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How is biodiversity measured and how are living things classified?

The meaning and measurement of biodiversity at species, genetic and habitat levels, the use of the index of diversity, and the classification and naming of organisms.

An Edexcel A-Level Biology B (Salters-Nuffield) answer on biodiversity and classification, covering species, genetic and habitat diversity, the index of diversity, and the classification, naming and grouping of organisms.

Generated by Claude Opus 4.89 min answerUpdated 2026-06-02

Reviewed by: AI editorial process; not yet individually human-reviewed

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What this dot point is asking
Measuring biodiversity
Classification and naming
Examples in context
Try this

What this dot point is asking

Edexcel wants you to define biodiversity at species, genetic and habitat levels, use and interpret the index of diversity, and describe how organisms are classified and named. Calculating and comparing the diversity index from sample data is a frequent exam task.

Measuring biodiversity

Species diversity is more than just counting species (richness); it also considers how evenly individuals are distributed among them.

Calculating the index of diversity

A field sample contains daisy $8$ , buttercup $5$ and clover $2$ individuals.

step 1 Find N and N(N-1)

Total $N = 8 + 5 + 2 = 15$ , so $N(N-1) = 15 \times 14 = 210$ .

step 2 Find n(n-1) for each species

Daisy $= 8 \times 7 = 56$ ; buttercup $= 5 \times 4 = 20$ ; clover $= 2 \times 1 = 2$ .

step 3 Sum and divide

$\sum n(n-1) = 56 + 20 + 2 = 78$ , so $D = \frac{210}{78} = 2.69$ to 3 significant figures.

A second field with the same three species but $5$ of each would give $D = \frac{15 \times 14}{3 \times (5 \times 4)} = \frac{210}{60} = 3.5$ , higher because the individuals are spread more evenly. This shows the index responds to evenness, not just richness.

Sampling for these data uses random quadrats for plants and slow-moving animals, and mark-release-recapture for mobile animals, to estimate population sizes fairly.

Classification and naming

Organisms are placed in a hierarchy of groups called taxa: domain, kingdom, phylum, class, order, family, genus and species. Each group contains the one below it, with no overlap.

The binomial system names a species with two Latin words: the genus (capitalised) and the species (lower case), both italicised, for example Homo sapiens.

Classification was originally based on observable features, but is now increasingly based on molecular evidence such as DNA base sequences and amino acid sequences of proteins, which gives a more accurate picture of evolutionary relationships (phylogeny). The three-domain system (Bacteria, Archaea, Eukarya), proposed by Carl Woese from ribosomal RNA sequences, is a direct result of molecular evidence reshaping older classifications.

Examples in context

Example 1. Comparing two habitats. An undisturbed woodland might give a diversity index of $D = 6.2$ while a nearby intensively farmed field gives $D = 1.5$ . The lower value reflects both fewer species and dominance by one crop. Conservationists use such comparisons to argue for protecting the woodland, linking this dot point directly to conservation.

Example 2. DNA reclassification of whales. Whales were once grouped on appearance with fish-like marine animals, but DNA and protein sequence comparisons place them firmly among the even-toed hoofed mammals, closest to hippopotamuses. This shows molecular evidence overturning a classification based on looks (convergent body shape) and revealing the true evolutionary relationship.

Try this

Q1. Explain why species evenness is included in a diversity index. [2 marks]

Cue. A community dominated by one species is less diverse than one with the same number of species spread evenly; the index reflects this.

Q2. State two levels at which biodiversity can be measured. [2 marks]

Cue. Any two of habitat diversity, species diversity and genetic diversity.

Exam-style practice questions

Practice questions written in the style of Pearson Edexcel exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.

Edexcel 20184 marksA meadow was sampled and three plant species were found with the following numbers of individuals: species A

11

, species B

7

, species C

2

. Calculate the index of diversity (D) for the meadow, showing your working.

Show worked answer →

A full worked calculation using $D = \frac{N(N-1)}{\sum n(n-1)}$ .

Total $N = 11 + 7 + 2 = 20$ , so $N(N-1) = 20 \times 19 = 380$ . For each species $n(n-1)$ : A $= 11 \times 10 = 110$ ; B $= 7 \times 6 = 42$ ; C $= 2 \times 1 = 2$ . Sum $= 110 + 42 + 2 = 154$ . Therefore $D = \frac{380}{154} = 2.47$ (to 3 significant figures).

Markers reward: correct $N(N-1) = 380$ , correct sum of $n(n-1) = 154$ , and the final $D = 2.47$ . A higher D would mean greater diversity.

Edexcel 20214 marksExplain why classification of organisms is now based on molecular evidence such as DNA and protein sequences rather than only on observable physical features.

Show worked answer →

Markers want the advantage of molecular data for showing relationships.

Observable features can mislead because unrelated species may look alike through convergent evolution (analogous structures), and similar-looking organisms may not be closely related. DNA and amino acid sequences reflect the actual genetic relatedness of organisms, so the more similar the sequences, the more recently the species shared a common ancestor. This gives a more objective, accurate picture of evolutionary relationships (phylogeny) and can reveal relationships not obvious from appearance.

Award marks for: physical features can mislead (convergent evolution); sequences show genetic relatedness; more similar sequence means more recent common ancestor; gives accurate evolutionary relationships.

Related dot points

Sources & how we know this

Pearson Edexcel A-Level Biology B (9BN0) specification — Pearson Edexcel (2015)