# Explain how the presence of a keystone species maintains biodiversity within a community.

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

## Mark scheme (4 marks)

1. A keystone species has a disproportionately large effect on community structure relative to its biomass or abundance.
2. The keystone species typically controls the population size of a dominant competitor or prey species, preventing competitive exclusion.
3. This allows multiple species that would otherwise be outcompeted to persist, increasing species richness/diversity.
4. Removal of the keystone species causes a trophic cascade or community collapse, with the dominant species increasing and biodiversity declining sharply.

## Key terms

- [keystone species](https://www.gradenine.co.uk/glossary/keystone-species)
- [biodiversity](https://www.gradenine.co.uk/glossary/biodiversity)

## Related

- [Revision notes for IB DP Biology Higher Level (2023 syllabus)](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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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-how-the-presence-of-a-7ac87f0a) · Published by Druglandscape Ltd.