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- Measuring Diversity

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**Simpson’s index of diversity**

Species diversity is described as the number of species in a community. The more species, the higher the diversity. What if there are two separate communities like this:

Community #1 has 150 individuals per each of 20 different species (3000 individuals in total)

Community #2 has 10 individuals per each of 19 species, and 2990 individuals of the last species (3000 individuals in total)

It doesn’t take a complex formula to figure out that community #1 is far more diverse compared to community #2, despite them having the same number of species and individuals. **The distribution of individuals to species** is important in determining a community’s diversity.

The above example is easy enough, but for most purposes a **formula **is needed. This formula measures the **index of diversity **a.k.a. **Simpson’s index of diversity**, which is simply a measure of diversity in a community. By calculating it and obtaining a **numerical value**, different communities can be easily compared.

Right, here it comes…

No, don’t run away yet! Wait and see how easy it is to work out.

D = Diversity index

N = total number of all organisms

n = total number of organisms of each species

Σ = sum of

Now it’s simply a matter of replacing numbers. Look, I made it all purple so you would enjoy looking at it. Let’s work out the index of diversity for community #1 (from above).

Firstly, we need a value for N. What’s the total number of organisms? 3000. Sorted.

Next, we need a value for N – 1. No calculators! …2999, sorted.

Finally, we need a value for n and n – 1. n = 150, while n – 1 = 149.

Drawing up a table helps:

species | n |
n – 1 |
n(n – 1) |

a | 150 | 149 | 22350 |

b | 150 | 149 | 22350 |

c | 150 | 149 | 22350 |

d | 150 | 149 | 22350 |

e | 150 | 149 | 22350 |

f | 150 | 149 | 22350 |

g | 150 | 149 | 22350 |

h | 150 | 149 | 22350 |

i | 150 | 149 | 22350 |

j | 150 | 149 | 22350 |

k | 150 | 149 | 22350 |

l | 150 | 149 | 22350 |

m | 150 | 149 | 22350 |

n | 150 | 149 | 22350 |

o | 150 | 149 | 22350 |

p | 150 | 149 | 22350 |

q | 150 | 149 | 22350 |

r | 150 | 149 | 22350 |

s | 150 | 149 | 22350 |

t | 150 | 149 | 22350 |

Total |
3000 | 2980 | 447000 |

3000*2999 8,997,000

20 in this case is **maximum diversity** (there are 20 different species). If the index was 1, then diversity would have been non-existent. An index of 10 would indicate moderate diversity.

Now work out the index of diversity for community #2 using the table above and the walk through as a guide. You should get a pretty low value. I know it’s a bit confusing that the above numbers are identical in all the columns, but if you work out community #2 then the values for 1 species should be different to the other 19.

Most of the time all species will have different values. The working of it is the same though.

**Plants** are difficult to count in individuals, so the **percentage cover** in a quadrat is usually used instead.

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