# Explain why a habitat with a high species richness does not necessarily have a high biodiversity index.

> OCR A-Level Biology A (H420) — 4.2 Biodiversity · Explain · 4 marks

> Ecologists studying two woodland sites recorded the same number of species at each site. However, when they calculated the biodiversity index for each site using Simpson's Index of Diversity, the two values were very different.

## Mark scheme (4 marks)

1. Species richness only measures the number of different species present, not how individuals are distributed among those species
2. Biodiversity indices such as Simpson's Index take into account the evenness / relative abundance of each species
3. If one species dominates — i.e. has a very high proportion of all individuals — the biodiversity index will be low despite high species richness
4. A high biodiversity index requires both high species richness AND high evenness — an equal / more even distribution of individuals across species

## Key terms

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

## 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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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-a-habitat-with-a-0aa08bca) · Published by Druglandscape Ltd.