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, it does not consider how many individuals each species has. A biodiversity index such as Simpson’s also depends on the relative abundance of each species. If one species dominates the community, the index will be low even though the species richness is high. Therefore a habitat can have many species but still have a low biodiversity index if the individuals are unevenly distributed among those species. A truly high biodiversity index requires both a large number of species and a roughly equal number of individuals in each species.

Examiner tips

  • Mention that richness is a count of species, while Simpson’s incorporates evenness; give the example of a dominant species lowering the index; use the term ‘species evenness’ and ‘relative abundance’
  • Show the logical link: high richness + uneven distribution → low index; high richness + even distribution → high index

Common mistakes

  • Confusing species richness with species evenness; writing that richness alone determines biodiversity; ignoring the role of relative abundance in the index

Mark scheme (4 marks)

  1. Species richness only measures the number of different species present, not how individuals are distributed among those species
  2. A high biodiversity index also requires even distribution of individuals across species (species evenness)
  3. If one species dominates (has a much greater number of individuals than others), the diversity index will be low despite high species richness
  4. A truly high biodiversity index requires both a large number of species AND roughly equal numbers of individuals in each species

Key terms in this question

species richness · biodiversity index

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