| MadSci Network: Environment |
Simpson's index:
You have written this index upside-down which would lead to mathematical error should the number of each individuals in each species be only one.
Imagine that you have a finite population of ten individuals. The table below shows a few different possibilities that might exist and the value of simpson's index in each case.
|
# of species |
distribution |
Index value |
|
10 |
1,1,1,1,1,1,1,1,1,1 |
0 |
|
5 |
2,2,2,2,2 |
0.11 |
|
5 |
6,1,1,1,1 |
0.33 |
|
1 |
10 |
1 |
You can see from the table that when the maximum diversity occurs, the value of the index is zero and when the minimum diversity occurs, the value of the index is 1. The range is therefore 0 - 1 with an inverse relationship between diversity and the index value.
The index works quite well in showing when one species dominates in an ecosystem as is evident from the values of 0.33 and 0.11 that occur for different distributions of individuals within 5 species. This quality of an ecosystem is referred to as the evenness. By comparing the value for 5 and 10 species with the same degree of evenness, you can see that the index also provides a measure of diversity
Richard Kingsley
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