By the end of this lesson
- Name and define the trophic levels.
- Assign organisms to trophic levels from a food chain or web.
- Explain the role of decomposers.
BIG QUESTION
Can the same animal be a primary and a secondary consumer?
Trophic level describes a feeding position in a particular chain, so an omnivore can occupy more than one.
Producers start food chains
A producerAn organism, usually a green plant, that makes its own food by photosynthesis; the first trophic level of a food chain. makes its own organic molecules, usually by photosynthesis, using light energy. Green plants, algae and some bacteria are producers.
Producers occupy the first trophic levelThe feeding position of an organism in a food chain, such as producer or primary consumer.. All the energy in a typical food chain entered through them.
Consumers are numbered by feeding position
A primary consumerAn organism that feeds on producers; a herbivore. eats producers and is a herbivore in that chain. A secondary consumerAn organism that feeds on primary consumers. eats primary consumers; a tertiary consumerAn organism that feeds on secondary consumers. eats secondary consumers.
An omnivore such as a fox that eats both berries and rabbits is a primary consumer in one chain and a secondary consumer in another. The label belongs to the feeding link, not permanently to the species.
Decomposers recycle every level
decomposerAn organism, such as a bacterium or fungus, that breaks down dead organic matter and waste, returning nutrients to the soil., mainly bacteria and fungi, feed on dead organisms, faeces and other waste by secreting enzymes and absorbing the products. They respire, releasing carbon dioxide, and release mineral ions such as nitrates back into the soil.
Decomposers are not usually drawn as one step in a food chain, because they feed on material from all trophic levels.
Trophic levels in a woodland chain
Name each organism by what it eats in this chain.
| Trophic level | Feeding role | Example |
|---|---|---|
| Producer | Makes organic molecules by photosynthesis. | Oak tree. |
| Primary consumer | Eats producers. | Caterpillar. |
| Secondary consumer | Eats primary consumers. | Blue tit. |
| Tertiary consumer | Eats secondary consumers. | Sparrowhawk. |
| Decomposer | Breaks down dead material and waste from every level. | Soil fungi and bacteria. |
Language that earns precision
- trophic level
- The feeding position of an organism in a food chain.
- producer
- An organism that makes its own organic nutrients, usually by photosynthesis.
- consumer
- An organism that obtains organic nutrients by feeding on other organisms.
- decomposer
- An organism, such as a bacterium or fungus, that breaks down dead material and waste.
Common misconceptions
Claim: The tertiary consumer is always the biggest animal.
Correction: Trophic level depends on feeding position, not size.
Claim: Decomposers are the last link in a food chain.
Correction: They break down material from all trophic levels.
Claim: An animal always has one trophic level.
Correction: Omnivores can feed at more than one level.
Match the answer to the command word
Read the arrows: the arrow points from the food to the organism that eats it.
For decomposers, mention enzymes, respiration (CO₂ release) and return of mineral ions.
Recall, apply, then justify
In the food chain grass → rabbit → fox, identify the primary consumer and the producer.
Hint
Which organism photosynthesises? Which eats it?
Suggested marking points
- Producer: grass.
- Primary consumer: rabbit.
Explain the importance of decomposers in an ecosystem.
Hint
Breakdown, respiration and minerals.
Suggested marking points
- Decomposers break down dead organisms and waste.
- They respire, releasing carbon dioxide for photosynthesis.
- They release mineral ions (such as nitrates) that plants absorb.
Herring gulls eat fish, crabs and also bread and chips from people. Explain why it is difficult to assign the gull to one trophic level.
Planning hint
Different foods sit at different levels.
What a strong design includes
- Trophic level depends on what an organism eats in a given chain.
- Bread and chips come from plants, so the gull acts as a primary consumer.
- Crabs that eat algae make the gull a secondary consumer.
- Fish that eat other animals can place the gull at a tertiary or higher level.
- So the gull occupies several levels in a food web.
- A food web shows this better than a single food chain.