Simpson's Index of Diversity
- The Simpson's Index of Diversity is used to study the composition of communities
- Once the abundance of different species in an area has been recorded the results can be used to calculate the species diversity or biodiversity for that area
- Simpson's diversity takes into account the community effects of species cohabiting an area and interacting with each other
- Species diversity looks at the number of different species in an area (species richness) but also the evenness of abundance across the different species in that area (species evenness)
- Simpson’s index of diversity (D) can be used to quantify the biodiversity of an area
Simpson's index
- The formula is:
where:
- n = total number of organisms for a single species
- N = total number of organisms for all species
- To calculate Simpson’s Index:
- Step 1: First calculate n / N for each species
- Step 2: Square each of these values
- Step 3: Add them together and subtract the total from 1
- To understand what the value of D means you need to know the following:
-
- 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
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Worked example
Samples of different insect species in a backyard were collected using sweep nets and identification keys. Use the data to calculate Simpson’s Index.
Using the formula:
The results and working out are seen in the table below. The figures have been rounded to three decimal places for columns 3 and 4
Species | Number of individuals (n) | n/N | (n/N)2 |
Northern brown Argus butterfly | 7 | 0.035 | 0.001 |
Ladybug | 34 | 0.168 | 0.028 |
Forester moth | 6 | 0.030 | 0.001 |
Wasp | 21 | 0.104 | 0.011 |
Grass spider | 12 | 0.059 | 0.003 |
Bee | 37 | 0.183 | 0.033 |
Hornet | 7 | 0.035 | 0.001 |
Fly | 19 | 0.094 | 0.009 |
Highland midge | 59 | 0.292 | 0.085 |
Total number of organisms (N) | 202 | ∑(n/N)2 = 0.172 |
D = 1 - 0.172 = 0.828
because the value of D is much closer to 1 than 0, it can be said that this is a relatively high value for biodiversity
Exam Tip
Remember, you will be provided with the formula for Simpson’s Index in the exam so you do not need to recall this.