In a food pyramid, which best explains why the number of organisms decreases from one trophic level to the next?

• A. Consumers at the top level require more energy than the lower-level consumers.
• B. Consumers at the lower level require more energy than the top-level consumers.
• C. The consumers are feeding on larger organisms that have less energy.
• D. The consumers are feeding on smaller organisms that have less energy.

Answer: (A)

Energy moves through an ecosystem, but not all of it moves with each transfer. When organisms eat, much of the energy they obtain is used for daily activities and biology, and a large portion is lost as heat. Only a small fraction is stored in the bodies of organisms that are one step higher in the food chain. Because top-level consumers typically need more energy to fuel larger bodies and more active lifestyles, there isn’t enough energy left from the level below to support many of them. That’s why the number of organisms tends to decrease as you go up each trophic level. The option that top-level consumers require more energy than those below best explains this drop in numbers. The other ideas don’t directly address why there are fewer individuals at higher levels, since prey size isn’t the main limiting factor and energy transfer efficiency—not prey size—controls how many organisms can be sustained at each level.