Ecological efficiency is the term used to describe how much energy will be passed from lower food pyramid level into the higher level. The amount of energy in primary level will be much higher than tertiary. The difference of level of tertiary(3) with primary(1) would be two level.
Assuming that the ecological efficiency is 10%, then the primary consumer level would be: 50 kcal * 10^(3-1)= 5000 kcal
