Simpson's Index (Cambridge International A-Level Biology)

Spec point — use Simpson’s index of diversity (D) to calculate the biodiversity of an area, and state the significance of different values of D

Simpson's index

• Once the abundance of different species in an area has been recorded, the results can be used to calculate the species diversity for that area

• Species diversity looks at the number of different species in an area but also the evenness of abundance across the different species

  • These values are also known as species richness and species evenness

• An index of diversity is a numerical value that takes into account both both species richness and species evenness; Simpson’s index of diversity (D) is one type of index that can be calculated

Calculating Simpson's index

• The formula for Simpson's index is:

$\mathrm{d} =1−( \sum_{ }^{ } ( \mathrm{n} / \mathrm{N} )²)$

• Where:

  • n = total number of organisms of a particular species
  • N = total number of organisms of all species added together
  • ∑ = 'sum of'

• To calculate Simpson’s Index:

  • Step 1: The first step is to calculate (n ÷ N) for each species
  • Step 2: Square each of these values
  • Step 3: Add them together and subtract the total from 1

• The possible values of D are significant:

  • The value of D can fall between 0 and 1
  • Values near 1 indicate high levels of biodiversity
  • Values near 0 indicate low levels of biodiversity

Worked Example Samples of different insect species in a back garden were collected using sweep nets and identification keys.

Use the data to calculate Simpson’s Index.

The results and calculations are seen in the table below. The figures have been rounded to four decimal places for columns 3 and 4

D = 1 - 0.1737 = 0.826 (3dp)

As the value of D is much closer to 1 than 0, it can be said that this is a relatively high value of biodiversity.

Exam Hint Remember, you will be provided with the formula for Simpson’s Index in the exam. You need to be able to carry out the calculation of D, as you could be asked to do this in the exam, and to state what high and low values of D mean for the biodiversity of an area.