What is biodiversity and why is it important in the study of Biology?

Biodiversity refers to the variety of life forms within a given ecosystem, biome, or the entire Earth, encompassing the diversity of species, genetic variation, and ecosystems. It is crucial in Biology because it helps maintain ecosystem balance, supports life processes such as pollination and nutrient cycling, and provides resources for food, medicine, and economic development. Understanding biodiversity is essential for conservation efforts and sustainable development.

How is biodiversity measured and classified in the Cambridge International A Levels curriculum?

In the Cambridge International A Levels Biology curriculum, biodiversity is measured using various indices such as species richness, which counts the number of different species in a given area, and the Simpson's Diversity Index, which considers both species richness and evenness. Classification involves organizing species into hierarchical categories based on shared characteristics, using taxonomic ranks such as kingdom, phylum, class, order, family, genus, and species.

What are the main threats to biodiversity and how do they impact ecosystems?

The main threats to biodiversity include habitat destruction, climate change, pollution, overexploitation, and invasive species. These threats can lead to loss of species and genetic diversity, disrupting ecosystem services such as pollination, water purification, and climate regulation. This can result in reduced resilience of ecosystems to environmental changes and can negatively impact human livelihoods and health.

How does conservation biology aim to protect biodiversity?

Conservation biology focuses on understanding and preserving biodiversity through strategies such as establishing protected areas, restoring habitats, and implementing laws and policies to prevent overexploitation and pollution. It also involves ex-situ conservation methods like seed banks and captive breeding programs. The goal is to maintain healthy ecosystems and ensure the survival of species for future generations.

What role do humans play in both the decline and conservation of biodiversity?

Humans play a dual role in biodiversity. On one hand, activities such as deforestation, pollution, and urbanization contribute to biodiversity loss. On the other hand, humans can also aid in conservation through sustainable practices, supporting conservation policies, and participating in restoration projects. Education and awareness are key to fostering a culture of conservation and ensuring that biodiversity is preserved for future generations.

Why is "Reporting Simpson's index as the sum $\sum (n/N)^2$ instead of $1 - \sum (n/N)^2$" a common mistake in Biodiversity ?

Why it happens: Candidates calculate the sum correctly but forget the final '1 −' step. How to avoid it: Always write the full formula at the start: D = 1 - (n/N)^2. After calculating the sum, the **last step is always** to subtract from 1. If your D > 1 or D < 0, you have made a mistake. Source: 9700 Examiner Reports 2024

Why is "Writing Lincoln formula upside down ($N = m_2 / (n_1 \times n_2)$) or forgetting which sample is which" a common mistake in Biodiversity ?

Why it happens: The formula has three variables and three places. How to avoid it: Remember: **N (whole population)** is on the **top**. The two captures multiply on the top; marked recaptures divide on the bottom. Sanity check: N should be **larger** than either sample, so multiplying must dominate. Source: 9700 Examiner Reports 2024

Why is "Giving vague mark-release-recapture assumptions like 'population is constant'" a common mistake in Biodiversity ?

Why it happens: Candidates know an assumption exists but can't name it precisely. How to avoid it: Three precise assumptions: (1) **marks don't affect survival or behaviour**; (2) **closed population** — no births, deaths, immigration or emigration between samples; (3) **sufficient time for full mixing** of marked individuals before recapture. Source: 9700 Examiner Reports 2023-2024

Why is "Defining species diversity as just the number of species (only richness)" a common mistake in Biodiversity ?

Why it happens: Common sense suggests 'more species = more diverse', but it's incomplete. How to avoid it: Species diversity = **richness AND evenness**. A community with 10 species, 9 of which are rare, is **less diverse** than one with 10 species each accounting for ~10% of individuals. Simpson's index responds to both. Source: 9700 Examiner Reports 2024

Why is "Placing quadrats 'randomly' by throwing them over your shoulder" a common mistake in Biodiversity ?

