Answer:
because 90% of energy is lost in the environment as heat.
Step-by-step explanation:
According to the 10% rule, only 10% of energy is transferred from one trophic level to the next trophic level. It means that if the producers have 100% percent energy than only 10% of energy will be passed from producers to primary consumers.
So only a small amount of energy is transferred to the next trophic level because most of the energy is lost as heat during the oxidation of food. Therefore a very small amount of energy is transferred to the highest trophic level.
As less energy is available to support the organisms present on higher trophic level so their number is lower than the number of organisms present at lower trophic levels.