Food Chains and Predator–Prey Cycles

Following a food chain

A food chain represents a sequence of feeding relationships. It begins with a producer, followed by consumers: organisms that obtain food by eating other organisms.

For example, a pond food chain is:

Algae → tadpoles → small fish → kingfisher

The algae are the producer. Tadpoles eat the algae, so they are primary consumers. Small fish eat the tadpoles, making them secondary consumers. The kingfisher eats the small fish, so it is a tertiary consumer in this chain.

Each arrow points from the food to the organism eating it. The arrow from tadpoles to small fish therefore means that the fish eat the tadpoles. It shows the direction in which material and energy are transferred, not the direction in which a predator moves when hunting.

A shorter chain is grass → rabbit → fox. Here, grass is the producer, the rabbit is the primary consumer and the fox is the secondary consumer. A food chain does not have to contain a tertiary consumer: the labels describe positions in the particular feeding sequence.

Predators and prey depend on each other

A predator is a consumer that kills and eats other animals. The animals it eats are its prey. In the grass–rabbit–fox chain, the fox is a predator and the rabbit is its prey. In the pond chain, a small fish is a predator of tadpoles but also prey for a kingfisher. An animal can therefore be both predator and prey.

Prey numbers affect predators because prey provide food. Predator numbers affect prey because predators kill them. These two effects link the populations together.

Why the populations rise and fall

In a stable community, predator and prey populations can rise and fall in repeating cycles. Stability does not mean that the number of every organism stays unchanged.

Imagine that rabbit numbers increase. More rabbits provide more food for foxes, allowing more foxes to survive and reproduce. The fox population increases, but this takes time: extra food does not instantly produce more adult foxes.

With more foxes hunting, more rabbits are eaten and rabbit numbers fall. There is then less food available for the foxes, so fewer survive or reproduce and fox numbers fall too. With fewer foxes, fewer rabbits are eaten. The rabbit population can recover, providing more food for foxes again, and the cycle repeats.

This delay means that the predator population usually reaches its peak after the prey population in a simple predator–prey model.

Reading a predator–prey graph

The graph below is an illustration of this pattern. Its horizontal axis shows time in months, and its vertical axis shows the number of organisms in each population.

Data for A simplified predator–prey population cycle

Begin with the labels and key so that you know which line represents each population. Then follow each line from left to right, looking for increases, decreases, peaks and troughs.

In this illustration, the prey population reaches a peak of 120 organisms at month 2. The predator population peaks later, at 40 organisms at month 4: a delay of two months. The prey peak provides more food, supporting the later increase in predators. By month 4, prey numbers have already fallen to 60 while predator numbers are at their highest.

The prey population reaches a trough at month 5. The predator population reaches its next trough at month 7, after the reduced food supply has affected its numbers. Prey numbers then recover as there are fewer predators eating them.

The same sequence repeats, with prey peaking at month 8 and predators at month 10. To interpret another graph, use its actual times and values rather than assuming this exact delay or cycle length. Real communities contain other influences, so this simple model helps explain a relationship without predicting every population change.

• Photosynthetic producers make glucose and build biomass, supporting food chains.
• Chains begin with a producer, usually a green plant or alga.
• Producer → primary consumer → secondary consumer → tertiary consumer.
• Arrows show transfer of material and energy from the organism eaten to the eater.
• Example: algae → tadpoles → small fish → kingfisher.

Predator–prey cycles

More prey → more food for predators → more predators → more prey eaten → fewer prey → less food for predators → fewer predators → prey recover.

A predator kills and eats other animals; those animals are its prey. An animal can be both predator and prey.

Graph interpretation

• Check axes, scales and the key.
• Identify peaks, troughs and repeating changes.
• In a simple model, predator peaks follow prey peaks because population growth takes time.
• Support explanations with the graph's values and times.
• Stable communities can have fluctuating populations.