This is a calculation done by ecologists that is a measure of diversity which takes into account both richness and evenness of species. The term ‘Simpson’s Diversity Index’ can actually refer to any one of a range of closely related indices (Simpson’s Diversity Indices), but QCAA specifically deals with the Simpson’s Index (D).
The Simpson’s Diversity Index (SDI) gives a numerical value between 0 and 1 that describes how diverse a community is. It measures the probability that two individuals randomly selected from a sample will belong to the same species. An SDI value of 0.68 means the community has a moderate to high level of diversity. In practical terms, it suggests there is about a 68 percent chance that two individuals chosen at random will belong to different species.
•A value close to 1 indicates high diversity, meaning many species are present and their populations are fairly even in size. In such communities there is a high chance that two individuals chosen at random will belong to different species because no single species dominates the habitat.
•A value close to 0 indicates low diversity. This usually happens when only a few species are present or when one species is far more abundant than the others. In these cases there is a high chance that two individuals selected at random will be the same species.
•SDI therefore helps ecologists compare the structure of different communities and track changes in diversity over time.
This is the version of the equation in the QCAA syllabus:
∑ = sum of
n = number of individuals
N = total number of individuals
You may see this equations in exactly the same form, just omitting the 1 -. With this index, 0 represents infinite diversity and 1, no diversity. This is neither intuitive nor logical, so that’s where the 1 – D comes from.
The following pages will provide step-by-step instructions to show you how to calculate Simpson’s Diversity Index.