Food chains and food webs: producers, consumers and energy
Read food chains and food webs, name trophic levels and calculate how much energy passes between them, with worked examples.
KS3GCSEMiddle SchoolHigh SchoolCurriculum fit: GCSE Biology and Combined Science (ecology). US: middle and high school life science (ecosystems: interactions, energy and dynamics).
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What is it?
An ecosystem is a community of living things and the non-living environment they interact with. Food chains show who eats whom, starting with a producer.
Producers, such as green plants, make their own food by photosynthesis. Consumers eat other organisms. Energy from the Sun enters the ecosystem through producers and is passed along the food chain, with much of it lost at each step.
Key formulas and ideas
- Producer → primary consumer → secondary consumer → tertiary consumer.
- Arrows in a food chain show the direction of energy flow, pointing to the eater.
- Only about 10% of energy is passed on at each trophic level (a commonly used estimate).
- Efficiency of transfer = energy transferred ÷ energy available × 100%.
Worked examples
Example 1: Reading a food chain
Question: In the food chain grass → rabbit → fox, name the producer, primary consumer and secondary consumer.
- Grass makes its own food, so it is the producer.
- The rabbit eats the grass, so it is the primary consumer.
- The fox eats the rabbit, so it is the secondary consumer.
Answer: Producer: grass. Primary: rabbit. Secondary: fox
Example 2: Energy transfer
Question: Grass stores 10 000 kJ of energy. About how much reaches the rabbits if 10% is passed on?
- 10% of 10 000 = 1000 kJ.
- The rest is lost as heat, or in waste and movement.
Answer: About 1000 kJ
Example 3: Population change
Question: In a food web, the number of rabbits falls sharply. Predict the effect on foxes and grass.
- Foxes have less food, so their numbers may fall.
- Fewer rabbits eat grass, so grass may increase.
Answer: Foxes may decrease and grass may increase
Common mistakes
- Drawing arrows from the eater to the food.
- Saying all the energy is passed on between levels.
- Calling a plant a consumer.
- Forgetting that decomposers break down dead matter and return nutrients.
Practice questions
Frequently asked questions
Why are food chains short?
Because so much energy is lost at each level that little remains to support more levels.
What do decomposers do?
They break down dead organisms and waste, returning nutrients to the soil.
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Reviewed by StudyMorrow editorial review on 5 Nov 2026.