Decomposers are the final consumers in every food chain or food web. At each higher feeding level the amount of energy available in a food web decreases. Only a small fraction of the energy taken in is stored as new tissue. Much of the food is not digested or absorbed. A large part of the energy in food is used for respiration and maintenance.
Much energy is lost as heat.
Only about ten percent of energy taken in at any feeding level is passed upward. The pyramid of energy is the amount of available energy in an ecosystem. The greatest amount of energy is at the base of the pyramid with the producers. The least amount of energy is present at the top of the pyramid. Energy decreases steeply so there are usually no more than four or five feeding levels in an ecosystem.