Answer:
Because some of the energy is dissipated in the metabolic activities such as digestion and maintaining the warmth of the body of the organism at each level.
Step-by-step explanation:
Food chain is the sequential chain of energy transfer from one onrgaanism at a trophic level to another organism at the other trophic level. Autotrophs receive the energy from the sun and prepare their own food. Autotrophs then get consumed by the primary heterotrophs and further primary heterotrophs by the secondary heterotrophs. When the secondary heterotrophs die they are then consumed by the scavangers and decayed by the micro-organisms in to the soil which finally help the autotrophs (plants) in their growth.
At each trophic level of the food chain the energy does not gets completely transferred because the organisms also spend some energy in search of the food and the processing of food is also an energy consuming process.
The energy is also used in combating the climatic conditions and predators for the survival.