A food chain shows what eats what, with arrows pointing from the food to the eater. A food web joins chains together, because one organism usually feeds on and is eaten by more than one other.
This lesson opens the SPM Biology ecosystems chapter. The next lesson follows the energy along the chains.
How do you build a chain?
- Find the producer, which is the green plant.
- Find what eats the producer. This is the primary consumer.
- Keep asking “what eats this?” to add secondary and tertiary consumers.
- Draw arrows from each food to the organism that eats it.
Worked example: a paddy field
A student records these observations in a paddy field:
- Grasshoppers eat rice plants.
- Rats eat rice grains.
- Frogs eat grasshoppers.
- Snakes eat frogs and rats.
- Owls eat rats and snakes.
One chain: rice plant → grasshopper → frog → snake → owl.
The food web joins all the links:
| Organism | Eats | Eaten by |
|---|---|---|
| Rice plant | Makes its own food | Grasshopper, rat |
| Grasshopper | Rice plant | Frog |
| Rat | Rice grains | Snake, owl |
| Frog | Grasshopper | Snake |
| Snake | Frog, rat | Owl |
| Owl | Rat, snake | Nothing in this web |
The rice plant is the producer, the grasshopper and rat are primary consumers, and the owl is at the top of the chain.
The mistake that costs marks
The common slip is to draw the arrows from the eater to the food, so the arrow means “eats”. In a food chain, the arrow shows where the energy goes.
| Wrong | Right | |
|---|---|---|
| Arrow | Frog → grasshopper (frog eats grasshopper) | Grasshopper → frog (energy moves to the frog) |
| Start of chain | A consumer | A producer |
| Web | One line only | All feeding links drawn |
Every chain starts with a producer, and the first arrow leaves the plant.
Check yourself
Write a food chain with four organisms from these observations: hawks eat snakes, snakes eat mice, mice eat seeds. Name the producer and the secondary consumer.
Answer
The chain is: seed → mouse → snake → hawk.
The producer is the seed, from the plant. The secondary consumer is the snake, because it eats the mouse, which is the primary consumer.
What to study next
Go on to explaining energy flow, and then read about ecological evidence and population relationships. Practise with the biological data interpretation trainer, and log slips in the mistake log and paper-error review.
If you want a teacher to check your diagrams, see online one-to-one Biology tuition.