Not all organic matter moves directly from living prey to living consumers. Dead organisms, shed tissues, feces and other organic remains form detritus.

A decomposer is a heterotroph—especially many fungi and microorganisms—that chemically breaks down dead organic matter and absorbs the resulting small molecules. Detritivores such as earthworms or many arthropods ingest detrital material and physically fragment it, often making it more accessible to decomposers.

Decomposition has two major ecosystem effects.

First, decomposers use organic molecules as sources of carbon and energy. Through cellular respiration, much of the organic carbon is returned to the environment as $CO_2$.

Second, decomposition converts organically bound nutrients into inorganic forms that can again be taken up by primary producers. This release is often called mineralization.

For example, nitrogen contained in proteins and nucleic acids of dead tissue can eventually be converted into inorganic nitrogen compounds that re-enter biological uptake pathways.

Decomposition rate depends on temperature, moisture, oxygen availability and the chemical composition of detritus. Organic material rich in resistant polymers can decompose much more slowly than soft, nutrient-rich tissue.

The detrital pathway is therefore not an ecological afterthought. It is a major route through which both energy and chemical elements pass after organisms die or produce waste.