Explain why the size of a prey population in an ecosystem tends to fluctuate in a cyclical pattern when a predator population is also present.

OCR A-Level Biology A (H420) — 6.6 Populations and sustainability · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

In many natural ecosystems, ecologists have observed that prey and predator populations oscillate over time, with the peaks of the predator population lagging slightly behind the peaks of the prey population. These oscillations can persist for many generations.

Model answer (4 marks)

An increase in the prey population provides more food for predators, so the predator population rises.
The larger predator population exerts greater predation pressure, reducing the prey numbers.
The decline in prey causes a food shortage for predators, so the predator population falls.
With fewer predators the predation pressure is lowered, allowing the prey population to recover and the cycle restarts.

Examiner tips

Use the sequence of cause and effect; mention food availability and predation pressure.
Show the lag by noting predator response follows prey change.
Use terms like ‘predation pressure’ and ‘food shortage’.
Keep each point brief but complete.

Common mistakes

Mixing up the order of cause and effect; saying predators rise before prey rise.
Using vague terms like ‘more food’ without linking to predator increase.
Omitting the predator decline step or the recovery of prey.

Mark scheme (4 marks)

An increase in prey population leads to an increase in food availability for predators, causing the predator population to increase
The increased predator population causes greater predation pressure, so prey numbers decline
The decline in prey population leads to food shortage for predators, causing predator numbers to fall
With fewer predators, predation pressure is reduced and the prey population can recover, restarting the cycle

Key terms in this question

prey population · predator population · cyclical pattern · fluctuate