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 Simpson's Diversity Index for each site, the values were markedly different.

Model answer (4 marks)

Species richness only counts how many different species are present, but it does not consider how many individuals of each species there are. A diversity index such as Simpson’s also takes account of the relative abundance of each species. If one species is very common and the others are rare, the index will be low even though many species are present. Therefore a habitat can have high species richness yet a low biodiversity index if the individuals are unevenly distributed among the species.

Examiner tips

  • Use the word ‘richness’ for the number of species and ‘diversity index’ for richness plus evenness
  • Show the link between dominance of one species and a low index value
  • Mention that a high index needs both many species and an even distribution

Common mistakes

  • Confusing species richness with species evenness
  • Saying the index is high because many species are present without noting abundance
  • Using the wrong diversity index or ignoring that Simpson’s is sensitive to dominance

Mark scheme (4 marks)

  1. Species richness only measures the number of different species present, not their relative abundance
  2. A diversity index also takes into account the abundance / proportion of individuals belonging to each species
  3. If one species dominates (high proportion of individuals), the diversity index will be low even if many species are present
  4. 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

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