Diversity is one of the most well known and common used indices in ecological research. It is often used as an indicator of good environmental quality and this is very true in the case of plant and animal communities. BUT in areas in the Midwest where earthworms are non-native, earthworm diversity is a bad thing. The more diverse an earthworm community the more species have successfully invaded the habitat, this has significant negative implications on the native plant and animal communities. High earthworm diversity often means a low plant and animal diversity!
To work out diversity of a community you simply count the number of species, right? Well, yes and no, the majority of times you also need the number of earthworms in each species. For example, say there are two sites that each contains 7 plant species. In one site, two of those species account for 90% of the total plant community with the other 5 species being present at very low levels, while in the other site; all 7 species are about equally abundant. Just because there have the same number of species they do not have the same species diversity. It is often best to have a measure of diversity that includes both richness (number of species present) and their relative abundance.
Several indices of diversity have been developed to help you do just that (see table below). One that is very commonly used in animal and plant communities is the Simpsons Diversity Index anther common index is Shannon. Because of Simpson’s wide range of applications it will be explained in more detail. It can be interesting to see how the different measures of diversity can sometimes give different impressions of the same community! So if you are good at statistics or feel like a challenge have a go at using more than one and see how they differ.
Diversity is most commonly used as an indicator of your site as a whole using ALL of your pot data you can also calculate it on a plot by plot basis (i.e. for each individual plot) which allows you to calculate an average diversity per plot.
Shannon's H' =[-Σp)]-[(s-1)/2N]
Shannon and Weaver 1964; Poole 1974
Integrates the number of species and relative abundance; derived from information theory; measure of entropy for the sample; full formula is and expanding series; the first two terms are shown here.
Simpson's D= Σ²
A measure of dominance by one or a few species; the probability that two individuals selected from the population at random will be of the same species.
Adapted from ‘Hale et al 1999’
Simpson’s Diversity index produces a value between 0 and 1, values around 1 have high diversity and values around 0 have low diversity. Compared to other diversity indexes calculating Simpson’s diversity index is actually quite simple, if you are good at maths you can even work it out without using a calculator! Although it is mathematically quite simple it’s equation (see below) does look quite complex, but don’t let the algebra scare you it is actually quite straightforward to calculate.
D = (∑n(n-1))/(N(N-1))
It is easiest to calculate this by hand using a table format, you can do this by hand but we have also provided excel spreadsheet. Instructions for calculating Simpson’s Diversity index from your earthworm species list using both methods is provided below. It is important to understand the basics of the mathematics, so try the mathmatics behind the statistic, so try following the example in ‘Calculating Species by Hand’ before you jump to the excel method’.
To calculate species diversity can be done using a species table with the names and count of each species of earthworm. For the sake of simplicity in the example below our species are called EarthwormSP 1, 2, 3, 4, 5 and 6 but your earthworm species names will be more complex.
Step 1: Look at your species list. You should have a column for species name and the number/count of earthworms that was collected in this individual species (otherwise known a n).
|Species Name||Counts of species (n)|
Step 2: In a new column entitled n-1, you have to minus 1 from the number/count of individual species (in algebra this is written n-1!).
Step 3: Times the n-1 column by your original n value. If you have done this correctly you should have something looking similar to the table below.
Step 4: The next step is to calculate ∑n(n-1), the sigma (∑) means ‘sum’ so this is simply the n(n-1) column added up!
E.g. If you have done this correctly in the example your ∑n(n-1) should equal 154
Step 5: If you look at the equation D = (∑n(n-1))/(N(N-1)). You can see that you already calculated the first part (∑n(n-1)) in step 4, now all you need is the N(N-1) value. N is the total number of worms in your sample otherwise, to calculate this you need to the sum of the count of earthworms (or the sum of n). Once you have this N value you can work out N(N-1).
e.g. So for our example the sum of n equals 36, therefore N = 36. If N = 36, N(N-1) is 36(36-1) or 36 x 35, therefore N(N-1) = 1260
Step 6: As now all you have to do is put into Simpson’s equation. This last step involves dividing your ∑n(n-1) value by your N(N-1) value.
E.g.This means you divide 154 by 1260, which equals 0.12222 (to five decimal places).
If you have done a Great Lakes Worm Watch study and sent in your sampled earthworms you will most likely have a list of the species of earthworms and their numbers found in your site. You can use this species list alongside an Excel spreadsheet to easily calculate the diversity of your site.
Step 1: Once you have downloaded this spreadsheet all you have to do is copy the counts of each species into their respective species box.
You may have noticed the cells highlighted in pink entitled ‘n-1’ and ‘n(n-1)’ have been changing as you enter data. It is important that you DO NOT CLICK ON THE PINK HIGHLIGHTED CELLS! These contain excel formulars that can easily be edited simply clicking them by accident.
Step 2: Once you have done this for all you species you can delete the extra rows for species that are not apparent at your site, you do this by right clicking next the row number (grey box to the left of ‘Species’) and selecting ‘Delete’. You have to repeat this for all absent species otherwise they will affect your diversity index.
Step 3: Next to the cell labeled ‘Simpsons Diversity Index’ a value will appear this is your Simpson’s value! Keep this value safe by writing it down, copying into another document or by saving the spreadsheet.
If you have carried out a plot study between to habitat types along a transect it may be interesting to see how diversity changes along this transect. You can then work out the species diversity for each plot and see how the species structure (i.e. diversity) changes between the habitats.