Index of Diversity (AQA AS Biology): Revision Note

The index of diversity

• An index of diversity describes the relationship between the number of species in a community and the number of individuals in each species

• It gives a numerical measure of species diversity in a community and takes into account both:

  • species richness – how many different species are present

  • species evenness – how evenly individuals are distributed among those species

• Using species richness alone can be misleading:

  • Habitat A has 10 species, but one species dominates (e.g. 90% of all individuals)

  • Habitat B has 8 species, all in similar numbers

  • Even though A has more species, B is more diverse overall — and the index of diversity shows this

Calculating the index of diversity

• The formula is:

• Where:

  • n = total no. of organisms for a single species in the community

  • N = total no. of organisms in the community

  • Σ = sum of

•  The larger the number obtained, the higher the level of diversity

• To calculate:

  • Step 1: Calculate N(N-1) to find value A

  • Step 2: Calculate n(n-1) for each species

  • Step 3: Add these numbers together to find value B

  • Step 4: Divide value A by value B

• The possible values of index diversity are significant:

  • The larger the number obtained, the higher the levels of diversity