As you just learned, in real ecosystems, trophic efficiencies usually vary from about 5% to about 20%. as a result, net production diagrams for ecosystems have a pyramid shape. two key factors explain why trophic efficiencies are relatively low, and thus why net production diagrams are shaped like pyramids.

When energy passes from one trophic level to the next, I would guess that the two factors which decrease the total amount of energy from being passed up are: 
1. An organism does not assimilate all the energy of food consumed. Within a consumer, digestion and assimilation of energy is not 100% efficient: some of the energy is lost.
2. A large proportion of energy assimilated by a producer and consumer is lost through respiration, i.e., day-to-day maintenance of metabolic processes.