Index of Diversity

An index of diversity is a single number that describes the biodiversity of a habitat, taking into account both the number of species (species richness) and the number of individuals of each species (abundance). Edexcel uses:D = N(N − 1) / Σn(n − 1)where D = index of diversity, N = total number of organisms of all species, n = number of organisms of each species, and Σ means 'the sum of'.Worked example. A sample from a hedgerow contains three species:Species A: 10   Species B: 5   Species C: 51. Total, N = 10 + 5 + 5 = 20.2. N(N − 1) = 20 × 19 = 380.3. For each species, n(n − 1): 10 × 9 = 90; 5 × 4 = 20; 5 × 4 = 20.4. Σn(n − 1) = 90 + 20 + 20 = 130.5. D = 380 / 130 = 2.92 (to 3 significant figures).The higher the value of D, the greater the diversity. The lowest possible value is 1, when all the organisms belong to a single species. D increases both when there are more species and when individuals are spread more evenly between them.The index is useful because it gives a fair comparison between habitats: a habitat dominated by one species gets a lower D than one with the same number of species in similar numbers. Rare species contribute little to the value, so a high D reflects a community where many species are reasonably common. It also helps to set conservation priorities and to monitor whether a habitat's biodiversity is changing over time.