1.10 Energy Flow and the 10% Rule

Energy Flows One Way

Energy enters ecosystems as sunlight, is captured by producers through photosynthesis, and flows through trophic levels (producer → primary consumer → secondary consumer, etc.). Unlike matter, energy is not recycled — it flows in one direction and is ultimately lost as heat through respiration and metabolic activity at every level.

The 10% Rule

On average, only about 10% of energy at one trophic level transfers to the next level; the other ~90% is lost as heat during respiration, movement, and other life processes, or is not consumed/digested. This means energy available decreases sharply at each higher trophic level, limiting the number of levels an ecosystem can support.

Ecological Pyramids & Biomass

Because of the 10% rule, biomass and energy pyramids are widest at the producer base and narrow toward top consumers. Fewer organisms and less energy can be supported at higher trophic levels, which is why food chains rarely exceed 4-5 levels and why eating lower on the food chain is more energy-efficient for humans.

Key terms

10% Rule

On average, only about 10% of energy at one trophic level transfers to the next; the rest is lost mostly as heat.

Trophic level

A position in a food chain (e.g., producer, primary consumer, secondary consumer) based on feeding relationships.

Energy pyramid

A diagram showing the amount of energy available at each trophic level, widest at producers and narrowing toward top consumers.

Why energy is lost between levels

Energy is used for metabolism, movement, and growth, and much of it is released as heat during cellular respiration.

Why food chains have limited trophic levels

Because only ~10% of energy passes to each successive level, there's eventually too little energy to support additional levels.

Try an AP-style question

Producers capture 10,000 kcal. About how much energy reaches tertiary consumers?

1. 10 kcal
2. 1,000 kcal
3. 1 kcal
4. 100 kcal