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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
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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