CHAPTER 9: Introduction to the Biosphere

(n). Organic Decomposition and the Detritus Food Chain

The detritus food chain differs from the grazing food chain in several significant ways. First, the organisms making it up are in general smaller in size. Second, the functional roles of the different organisms do not fall as neatly into categories like the grazing food chain's trophic levels. Finally, detritivores live in environments (soil, sea bed, etc.) that are rich in scattered food particles. As a result, decomposers are less mobile than herbivores or carnivores.

The organisms of the detritus food chain include members of many different species of animals and plants, such as algae, bacteria, slime molds, fungi, protozoa, insects, mites, crustaceans, centipedes, mollusks, worms, sea cucumbers, and even some vertebrates (Figure 9n-1). These organisms consume organic wastes, shed tissues, and the dead bodies of both plants and animals.

Figure 9n-1: Earthworms are one of the most important soil decomposers. These organisms consume vast amounts of organic matter and mineral soil. As the organic matter passes through their digestive system, it is subjected to digestive enzymes and the grinding action of mineral soil particles. The amount of material consumed per day is often equal to their body weight.


Decomposers tend to always be active, processing large amounts of organic matter and releasing a great deal of energy mostly as heat from metabolic activities (e.g., compost heap). The end result of decomposition is the conversion of organic matter back into its original inorganic nutrient form. In mature forest and grassland soils, the decomposition process establishs an equilibrium over time where litterfall additions equal the amount of organic matter decomposed.

The rate of decomposition in a detritus food chain is controlled by many factors. In most terrestrial ecosystems, temperature and soil oxygen and moisture content tend to be the primary variables controlling the activities of decomposers. In some ecosystems, oxygen may not be readily available. In these cases aerobic respiration cannot take place, and the breakdown of organic compounds and energy extraction must then proceed by anaerobic means like fermentation. Organisms involved in fermentation include bacteria and yeast. Anaerobic decomposition releases much less energy from organic matter than does aerobic respiration. The end products of anaerobic decomposition include molecules such as carbon dioxide, water, and humus. They also include small-molecule alcohols, organic acids, ptomaines, amines, and other products, as well as gaseous substances such as methane.

Because respiration is so much more efficient at releasing the energy contained in organic molecules, the activity of the detritus food chain is much higher in an aerobic environment, and the breakdown of materials more complete. Organic matter breakdown is substantially slower and less complete in anaerobic environments. It also results in the accumulation of undegraded organic matter in the form of peat, organic soils, and highly organic sediments.

In conclusion, there is no waste in a properly functioning natural ecosystem. Everything once living or alive will be consumed by decomposers at some time and returned to inorganic form.



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Created by Dr. Michael Pidwirny & Scott Jones University of British Columbia Okanagan

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04/17/2006 7:28