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 · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

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.

Model answer (4 marks)

Species richness only counts how many different species are present, it says nothing about how many individuals of each species there are.
Simpson's Index of Diversity incorporates both richness and evenness – it is lowered when one species dominates the community.
Thus a habitat can have many species but if one species makes up most of the individuals the index will be low.
A high biodiversity index therefore requires not just many species but also a relatively even distribution of individuals among them.

Examiner tips

Use the word ‘evenness’ or ‘relative abundance’ when describing Simpson’s Index.
Show the link between dominance of one species and a low index value.
Mention that both richness and evenness are needed for a high index.

Common mistakes

Confusing species richness with species diversity.
Failing to explain the role of evenness or dominance in Simpson’s Index.
Using the wrong index (e.g. Shannon instead of Simpson) in the explanation.

Mark scheme (4 marks)

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

Key terms in this question

species richness · biodiversity index