Understanding Biodiversity and its Calculation Revision Notes | A Level AQA Biology

1. Why Counting Species Is Not Enough

Imagine two meadows. Meadow A has five species of wildflower, each with roughly equal numbers of plants. Meadow B also has five species, but one species makes up 95% of all the plants. Which meadow is more biodiverse? Your instinct probably says Meadow A — and you would be right. The challenge is expressing that difference with a number.

Biodiversity is not a single, fixed thing. It can be assessed at any scale — from the organisms living in a single pond, to the species found across an entire country, to the full range of life on Earth. AQA expects you to appreciate this range of scales and to apply precise mathematical tools when comparing communities.

With that foundation in place, we can now look at the two main ways ecologists measure biodiversity — and why one of them is far more powerful than the other.

2. Species Richness — A Starting Point

The most straightforward measure of biodiversity is species richness: the total number of different species present in a community. If you survey a woodland and find 12 species of bird, the species richness for birds in that woodland is 12. It is quick to record and straightforward to compare between sites.

Here is the problem: species richness treats a community with one rare individual of a species the same as a community where that species is thriving with thousands of individuals. Two communities can share identical species richness yet differ enormously in their ecological stability and resilience. That is where the index of diversity becomes essential.

3. The Index of Diversity — A Richer Picture

The index of diversity (given the symbol $d$) solves the problem above by combining two pieces of information: how many species are present, and how individuals are distributed among those species. A community where individuals are spread evenly across many species will produce a high value of $d$; a community dominated by a single species will produce a low value.

Notice that the numerator uses the total count $N$, while the denominator sums a term calculated separately for each species. The subtraction of 1 in both $N(N-1)$ and $n(n-1)$ reflects the mathematical logic of counting pairs of individuals — you are effectively asking how many pairs of individuals could be drawn from the same species versus from the whole community.

A higher value of $d$ means greater biodiversity. There is no fixed upper limit, but comparing $d$ values between two communities sampled in the same way gives a meaningful, quantitative comparison — something species richness alone cannot provide.

4. Calculating the Index of Diversity — Step by Step

The formula looks more complex than it is. The key is to work through it systematically: find $N$, then calculate $n(n-1)$ for each species, sum those values to get $\Sigma n(n-1)$, and finally divide. Let's walk through a full worked example so you can see exactly how each step connects.

Worked example: Calculating the Index of Diversity for a Grassland Community

A student surveys a grassland plot and records the following data: - Grass species A: 40 individuals - Grass species B: 25 individuals - Grass species C: 10 individuals - Grass species D: 5 individuals Calculate the index of diversity ($d$) for this community. Show your working. [4 marks]

1. $N = 40 + 25 + 10 + 5 = 80$ — Add up all individuals across every species. This is your total N. [M1]
2. $N(N-1) = 80 \times (80 - 1) = 80 \times 79 = 6320$ — Subtract 1 from N, then multiply. 80 times 79: 80 times 80 = 6400, minus 80 = 6320. This is the numerator.
3. Species A: $40 \times 39 = 1560$ Species B: $25 \times 24 = 600$ Species C: $10 \times 9 = 90$ Species D: $5 \times 4 = 20$ — For each species, subtract 1 from its count and multiply. Work through every species in turn — do not skip any. [M1]
4. $\Sigma n(n-1) = 1560 + 600 + 90 + 20 = 2270$ — Add all the n(n-1) values together. This is the denominator.
5. $d = \frac{6320}{2270} \approx 2.78$ — Divide numerator by denominator. Give your answer to 2 or 3 significant figures unless told otherwise. [A1]
6. $d \approx 2.78$ — This relatively low value reflects that species A dominates the community with 40 out of 80 individuals. A more even distribution would give a higher d value.

Answer: $d \approx 2.78$ (no units)

Notice how species A, with half of all individuals, pulls the index of diversity down. If those 40 individuals were redistributed more evenly across all four species, $d$ would rise — reflecting a genuinely more balanced community.

5. Interpreting and Comparing Index of Diversity Values

Calculating $d$ is only half the skill — the other half is interpreting what the number tells you. AQA questions frequently ask you to compare two communities or to suggest reasons why one habitat has a higher index of diversity than another.

The table above shows something important: two communities can have identical species richness yet very different indices of diversity. The farmed field and the meadow both contain four species, but the meadow's more even distribution of individuals produces a much higher $d$ value. This is precisely why the index of diversity is a more informative measure than species richness alone.

When comparing communities, always comment on both the number of species and the distribution of individuals. A high $d$ value indicates that no single species dominates — the community is more resilient because it does not depend on the success of one species.

6. Biodiversity Across Scales

The AQA specification makes a point of stating that biodiversity can relate to a range of habitats, from a small local habitat to the entire Earth. This is worth pausing on, because the scale at which you measure biodiversity affects both the methods you use and the conclusions you can draw.

• Local scale: a single pond, hedgerow, or field — species richness and index of diversity can be measured directly by sampling
• Regional scale: a national park or biome — data from multiple surveys are combined; species lists become longer and sampling becomes more complex
• Global scale: the entire Earth — biodiversity is estimated from compiled databases; includes all known species across all ecosystems

At every scale, the same principles apply: greater variety of species and more even distribution of individuals both contribute to higher biodiversity. The index of diversity formula works at any scale where you have count data, though in practice it is most commonly applied at the local or community level in exam questions.

7. Exam Technique and Common Mistakes

Calculation questions on the index of diversity are reliable mark-earners — but only if you follow the method precisely. The most common errors are avoidable with a clear routine.

For extended-response questions, the mark scheme typically rewards a structured answer that defines the measure, describes the calculation method, and then interprets the result in the context of the habitat described. Practise writing answers that move through all three stages rather than stopping after the definition.

With the formula, the interpretation, and the common pitfalls all covered, you now have everything needed to tackle AQA questions on biodiversity and its calculation — whether they ask you to define, calculate, compare, or evaluate.