Explain how the predator–prey relationship between two species maintains a cyclical pattern of population fluctuation in a community.

IB DP Biology Standard 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).

In many ecosystems, population sizes of predators and their prey oscillate over time in a repeating cycle. The Canadian lynx and snowshoe hare are a well-documented example of this phenomenon.

Model answer (4 marks)

1. When prey numbers rise, predators have more food, so predator numbers increase.
2. The larger predator population exerts more predation pressure, reducing the prey population.
3. As prey become scarce, predators lose food and their numbers fall.
4. With fewer predators, the prey population can recover and grow again, restarting the cycle.

Examiner tips

Use the exact sequence of cause–effect steps; each step earns one mark.
Show the link between food availability and population change.
Keep the answer concise – one sentence per point.
Use UK spelling (e.g. ‘prey’).

Common mistakes

Mixing up the order of the steps; the cycle must start with prey increase.
Using vague terms like ‘more food’ without linking to predator growth.
Including unnecessary details such as specific species names or dates.

Mark scheme (4 marks)

An increase in prey population provides more food for predators, causing predator numbers to increase.
The increased predator population causes greater predation pressure, leading to a decline in prey numbers.
As prey numbers decline, food becomes scarce for predators, so predator numbers subsequently decrease.
With fewer predators, prey population recovers/increases again, restarting the cycle.

Key terms in this question

predator–prey relationship · population fluctuation · cyclical pattern · community