Simpson's Index (CIE A Level Biology)

• 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

Simpson's index

• The formula for Simpson's index is:

• • 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

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

Species	Number of individuals (n)	n/N	(n/N)2
Northern brown argus butterfly	7	0.0347	0.0012
Ladybird	34	0.1683	0.0283
Forester moth	6	0.0297	0.0009
Wasp	21	0.1040	0.0108
Grass spider	12	0.0594	0.0035
Bee	37	0.1832	0.0336
Hornet	7	0.0347	0.0012
Fly	19	0.0941	0.0088
Highland midge	59	0.2921	0.0853
Total no. of organisms (N)	202

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.