Food chains and energy flow

A food chain shows who eats whom: grass → deer → lion. Each step in the chain is a trophic level, simply its position in the chain:

Every food chain starts with a producer.

"Why do we eat?" For energy. So eating passes energy from one level to the next. It starts with the Sun: green plants capture only about 1% of the sunlight falling on their leaves and store it as food.

The 10 per cent rule. At each level, most of the energy is used up in living (moving, keeping warm, breathing) or lost as heat. On average only about 10% of the food at one level reaches the next. Going up the chain, divide by 10; going down, multiply by 10.

Example: producers hold 10,000 J. Second level: 1,000 J. Third level: 100 J. Fourth level: 10 J. A fifth level would get only 1 J, too little to live on. That is why food chains are short: generally only three or four steps.

Energy flows one way: from the Sun to plants to animals, never back. The grass doesn't get its energy back from the deer.

In real life, a deer can be eaten by a lion or a tiger, and a frog eats many kinds of insects. Food chains cross and join, forming a food web.

"What if one level disappears?" Suppose all the deer in a forest die. The animals that eat deer go hungry, and their numbers fall. The grass has nothing eating it, so it spreads. Removing one level affects the levels both above and below it, and in different ways.