Explain the flow of energy in an ecosystem. State the ten percent law with a diagram, and say why the flow is unidirectional.

1. 1 · Open with the source. The sun is the source of energy. Producers on land trap about 1% of the sunlight that falls on their leaves.
2. 2 · State the law. About 10% of the energy at each trophic level is passed to the next; the rest is used in life processes or lost as heat.
3. 3 · Draw the flow. [Sun → producers 10,000 J → herbivores 1,000 J → carnivores 100 J → top carnivores 10 J, with arrows for heat lost at each step]
4. 4 · Explain one-way flow. Energy taken in by herbivores does not go back to plants, and plants do not return it to the sun.
5. 5 · Explain short chains. So little energy is left after 3 or 4 levels that food chains stay short.
6. 6 · Real life. A rice and dal meal is one step from the producers, so it uses the sun's trapped energy well.
7. 7 · Close in one line. Energy flows one way and shrinks about tenfold at each step.

Scoring tip: Put energy values on your diagram. They show the examiner that you understand the ten percent law.