When plants and animals die, their bodies become resources for other organisms. Of course, in a sense, most consumers live on dead material - the carnivore catches and kills its prey, and the living leaf taken by a herbivore is dead by the time digestion starts. The critical distinction between the organisms in this chapter, and herbivores, carnivores and parasites, is that the latter all directly affect the rate at which their resources are produced. Whether it is lions eating gazelles, gazelles eating grass or grass parasitized by a rust fungus, the act of taking the resource harms the resource's ability to regenerate new resource (more gazelles or grass leaves). In contrast with these groups, saprotrophs (organisms that make use of dead organic matter) 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. Exceptions exist among necrotrophic parasites (see Chapter 12) that kill and then continue to extract resources from the dead host. Thus, the fungus Botrytis cinerea attacks living bean leaves but continues this attack after the host's death. Similarly, maggots of the sheep blowfly Lucilia cuprina may parasitize and kill their host, whereupon they continue to feed on the corpse. In these cases the saprotroph can be said to have a measure of control over the supply of its food resource.
We distinguish two groups of saprotrophs: decomposers (bacteria and fungi) and detritivores (animal consumers of dead matter). Pimm (1982) described the relationship that generally exists between decomposers or detritivores and their food as donor controlled: the donor (prey; i.e. dead organic matter) controls the density of the recipient (predator; i.e. decomposer or detritivore) but not the reverse. This is fundamentally different from truly interactive predator-prey interactions (see Chapter 10). However, while there is generally no direct feedback between decomposers/detritivores and the dead matter consumed (and thus donor-controlled dynamics apply), 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. In fact, it is in nutrient recycling that decomposers and detritivores play their most fundamental role (see Chapter 19). More generally, of course, the food webs associated with decomposition are just like food webs based on living plants: they have a number of trophic levels, including predators of decomposers (microbivores) and of detritivores, and consumers of these predators, and exhibit a range of trophic interactions (not just donor controlled).
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 with complex, energy-rich molecules being broken down by their consumers (decomposers and detritivores) into carbon dioxide, water and inorganic nutrients. Some of the chemical elements will have been locked up for a time as part of the body structure of the decomposer organisms, and the energy present in the organic matter will have been used to do work and is eventually lost as heat. Ultimately, the incorporation of solar energy in photosynthesis, and the immobilization of inorganic nutrients into biomass, is balanced by the loss of heat energy and organic nutrients when the organic matter is mineralized. Thus a given nutrient molecule may be successively immobilized and mineralized in a repeated round of nutrient cycling. We discuss the overall role played by decomposers and detritivores in the fluxes of energy saprotrophs: detritivores and decomposers...
... do not generally control their supply of resources - 'donor control'
decomposition defined and nutrients at the ecosystem level in Chapters 17 and 18. In the present chapter, we introduce the organisms involved and look in detail at the ways in which they deal with their resources.
It is not only the bodies of dead animals and plants that serve as resources for decomposers and detritivores. Dead organic matter is continually produced during the life of both animals and plants and can be a major resource. Unitary organisms shed dead parts as they develop and grow - the larval skins of arthropods, the skins of snakes, the skin, hair, feathers and horn of other vertebrates. Specialist feeders are often associated with these cast-off resources. Among the fungi there are specialist decomposers of feathers and of horn, and there are arthropods that specialize on sloughed off skin. Human skin is a resource for the household mites that are omnipresent inhabitants of house dust and cause problems for many allergy sufferers.
The continual shedding of dead parts is even more characteristic of modular organisms. Some polyps on a colonial hydroid or coral die and decompose, while other parts of the same genet continue to regenerate new polyps. Most plants shed old leaves and grow new ones; the seasonal litter fall onto a forest floor is the most important of all the sources of resource for decomposers and detritivores, but the producers do not die in the process. Higher plants also continually slough off cells from the root caps, and root cortical cells die as a root grows through the soil. This supply of organic material from roots produces the very resource-rich rhizosphere. Plant tissues are generally leaky, and soluble sugars and nitrogenous compounds also become available on the surface of leaves, supporting the growth of bacteria and fungi in the phyllosphere.
Finally, animal feces, whether pro... and of feces duced by detritivores, microbivores, herbivores, carnivores or parasites, are a further category of resource for decomposers and detritivores. They are composed of dead organic material that is chemically related to what their producers have been eating.
The remainder of this chapter is in two parts. In Section 11.2 we describe the 'actors' in the saprotrophic 'play', and consider the relative roles of the bacteria and fungi on the one hand, and the detritivores on the other. Then, in Section 11.3, we consider, in turn, the problems and processes involved in the consumption by detritivores of plant detritus, feces and carrion.
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