Why it happens: Looks random; quick. How to avoid it: True random sampling requires **random number coordinates** — never throw or guess. Examiners expect: lay out a grid, generate random coordinates from a random number table / calculator function, place quadrat at coordinate. Throwing is a textbook example of **biased sampling**. Source: 9700 Examiner Reports 2024

Why is "Confusing 'systematic' with 'biased' / 'researcher's choice'" a common mistake in Biodiversity ?

Why it happens: The word 'systematic' sounds like 'system' / 'pre-chosen'. How to avoid it: Systematic sampling means **evenly spaced** along a transect — not researcher's choice. It is **unbiased** for studying gradients. Researcher's-choice sampling is **subjective sampling** and is a different (poor) method. Source: 9700 Examiner Reports 2023

Classification, Biodiversity and ConservationCambridge International A Levels Biology (9700)

Biodiversity

Q: Why is "Writing Lincoln formula upside down ($N = m_2 / (n_1 \times n_2)$) or forgetting which sample is which" a common mistake in Biodiversity ?

Why it happens: The formula has three variables and three places. How to avoid it: Remember: **N (whole population)** is on the **top**. The two captures multiply on the top; marked recaptures divide on the bottom. Sanity check: N should be **larger** than either sample, so multiplying must dominate. Source: 9700 Examiner Reports 2024

Q: Why is "Giving vague mark-release-recapture assumptions like 'population is constant'" a common mistake in Biodiversity ?

Why it happens: Candidates know an assumption exists but can't name it precisely. How to avoid it: Three precise assumptions: (1) **marks don't affect survival or behaviour**; (2) **closed population** — no births, deaths, immigration or emigration between samples; (3) **sufficient time for full mixing** of marked individuals before recapture. Source: 9700 Examiner Reports 2023-2024

Q: Why is "Defining species diversity as just the number of species (only richness)" a common mistake in Biodiversity ?

Why it happens: Common sense suggests 'more species = more diverse', but it's incomplete. How to avoid it: Species diversity = **richness AND evenness**. A community with 10 species, 9 of which are rare, is **less diverse** than one with 10 species each accounting for ~10% of individuals. Simpson's index responds to both. Source: 9700 Examiner Reports 2024

Q: Why is "Placing quadrats 'randomly' by throwing them over your shoulder" a common mistake in Biodiversity ?

Why it happens: Looks random; quick. How to avoid it: True random sampling requires **random number coordinates** — never throw or guess. Examiners expect: lay out a grid, generate random coordinates from a random number table / calculator function, place quadrat at coordinate. Throwing is a textbook example of **biased sampling**. Source: 9700 Examiner Reports 2024

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Biodiversity — Cambridge International A Level Biology 9700 Study Notes (2025-2027 syllabus)

The three levels of biodiversity (genetic, species, ecosystem); sampling methods using quadrats — random and systematic with line/belt transects; mark-release-recapture for mobile animals using the Lincoln index $N = (n_1 n_2)/m_2$ ; Simpson's index of diversity $D = 1 - \sum (n/N)^2$ ; statistical tests appropriate to ecological data (Spearman's rank correlation, t-test, chi-squared) — spec 18.2.

At a glance

Biodiversity has three levels: genetic (within species), species (within community), ecosystem (across region).
Quadrats placed randomly (random number coordinates) for uniform habitats.
Transects (line or belt) for environmental gradients (shore zonation, soil moisture, edge effects).
Mark-release-recapture for mobile animals: $N = \dfrac{n_1 n_2}{m_2}$ (Lincoln index).
Three MRR assumptions: marks harmless, closed population, sufficient mixing.
Simpson's diversity index: $D = 1 - \sum (n/N)^2$ ; 0 = no diversity, ~1 = max diversity. Captures richness AND evenness.
Statistical tests: Spearman's rank (correlations), t-test (compare means), chi-squared (categorical data).
Biodiversity matters for ecosystem services, genetic resources, resilience and ethical/aesthetic reasons.

