Consumers don't eat or digest all plant or animal biomass. Some biomass is left behind during feeding. Other biomass is eaten but not digested, then leaves the body as waste.

Decomposers are organisms that break down the biomass left behind by consumers, including waste, uneaten food, and dead organisms. Many types of fungi and bacteria are decomposers. Detritivores are a particular type of decomposer that consume leftover biomass and break it down through digestion. Examples of detritivores include earthworms and dung beetles.

Just like consumers, decomposers use biomass from other organisms to obtain usable energy, in the form of ATP. They use this energy to grow, generate heat, and perform cellular and body functions.