Which statement about energy flow is correct?

• A. Energy is stored indefinitely at each trophic level.
• B. All energy is passed to the next trophic level without any loss.
• C. Energy is used to build biomass at the same level and never moves up.
• D. Energy flows through trophic levels and is largely lost as heat, with only a portion passed to the next level.

Answer: (D)

Energy flows through ecosystems in one direction: from sunlight to producers, then to consumers, and at each transfer most of that energy is released as heat or used for metabolism rather than being passed on. Only a fraction—roughly about 10% on average—of the energy at one trophic level makes it to the next level to be used for growth and maintenance. The rest is dissipated as heat through respiration, movement, digestion, and other life processes, or ends up in waste and is not stored as usable energy for the next level. Because of these losses, energy available to higher trophic levels decreases as you move up the chain, which is why food chains are relatively short and energy pyramids slope upward. The statement also reflects that energy can be used to build biomass at a given level, but that energy is not perpetually accumulated there; it continually moves upward in small fractions and is largely lost as heat with each transfer.