Why is substantially less energy generally available to organisms at higher trophic levels?

Core Concept

Energy transfer between trophic levels is inefficient because organisms use energy for metabolism and maintenance, some energy leaves as heat, and not all biomass is consumed or assimilated. The commonly taught '10% rule' is only a rough heuristic; actual ecological transfer efficiencies vary, and the current AP framework emphasizes the general consequence of declining energy availability rather than a fixed percentage.