Explain how predator–prey interactions can produce cyclical fluctuations in the population sizes of both species within a community.

IB DP Biology Higher Level (2023 syllabus) — C4.1 Populations and communities · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

An increase in prey population gives predators more food, so predator numbers rise (with a time lag). The larger predator population then exerts greater predation pressure, causing the prey population to fall. As prey become scarce, predators suffer starvation and reduced reproduction, so predator numbers decline. With fewer predators, predation pressure drops and the prey population can recover, restarting the cycle.

Examiner tips

Use the word "increase" and "decrease" clearly for both species.
Show the time‑lag relationship between prey and predator changes.
Mention predation pressure and its effect on reproduction/food availability.
Keep the answer concise and directly linked to the four points.

Common mistakes

Confusing the direction of change (e.g., saying predators increase when prey decrease).
Omitting the time‑lag between prey and predator responses.
Using vague terms like "more food" without linking to predator population growth.

Mark scheme (4 marks)

An increase in prey population size provides more food/resources for predators, so predator numbers increase (with a time lag).
Rising predator numbers increase predation pressure, causing prey population size to decline.
As prey numbers fall, food becomes scarce for predators, so predator population declines due to starvation/reduced reproduction.
Reduced predation pressure allows prey population to recover/increase again, restarting the cycle.

Key terms in this question

predator–prey interaction