What you’ll learn

Mapped to the Cambridge IGCSE 9700 syllabus (2025-2027).

18.2.1 — Define biodiversity in terms of three levels: genetic, species and ecosystem diversity.
18.2.2 — Describe how to use frame quadrats and random sampling to assess plant or sessile animal populations.
18.2.3 — Describe how to use line and belt transects to assess change in species distribution across an environmental gradient.
18.2.4 — Describe the mark-release-recapture method (Lincoln index) for estimating the population of mobile animals and state its assumptions.
18.2.5 — Calculate and interpret Simpson's index of diversity $D = 1 - \sum (n/N)^2$ for a community.
18.2.6 — Identify appropriate statistical tests for ecological data: Spearman's rank correlation, t-test, chi-squared test.
18.2.7 — Discuss the importance of biodiversity to ecosystems, agriculture, medicine and human society.

The three levels of biodiversity

Genetic (within species) < species (within community) < ecosystem (across region).

Biodiversity is the variety of life. It is measured at three nested levels:

(1) Genetic diversity — variety of alleles within a species.

Different alleles at each locus → different phenotypes → adaptive potential.
Measured by:
- Number of alleles per locus.
- Heterozygosity (proportion of loci at which individuals are heterozygous).
- DNA sequence variation (single nucleotide variants).
High genetic diversity is essential for adaptation to environmental change and disease resistance.
Loss of genetic diversity (e.g. in inbred captive populations or after population bottlenecks) reduces adaptive flexibility. The cheetah is the textbook example — its Pleistocene bottleneck left so little genetic diversity that tissue grafts between unrelated individuals are accepted with little immune rejection.

(2) Species diversity — variety of species in a community.

Has two components:

Species richness: the number of different species present.
Species evenness: the relative abundance of each species.

Both matter:

A community of 10 species, 9 of which are rare and 1 dominant, is less diverse than a community of 10 species with even abundances.
Captured quantitatively by Simpson's index of diversity (see formula section).

Examples.

Tropical rainforest in Borneo: 200+ tree species per hectare, no single dominant species → very high species diversity.
Oil palm plantation: 1 tree species, monoculture → species diversity ≈ 0.

(3) Ecosystem diversity — variety of habitats / ecosystems in a region.

A landscape with rivers, wetlands, forests, grasslands and coastal areas has more ecosystem diversity than a uniform plain. Different ecosystems support different species communities and different gene pools, so ecosystem diversity supports the other two levels.

Examples.

A national park with mountains, lakes, forests and meadows has high ecosystem diversity.
An agricultural region of uniform wheat fields has low ecosystem diversity.

Why distinguish all three levels? Conservation needs to act at every level. Saving a single endangered species in a zoo (species level) is ineffective if its captive population lacks genetic diversity (genetic level) and its wild habitat is destroyed (ecosystem level).

Three levels: genetic (within species), species (within community), ecosystem (across region).
Genetic diversity → adaptive potential; loss → vulnerability (cheetah).
Species diversity = richness + evenness (Simpson's D combines both).
Ecosystem diversity = variety of habitats; supports the other two levels.
Conservation must protect all three levels simultaneously.

See the full worked example for biodiversity →

Quadrat sampling — random and systematic

Random quadrats for uniform habitat; systematic (transects) for environmental gradients.

A quadrat is a square sampling frame placed on the ground to sample plants or sedentary animals (e.g. limpets on a rocky shore). Quadrats come in different sizes:

0.25 m × 0.25 m — for dense, small organisms (e.g. small herbs, lichens).
0.5 m × 0.5 m — common general purpose.
1 m × 1 m — for larger plants or lower densities.

What you record in each quadrat depends on the question:

Species count — list of species present.
Density — number of individuals (for countable plants/animals).
Percentage cover — for plants that can't be counted as individuals (e.g. grasses, mosses).
Frequency — proportion of quadrats in which the species occurs.
Abundance scale — semi-quantitative (e.g. ACFOR: Abundant, Common, Frequent, Occasional, Rare).

