# Explain how keystone species contribute to maintaining biodiversity within an ecosystem, and outline why their removal can trigger a trophic cascade.

> IB DP Biology Higher Level (2023 syllabus) — A4.2 Conservation of biodiversity · Explain · 4 marks

## Mark scheme (4 marks)

1. A keystone species has a disproportionately large effect on ecosystem structure relative to its abundance/biomass.
2. Keystone species regulate prey/competitor populations, preventing any one species from dominating, which maintains species richness/diversity.
3. Removal of a keystone species leads to a trophic cascade — populations at lower trophic levels increase unchecked, causing overexploitation of resources at the next trophic level down.
4. The trophic cascade results in the loss of species / reduction in biodiversity because habitat structure or resource availability is drastically altered.

## Key terms

- [keystone species](https://www.gradenine.co.uk/glossary/keystone-species)
- [trophic cascade](https://www.gradenine.co.uk/glossary/trophic-cascade)
- [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-keystone-species-contribute-to-d8554f6f) · Published by Druglandscape Ltd.