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Organisation of an Ecosystem » Food Chains and Predator-Prey Cycles

What you'll learn this session

Study time: 30 minutes

AQA spec: 4.7.2.1

Why photosynthetic organisms are the producers of biomass for life on Earth
How to read and build a food chain, with producers and primary, secondary and tertiary consumers
What predators and prey are
Why predator and prey numbers rise and fall in cycles, and how to interpret the graphs

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Producers: where the biomass comes from

Living things are made of biomass. Green plants and algae make glucose by photosynthesis and use it to build the other molecules they need. Animals cannot do this.

Because of this, photosynthetic organisms are the producers of biomass for life on Earth. Every animal gets its biomass, directly or indirectly, from a producer.

Key terms:

Biomass: the mass of living material.
Producer: an organism that makes (synthesises) its own molecules, usually a green plant or alga making glucose by photosynthesis.
Food chain: a diagram showing the feeding relationships between organisms in a community.
Consumer: an organism that eats other organisms.
Predator: a consumer that kills and eats other animals.
Prey: an animal that is eaten by a predator.

Food chains

Feeding relationships within a community can be shown as a food chain. Every food chain begins with a producer. The arrows show which organism is eaten by which. Each arrow means "is eaten by".

Here is a food chain from a meadow:

grass → grasshopper → frog → snake

The grass is the producer. It makes glucose by photosynthesis.
The grasshopper eats the producer, so it is a primary consumer.
The frog eats the primary consumer, so it is a secondary consumer.
The snake eats the secondary consumer, so it is a tertiary consumer.

🌱 Producer

Makes its own food. It is always at the start of the chain.

🐛 Primary consumer

Eats the producer. The grasshopper eats grass.

🐍 Secondary and tertiary consumers

Secondary consumers eat primary consumers. Tertiary consumers eat secondary consumers.

Predators and prey

Consumers that kill and eat other animals are predators. The animals they eat are prey. In the meadow chain, the frog is a predator of the grasshopper, so the grasshopper is its prey. The same frog is also prey for the snake.

An animal can be a predator and prey at the same time. It depends on which link in the chain you are looking at.

Common mistakes

Writing the arrows the wrong way round. The arrow points to the organism that does the eating, so grass → grasshopper is correct, not grasshopper → grass. Also, do not call a rabbit a producer just because it eats plants. Only organisms that make their own glucose by photosynthesis are producers. A rabbit is a primary consumer.

Predator-prey cycles

In a stable community the numbers of predators and prey rise and fall in cycles. The numbers are not fixed, but they follow a repeating pattern.

Take rabbits (prey) and foxes (predators) in a wood. The cycle works like this:

There is plenty of grass, so the number of rabbits rises.
More rabbits means more food for foxes. The number of foxes rises, but a little later.
The many foxes eat lots of rabbits, so the number of rabbits falls.
There are now fewer rabbits, so some foxes starve and the number of foxes falls.
With fewer foxes, fewer rabbits are eaten and their numbers rise again.

The cycle then repeats.

📈 Prey go first

The prey peak comes before the predator peak, because the predators need the prey to be plentiful before their own numbers can rise.

⏱ Predators follow

The predator curve has a similar shape to the prey curve, but it is slightly behind it.

Interpreting predator-prey graphs

Scientists use graphs to model these cycles. The horizontal axis shows time. The vertical axis shows the number of animals. There are two lines, one for the prey and one for the predator.

When you read one of these graphs:

Decide which line is the predator. It is the one that peaks second.
Say what the prey line does first, then what the predator line does after it.
Use the word "because" to link the two lines.

Worked example

A graph shows rabbits and foxes. Rabbits peak at 800 in year 2. Foxes peak at 90 in year 3. Foxes fall to 20 in year 5.

Question: Explain why the number of rabbits falls after year 2.

Answer: The number of foxes is rising, so more rabbits are eaten. Fewer rabbits survive, so the number of rabbits falls.

Question: Which is the predator?

Answer: The foxes. Their peak comes after the rabbit peak.

Common mistakes

Saying that only one population causes the cycle. Both populations affect each other. Another mistake is to describe only one line. A full answer says what happens to the prey, then what happens to the predator, and links them.

Exam-style question

A graph for a wood shows the number of rabbits and the number of foxes over 10 years. The rabbit number rises to a peak. A little later the fox number rises to a peak. The rabbit number then falls, and the cycle repeats.

(a) Name the producer in a food chain that includes rabbits and foxes. [1 mark]

(b) State the type of consumer a rabbit is. [1 mark]

Model answer

(a) Grass (or another green plant). (1)

(b) Primary consumer. (1)

(c) There are fewer rabbits, so there is less food for the foxes (1), so some foxes starve and the number of foxes falls (1).

Exam tip

In part (c), do not just say "there is less food". Name the food (rabbits) and say what happens to the foxes as a result.

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