Questions: Energy Flow in Ecosystems

Question 1

Why are there typically fewer top predators than herbivores in an ecosystem?

• A. Because predators are naturally rarer due to genetics
• B. Because 90% of energy is lost at each level, so less energy is available to support organisms at higher levels
• C. Because herbivores reproduce faster than predators
• D. Because predators need less food

Answer: (B)

At each level of the food chain, about 90% of the energy is used for life processes and lost as heat. Only about 10% is stored in body tissue and available to the next level. This means each successive level has far less energy to support its organisms. A large base of producers supports a smaller number of herbivores, which supports an even smaller number of predators.

Question 2

Energy is recycled in ecosystems just like nutrients are.

• T. True
• F. False

Answer: False

Nutrients (like nitrogen and phosphorus) are recycled — decomposers return them to the soil, and producers take them up again. But energy is NOT recycled. It enters the ecosystem as sunlight, is converted to chemical energy by producers, and is gradually lost as heat at each trophic level. The ecosystem must receive a continuous supply of new energy from the sun.

Question 3

If a field of grass captures 10,000 kcal of energy from the sun, approximately how much energy is available to secondary consumers (like snakes that eat mice)?

Model answer: About 100 kcal. The grass passes about 10% (1,000 kcal) to primary consumers (mice). The mice pass about 10% of that (100 kcal) to secondary consumers (snakes). Each level loses about 90% of the energy as heat.

Applying the 10% rule across two transfers: 10,000 → 1,000 → 100. This dramatic reduction explains why ecosystems are shaped like pyramids — wide at the base (many producers) and narrow at the top (few top predators).