavenger and detritivores can feed on the carcasses or litter, but they will inevitably leave behind a considerable amount of unused energy and nutrients. Unused energy and nutrients will be present both in the unconsumed portions (bones, feathers or fur in the case of animals, wood and other indigestable litter in the case of plants) and in the feces of the scavengers and detritivores. Decomposers complete decomposition by breaking down this remaining organic matter. Decomposers eventually convert all organic matter into carbon dioxide (which they respire) and nutrients. This releases raw nutrients (such as nitrogen, phosphorus, and magnesium) in a form usable to plants and algae, which incorporate the chemicals into their own cells. This process resupplies nutrients to the ecosystem, in turn allowing for greater primary production.

An example of a decomposer is saprotrophic fungus; it breaks down dead trees, harvesting the energy stored therein.

Although decomposers are generally located on the bottom of ecosystem diagrams such as food chains, food webs, and energy pyramids, decomposers in the biosphere are crucial to the environment. By breaking down dead material, they provide the nutrients that other organisms need to survive. As decomposers feed on dead organisms, they leave behind nutrients. These nutrients become part of the soil. Therefore, more plants can grow.

Bacteria are the primary decomposers of dead animals (carrion) and are the primary decomposers of dead plant matter (litter) in some ecosystems. In soils, where decomposition occurs in terrestrial ecosystems, bacteria are capable of rapid growth and reproduction. This allows bacteria to rapidly utilize and decompose available organic matter, especially if the organic matter has relatively simple chemical bonds. Bacteria were traditionally believed to be less abundant in soils than fungi, and therefore less important as decomposers. In some grasslands, however, active bacteria can be more abundant than active fungal hyphae, and bacteria in such ecosystems are much more important in the recycling of nutrients. Bacteria can also be very important in agricultural fields, because tillage usually increases the abundance of bacteria relative to fungi.

Fungi are the primary decomposers of litter in many ecosystems. Unlike bacteria, which are unicellular, most saprotrophic fungi grow as a branching network of hyphae. While bacteria are restricted to growing and feeding on the exposed surfaces of organic matter, fungi can use their hyphae to penetrate larger pieces of organic matter. Additionally, only fungi have evolved the enzymes necessary to decompose lignin, a chemically complex substance found in wood. These two factors make fungi the primary decomposers in forests, where litter has high concentrations of lignin and is often in large pieces.

Some animals, like millipedes and woodlice are commonly called decomposers, because such animals consume dead organic matter and contribute to the process of decomposition. Ecologists, however, generally refer to such organisms as detritivores. This distinction is made because bacteria and fungi are capable of digesting many complex chemical molecules that animals are incapable of digesting. Additionally, bacteria and fungi digest and decompose organic matter more fully than detritivores, reducing it to inorganic material. For these reasons, bacteria and fungi play a more fundamental role in the processes of decomposition and nutrient recycling than animals.

Organisms such as millipedes, woodlice, and earthworms are not considered decomposers in the modern scientific world.

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• Beare MH, Parmelee RW, Hendrix PF, Cheng W (1992) Microbial and faunal interactions and effects on litter nitrogen and decomposition in agroecosystems. Ecological Monographs 62: 569-591

• Hunt HW, Coleman DC, Ingham ER, Ingham RE, Elliot ET, Moore JC, Rose SL, Reid CPP, Morley CR (1987) "The detrital food web in a shortgrass prairie". Biology and Fertility of Soils 3: 57-68

• Smith TM, Smith RL (2006) Elements of Ecology. Sixth edition. Benjamin Cummings, San Francisco, CA.

• Swift MJ, Heal OW, Anderson JM (1979) Decomposition in Terrestrial Ecosystems. University of California Press, Berkeley, CA.