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Measuring diversity

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 nn - 1  n(n - 1)
 a150149 22350
 b150  149   22350
 c150  149 22350
 d150  149 22350
 e150  149  22350
 f150  149  22350
 g150  149  22350
 h150  149  22350
 i150  149  22350
 j150  149  22350
 k150  149 22350 
 l150  149 22350
 m150  149 22350 
 n150  149 22350 
 o150  149 22350 
 p150  149 22350 
 q150  149 22350 
 r150  149 22350 
 s150  149 22350 
 t150 149  22350
 Total 3000 2980 447000

                3000*2999      8,997,000
So, D = ----------------- =  --------------- = 20
                  447000            447000

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.

Ok byeeeee

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