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Detritus Food Chain

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Bacteria and fungi, however, are eaten by organisms and they in turn are eaten by other organisms.  Since the source of energy is not the sun but detritus, this linear feeding relationship is called a detritus food chain. Thus, ecosystems can have both grazing food chains and detritus food chains.  In some ecosystems, detritus food chains may move very large amounts of energy through them.  

Consider a deciduous forest where the leaves topple to the ground in the fall.   Bacteria and fungi begin to attack the plant matter, thereby triggering the decomposition process.  As nutrients are made available in the soil, its fertility increases.  Thus, plant seeds are able to germinate.  Organisms such as earthworms feed on the detritus.  In turn, other organisms such as moles feed on the earthworms.
Since we do not see what is happening below ground, we often do not appreciate the fact that a grazing food chain even exists possibly right under our own feet.
Decomposition process
Decomposition Process

Detritus food chain

Simplified Detritus Food Chain

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Credits: Decomposition Process (Wlliam Donahue) / Mole, Worm (Dorling Kindersley)