# 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

> 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.

## Mark scheme (4 marks)

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

## Key terms

- [prey population](https://www.gradenine.co.uk/glossary/prey-population)
- [predator population](https://www.gradenine.co.uk/glossary/predator-population)
- [cyclical pattern](https://www.gradenine.co.uk/glossary/cyclical-pattern)
- [fluctuate](https://www.gradenine.co.uk/glossary/fluctuate)

## Related

- [Revision notes for OCR A-Level Biology A (H420)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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