The biomass at lower trophic levels provides food for higher trophic levels. Most of the food consumed by animals is assimilated and is used first to satisfy metabolic demands. The excess can be used for growth and reproduction. Energy used for metabolic processes is lost as heat and will not be available to higher trophic levels. Thus the amount of energy available for growth and reproduction decreases at successive trophic levels.
Unassimilated material is passed out as feces.