8.2 Energy Flow Through Ecosystems

Trophic levels

Energy enters ecosystems and flows through trophic levels. Producers (autotrophs, like plants) make their own food. Consumers eat others: primary (herbivores), secondary, and tertiary. Decomposers break down dead matter, recycling nutrients.

The 10% rule

Energy flow is one-way. Only about 10% of the energy at one trophic level passes to the next — the rest is lost, mostly as heat. This is why food chains rarely have more than a few levels, and why energy pyramids narrow toward the top.

Key terms

Trophic level

A feeding level in an ecosystem (producers, consumers, decomposers).

Producer (autotroph)

An organism that makes its own food, e.g., by photosynthesis.

10% rule

Only about 10% of energy passes from one trophic level to the next.

Decomposer

An organism that breaks down dead matter and recycles nutrients.

Energy pyramid

A diagram showing energy decreasing at each higher trophic level.

Try an AP-style question

Using the 10% rule, about how much energy reaches the hawks?

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