We distinguish two groups of organisms that make use of dead organic matter (saprotrophs): decomposers (bacteria and fungi) and detritivores (animal consumers of dead matter). These do not control the rate at which their resources are made available or regenerate; they are dependent on the rate at which some other force (senescence, illness, fighting, the shedding of leaves by trees) releases the resource on which they live. They are donor controlled. Nevertheless, it is possible to see an indirect 'mutualistic' effect through the release of nutrients from decomposing litter, which may ultimately affect the rate at which trees produce more litter.

Immobilization occurs when an inorganic nutrient element is incorporated into an organic form - primarily during the growth of green plants. Conversely, decomposition involves the release of energy and the mineralization of chemical nutrients - the conversion of elements from an organic to inorganic form. Decomposition is defined as the gradual disintegration of dead organic matter and is brought about by both physical and biological agencies. It culminates, often after a reasonably predictable succession of colonizing decomposers, with complex energy-rich molecules being broken down into carbon dioxide, water and inorganic nutrients.

Most microbial decomposers are quite specialized, as are the tiny consumers of bacteria and fungi (microbivores), but detritivores are more often generalists. The larger the detritivore, the less able it is to distinguish between microbes as food and the detritus on which these are growing. We discuss the relative roles in decomposition of decomposers and detritivores in terrestrial, freshwater and marine environments.

The rate at which dead organic matter decomposes is strongly dependent on its biochemical composition and on the availability of mineral nutrients in the environment. Two of the major organic components of dead leaves and wood are cellulose and lignin. These pose considerable digestive problems for animal consumers, most of which are not capable of manufacturing the enzymatic machinery to deal with them. Most detritivores depend on microbial organisms to digest cellulose, in a variety of increasingly intimate associations. Dead fruit is a lot easier for detritivores to deal with.

Feces and carrion are abundant dead organic resources in all environments and, once again, microbial organisms and detriti-vores both play important roles. Many detritivores feed on feces, and the dung of herbivores (but not carnivores) supports its own characteristic fauna. Similarly, many carnivores are opportunistic feeders on carrion but there is also a specialized carrion-feeding fauna.

Decomposer communities are, in their composition and activities, as diverse as or more diverse than any of the communities more commonly studied by ecologists.

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Parasitism and Disease

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