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 use quantitative measures to compare the biodiversity of different habitats. Two woodland sites each contain 12 species of flowering plant, yet their calculated diversity indices differ considerably.

Model answer (4 marks)

A diversity index measures both the number of species and how evenly the individuals are distributed among them. Species richness only counts the different species, so a habitat can have many species but if a few dominate the population the index will be low. Therefore high species richness does not guarantee a high biodiversity index unless the individuals are also evenly distributed.

Examiner tips

Use the word ‘diversity index’ and mention ‘evenness’ or ‘relative abundance’
Show that richness alone is insufficient, give the example of dominance
Link the two points: richness + evenness = high index

Common mistakes

Confusing species richness with species diversity
Failing to mention evenness or relative abundance
Over‑simplifying by saying ‘more species = more diversity’

Mark scheme (4 marks)

Species richness only counts the number of different species present, not how individuals are distributed among those species
A diversity index (such as Simpson's) takes into account both the number of species AND the relative abundance / evenness of individuals across those species
If one or a few species dominate the habitat (very uneven distribution), the diversity index will be low despite high species richness
A high diversity index requires both high species richness AND an even distribution of individuals among those species

Key terms in this question

species richness · biodiversity index