Ecosystems and Energy Flow
Trophic levels and the ten-percent rule.
An ecosystem is a community of living things together with its physical environment, linked by flows of energy and cycles of matter. Energy enters mostly as sunlight captured by producers, passes to consumers through food chains and food webs, and leaves as heat. Matter, such as carbon and nitrogen, cycles and is reused.
Only about of the energy at one trophic level is passed to the next; the rest is used for life processes or lost as heat. That is why food chains rarely have more than four or five levels and why ecosystems support far fewer top predators than plants.
Key ideas
- Trophic levels: producers, primary consumers (herbivores), secondary and tertiary consumers.
- Decomposers return nutrients from dead matter to the soil.
- The 10% rule explains energy pyramids.
- Keystone species have effects far larger than their numbers suggest.
Worked example
Producers in a grassland capture kcal. Using the 10% rule, how much energy reaches tertiary consumers?
- Primary consumers: of kcal.
- Secondary consumers: kcal.
- Tertiary consumers: kcal.
Answer: kcal
Common mistakes
- Saying energy cycles; energy flows one way while matter cycles.
- Forgetting decomposers in food webs.
- Applying the 10% rule as an exact law rather than a rough average.
Practice questions
Try each one, then open the answer. Numerical answers on this page are verified by an automated calculation check.
1.With the 10% rule, energy reaching herbivores from 8000 kcal of plants?
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kcal
2.What role do fungi that break down dead leaves play?
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Decomposers
3.How many times more energy is at the producer level than at the third consumer level under the 10% rule?
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