Two contrasting deployment strategies:

Random sampling.

How it works:

Lay out a coordinate grid over the study area (often using two long tape measures along the edges of the site).
Generate random coordinates using a random number table, a calculator's RAND function, or a smartphone app.
Place a quadrat at each random coordinate.
Record data; repeat for a chosen sample size (often 10-30 quadrats).
Calculate mean density / cover and standard error.

When to use: Uniform habitats — a flat grassland field, the interior of a tropical forest, the middle of a sandy beach.

Strengths:

Eliminates conscious and unconscious bias.
Samples are statistically independent → can use parametric statistics (means, t-tests).

Systematic sampling — line and belt transects.

How it works:

Lay out a tape measure as a transect line across an environmental gradient.
Line transect: record every species touching the line at fixed intervals (e.g. every 1 m).
Belt transect: place a quadrat at fixed intervals (e.g. every 5 m) and record all species within it. The "belt" is the width of one quadrat. Continuous belt transects use abutting quadrats.

When to use: Environmental gradients —

Sea shore (high tide → low tide).
Pond edge to centre.
Forest interior to forest edge.
Up a hill (altitude gradient).
Across a soil moisture or pH gradient.

Strengths:

Reveals zonation — how species replace each other along the gradient (e.g. seaweed zonation on a rocky shore: Pelvetia high, Fucus mid, Laminaria low).
Reveals succession in space — pioneer to climax community along time-replaceable gradients.

Weakness: adjacent quadrats are correlated, so standard parametric tests are not strictly appropriate. Analysis typically focuses on visualising the gradient or correlating species cover with a measured environmental variable (using Spearman's rank).

Choosing your method. If the habitat looks uniform → random. If there's an obvious environmental gradient → systematic. Mixed habitats can use stratified random sampling (split into zones and sample randomly within each).

Quadrats: 0.25 m, 0.5 m or 1 m square frames.
Record: species count, density, % cover, frequency or abundance scale.
Random sampling: random number coordinates; for uniform habitats; eliminates bias.
Line transect: species touching the line at fixed intervals.
Belt transect: quadrats at fixed intervals along a line.
Systematic for gradients (shore zonation, soil moisture, edge effects).
Adjacent quadrats correlated → analysis focuses on gradient change.

See the full worked example for biodiversity →

Mark-release-recapture (Lincoln index)

$N = (n_1 \times n_2) / m_2$ . For mobile animals; needs harmless marks, closed population, sufficient mixing.

For mobile animals (woodlice, fish, butterflies, mice, snails), quadrat counts give the wrong answer — animals move in and out of the quadrat. Instead, use the mark-release-recapture method, analysed with the Lincoln index.

The procedure:

Step 1 — First capture and marking.

Capture a sample using a method appropriate to the species — pitfall traps for ground invertebrates, sweep nets for flying insects, Longworth traps for small mammals, nets for fish, light traps for moths.
Count the captured individuals: $n_1$ .
Mark each one harmlessly — a small dot of waterproof ink on the carapace for woodlice, a fin clip for fish, a numbered metal ring for birds, a microchip for mammals.
The mark must:
- Last until the recapture.
- Not affect survival or behaviour — not increase predation, not impede movement or feeding.
Release all marked individuals at the point of capture.

Step 2 — Wait for full mixing.

Several days to a week (depending on the species' mobility).
Allows marked individuals to redistribute randomly through the population.

Step 3 — Second capture.

Repeat the capture with the same method.
Let $n_2$ = total number captured; $m_2$ = number of those that are marked.

Step 4 — Apply the Lincoln index.

$N = \frac{n_1 \times n_2}{m_2}$

The logic. If marked animals mix randomly, the proportion of marked animals in the second sample equals the proportion of marked animals in the whole population:

$\frac{m_2}{n_2} = \frac{n_1}{N}$

Rearranging gives the Lincoln formula.

Worked example.

