Energy flow in the ecosystem follows the law of $10 \%$ energy flow proposed by Lindman. According to this law, only $10 \%$ of the energy available at each trophic level, gets transferred to the next trophic level, the rest is lost to the environment as heat.
The energy available to organisms keeps on decreasing as we move to higher trophic levels. Thus, the top carnivore gains the least energy in a food chain. Heat energy is lost during respiration.