Final answer:
Only about 10 percent of the energy at one trophic level actually passes on to the next higher trophic level. The rest of the energy is used up at the lower trophic level or lost to the environment as heat or incompletely digested food.
Step-by-step explanation:
Energy is passed up a food chain or food web from lower to higher trophic levels. However, as shown in the energy pyramid, only about 10 percent of the energy at one trophic level is actually passed up to the next higher trophic level. The other 90 percent of energy at each trophic level is used by organisms at that level for metabolism, growth, and repair. Metabolism generates heat (thermal energy), which is the energy that is lost to the environment. Some energy is also lost as incompletely digested food that is excreted. This decline in energy from one trophic level to the next explains why there are rarely more than four trophic levels in a food chain or food web.