Comparing habitats: index of diversity (D)Spec 4.18
In short
The index of diversity D = N(N−1) ÷ Σn(n−1) takes into account both the number of species and the number of individuals of each, where N is the total number of organisms and n is the number of each species. A higher D means a more diverse habitat. It allows different habitats to be compared.
Species richness does not tell us how evenly individuals are spread among species. The index of diversity (D) uses both the number of species and their relative abundance.
- N = total number of organisms of all species
- n = total number of organisms of each species
- Σ = sum of
A habitat with many species with similar numbers of individuals has a high D. A habitat dominated by one species has a low D, as does one with few species. D has a minimum of 1 for a single species.
Calculating D for a habitat
A quadrat survey of woodland A recorded five species: 12, 8, 5, 3 and 2 individuals. Calculate the index of diversity.
- N = 12 + 8 + 5 + 3 + 2 = 30
- N(N − 1) = 30 × 29 = 870
- Σn(n − 1) = (12 × 11) + (8 × 7) + (5 × 4) + (3 × 2) + (2 × 1) = 132 + 56 + 20 + 6 + 2 = 216
- D = 870 ÷ 216
Answer: 4.0 (to 2 significant figures)
If a second woodland, B, has 30 individuals in three species (25, 3 and 2), Σn(n − 1) = 600 + 6 + 2 = 608 and D = 870 ÷ 608 = 1.4. Woodland A is more diverse than woodland B, even though B also has several species, because B is dominated by one species.
Interpreting and evaluating D
- A high D means the habitat is more diverse, which often indicates a stable habitat with many niches.
- A low D suggests a habitat dominated by a few species, which may indicate pollution or disturbance.
- Results depend on sampling: use random sampling, with enough samples, from habitats of comparable area, at the same time of year.
- D uses numbers only, so it does not show how different the species are from one another, and it gives only a snapshot at one time.
Show N, N(N − 1), the sum Σn(n − 1) and the division. A final answer with no working often loses marks.
n(n − 1) for a species with 1 individual is 0, so single individuals do not add to the bottom line but are still counted in N.
Quick check
Name the three domains of life.
Show answer
Archaea, Bacteria and Eukarya.
What type of evidence led to the three-domain system?
Show answer
Molecular evidence, such as differences in rRNA and other nucleotide or protein sequences.
Define endemism.
Show answer
A species being found in only one geographical region and nowhere else.
What is species richness?
Show answer
The number of different species in a habitat.
In D = N(N − 1) ÷ Σn(n − 1), what does n stand for?
Show answer
The total number of organisms of each species.
Written and checked against the Edexcel International AS Level Biology (XBI11) specification · Updated October 2026