KS3 · Biology

Predator–prey relationships

Hunters and hunted are locked together: when one population changes, the other follows — round and round in a loop.

Predators and prey · The loop

Rebuild the loop: two populations that keep pushing each other

2 stages are missing. Use the rest of the loop to work out which goes where.

  1. Stage 1: The prey population rises

    There are fewer predators, so the prey are not hunted as much.

  4. Stage 4: The predator population falls

    Fewer prey means less food for the predators.

…and stage 4 leads back to stage 1.

Predict, then check

Now stretch the loop out over time.

Picture the loop drawn as a graph: population size up the side, time along the bottom, one line for the prey and one for the predators. Both lines rise and fall, again and again. What will the two lines do?

Food chains · Who eats whom

Where predators and prey sit: the food chain
Quaternary consumerTertiary consumerSecondary consumerPrimary consumerProduceris eaten byEach box = a whole population

Showing 1 layer: Food chain

Explore

A food chain diagram is a model — a simplified picture of what is happening in the real world. Each box doesn't stand for one organism: it stands for the whole population of that organism in the area. (A population is the number of one type of organism in a specific area.)

Tap each position to see what it's called and what it eats.

Predators and prey · Who's who

Predator, prey, both — or neither?

Use the food chain you've just explored. For each organism, tick every set it belongs to. Some belong to both sets — and one belongs to neither.

  • A Predator: hunts and eats other animals
  • B Prey: caught and eaten by a predator
  1. Producer
  2. Primary consumer
  3. Secondary consumer
  4. Tertiary consumer
  5. Quaternary consumer

Changing populations

Predators aren't the only thing: what makes a population grow or shrink?

Populations are always changing. For each factor, tick what it can do to a population. Then look hard at the last column.

Organisms reproducing
Migration into or out of the area
Predation by other animals
Disease
Natural disasters, such as floods and wildfires
Hunting, fishing, cutting down forests and pollution
Conservation

The knock-on effect

?

Reason it through

Humans hunt the quaternary consumer, and its population falls. Why does that change reach organisms it never eats?

Link 1 of 4

First link · your turn

The quaternary consumer hunts the tertiary consumer. Now there are fewer quaternary consumers. What happens to the tertiary consumers?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How the hunters and the hunted push each other's numbers up and down.

What you need to know

  • A population is the number of one type of organism in a specific area. It goes up when organisms reproduce or migrate in, and down when organisms die or migrate out.
  • A predator hunts and eats other animals; prey are the animals a predator catches and eats. A consumer can be both. A producer is neither, and the apex predator at the end of a food chain is not prey for any other organism.
  • Consumers are named by position: the primary consumer eats the producer, and the secondary, tertiary and quaternary consumers are the second, third and fourth consumers. A food chain diagram is a model in which each box is a whole population.
  • Predator and prey populations rise and fall in a repeating cycle, with the predator changes following the prey changes. Real populations behave in the same linked way but are more complex than the model.
  • Population size is also changed by disease, the seasons, natural disasters (such as floods and wildfires) and human activities such as hunting, fishing, cutting down forests and pollution. Conservation helps populations grow.

The big picture

A predator is an animal that hunts and eats other animals, and its prey are the animals it catches and eats. Predator and prey populations depend on each other in a repeating loop: more prey means more food, so predators increase; more predators means more prey are eaten, so prey fall; fewer prey means less food, so predators fall; fewer predators means prey are hunted less, so prey rise again. On a predator–prey model graph the predator changes follow the prey changes, and because every organism in a food chain is linked, a change in one population affects all the others.

Key points

1More prey → more food → more predators.
2More predators → more prey eaten → fewer prey.
3Fewer prey → less food → fewer predators.
4Fewer predators → prey hunted less → more prey.
5Prey change first; predators follow.
6A change in one population affects every organism in the food chain.

Worked example

Problem

In the food chain producer → primary consumer → secondary consumer → tertiary consumer, a disease makes the secondary consumer population fall. Predict what happens to the tertiary consumers and to the primary consumers, and give a reason for each.

⚠ Watch out

Thinking a change only affects the organisms right next to it in the food chain. A change in one population affects all the other organisms in the chain — the effect passes along link by link until it reaches every one.

🧠

Memory hook

Prey lead, predators follow — and every step has its reason: MORE FOOD, MORE EATEN, LESS FOOD, HUNTED LESS. Four reasons, one loop.

✓

Check yourself

A predator population suddenly falls. Using the loop, what happens next to its prey, and then what happens to the predators after that? Give the 'because' for each step.

Flashcards

(15)
What is a population?
The number of one type of organism in a specific area.
What is a predator, and what are its prey?
A predator is an animal that hunts and eats other animals. Prey are the animals the predator catches and eats.
Which organism begins every food chain, and what does it provide?
The producer. It provides the food for all the other organisms in the food chain.
What does a primary consumer always eat?
The producer.
What are the secondary, tertiary and quaternary consumers?
The second, third and fourth consumers in a food chain. They eat other animals.
How can one organism be both a predator and prey?
It hunts one organism and is hunted by another — like any consumer in the middle of a food chain.
Is a producer a predator, prey, both or neither?
Neither.
What is an apex (top) predator?
The organism at the end of a food chain. It is not prey for any other organism.
Why is a food chain diagram called a model?
It is a simplification of what happens in the real world, and each box represents a whole population, not one organism.
Predators increase. What happens to the prey, and why?
The prey population falls, because more prey are eaten.
Prey increase. What happens to the predators, and why?
The predator population increases, because there is more food available.
On a predator–prey model graph, which population changes first?
The prey. The predator changes follow the prey changes.
How does migration change a population?
Organisms migrating into an area make the population go up; organisms migrating out make it go down.
Which usually reduces a population faster: a natural disaster or a disease?
A natural disaster, such as a flood or wildfire, tends to reduce it very rapidly. Disease usually causes a smaller fall over a longer time.
How can humans help a population to grow?
Through conservation: restoring the places where organisms live.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How this lesson was checked. This KS3 Biologylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 1 October 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.