In a food chain, energy flows from producers to herbivores to carnivores. Why does the amount of usable energy decrease at each trophic level?

• A. Because energy is lost as heat and through metabolic processes; only a small portion is transferred between levels.
• B. Because producers are the only source of energy.
• C. Because energy increases at higher trophic levels.
• D. Because humans consume energy from the sun.

Answer: (A)

Energy flows through ecosystems in one direction from producers to herbivores to carnivores. At each step, most of the energy is used for the organism’s daily life activities or released as heat through metabolism, growth, movement, and digestion. Only a small portion of the energy captured by one level is stored in the tissues of the next level, so the amount available to the next consumer is reduced. This is why usable energy decreases as you move up the food chain, often summarized by the idea that only a fraction of energy is transferred to the next level. The other choices don’t fit: producers aren’t the only energy source in the system, energy does not increase at higher trophic levels, and humans don’t directly obtain energy from the sun.