Biodiversity is the variety of living organisms in an area. In AQA A-level Biology, you can describe biodiversity within a community using species richness and the index of diversity. A community is the populations of different species living in the same area. Species richness is the number of different species present; the index also takes account of abundance, meaning the number of individuals of each species.

Calculating the index

Use the formula:

d=\frac{N(N-1)}{\sum n(n-1)}

Here, $N$ is the total number of individuals of all species, and $n$ is the number of individuals of one species. The $∑\sum$ sign means “add up”: calculate $n (n - 1)$ separately for every species, then add the results. Only after finding that total should you divide $N (N - 1)$ by it.

Suppose a sample contains $5$ daisies and $5$ buttercups. There are $10$ individuals altogether:

d=\frac{10(10-1)}{5(5-1)+5(5-1)}=\frac{90}{40}=2.25

A second sample contains $9$ daisies and $1$ buttercup. It still has two species and $10$ individuals, but its index is different:

d=\frac{10(10-1)}{9(9-1)+1(1-1)}=\frac{90}{72}=1.25

The first sample has the higher index because individuals are distributed more evenly between the species. In general, a higher index of diversity indicates greater biodiversity. The index has no units. Notice that the single buttercup contributes $1 (1 - 1) = 0$ to the denominator; you still include it when calculating $N$ .

Comparing communities

For a fair comparison, collect data using comparable methods and sampling effort. For example, use randomly placed quadrats of the same size in each area, identify the species and count their individuals. A common exam mistake is to use species richness alone to decide which community is more diverse. These examples have the same species richness, but different indices because their abundances differ. Show your calculation of $N$ and each species’ contribution to the denominator so your working is clear.