60 woodlice captured, marked, released ( $n_1 = 60$ ).
One week later, 50 woodlice captured ( $n_2 = 50$ ), of which 12 are marked ( $m_2 = 12$ ).
$N = (60 \times 50) / 12 = 250$ woodlice.

Three assumptions (Cambridge mark schemes always credit these explicitly):

(1) Marks do not affect the animals. Mark must be visible to the researcher but invisible to predators; must not impede movement or change behaviour; must persist until recapture.

(2) Closed population. No births, deaths, immigration or emigration between the two samples. If individuals die, marked proportion declines; if new individuals arrive, marked proportion is diluted. The interval should be short relative to the species' lifespan.

(3) Sufficient time for marked individuals to mix randomly. If still clustered near release point, the second sample (taken elsewhere) catches too few marked individuals → overestimate of N.

Additional assumptions sometimes credited:

Catching method equally efficient for marked and unmarked.
No "trap-shy" or "trap-happy" individuals.
Equal capture probability across all age/sex/size classes.

Method for mobile animals (woodlice, fish, mice, etc.).
Lincoln index: $N = (n_1 \times n_2) / m_2$ .
Procedure: capture-mark-release, wait for mixing, recapture, count marked.
Assumption 1: marks don't affect survival or behaviour.
Assumption 2: closed population (no births, deaths, migration).
Assumption 3: sufficient time for full mixing before recapture.

See the full worked example for biodiversity →

Simpson's index of diversity (D)

$D = 1 - \sum (n/N)^2$ . 0 = no diversity; ~1 = maximum. Captures richness and evenness.

Simpson's index of diversity quantifies species diversity in a community. It takes account of both the number of species (species richness) and the relative abundance of each species (species evenness) — unlike a simple count, which only captures richness.

The formula:

$D = 1 - \sum \left(\frac{n}{N}\right)^2$

where:

$n$ = number of individuals of one species,
$N$ = total number of individuals of all species,
the sum is taken over all species.

Range and interpretation.

D value	Interpretation
Near 0	Low diversity — one species dominates, or only one species present
~0.5	Moderate diversity
Near 1	High diversity — many species, all relatively equal

Worked example — pond invertebrates.

Sample: 24 water beetles, 16 dragonfly nymphs, 8 mayfly nymphs, 2 freshwater shrimp.

Species	n	n/N	$(n/N)^2$
Water beetle	24	0.48	0.2304
Dragonfly nymph	16	0.32	0.1024
Mayfly nymph	8	0.16	0.0256
Freshwater shrimp	2	0.04	0.0016
Total	N = 50	1.00	$\sum = 0.36$

$D = 1 - 0.36 = 0.64$

This indicates moderately high diversity — four species present, with one dominant.

Why Simpson's captures evenness.

Consider two communities, each with 4 species and N = 100:

Community A (even): 25, 25, 25, 25 → $\sum (n/N)^2 = 4 \times 0.25^2 = 0.25$ , so $D = 0.75$ .
Community B (dominant): 90, 5, 3, 2 → $\sum (n/N)^2 = 0.9^2 + 0.05^2 + 0.03^2 + 0.02^2 = 0.81 + 0.0025 + 0.0009 + 0.0004 = 0.8138$ , so $D = 0.19$ .

Same number of species (4), but D differs dramatically because Community B is dominated by one species. Simpson's index gives different (and biologically meaningful) results for communities that a simple species count would treat as identical.

Using D in practice.

Compare D between sites to identify the more biodiverse one.
Compare D over time to track community change (e.g. before and after pollution, restoration, or invasive species).
Combined with species lists, D supports decisions about which habitats to prioritise for conservation.

A subtle note on notation. Cambridge syllabus uses $D = 1 - \sum (n/N)^2$ (Simpson's index of diversity). Some textbooks use $D = \sum (n/N)^2$ (the "dominance index") or $1/D$ (reciprocal Simpson). For Cambridge 9700, stick with the syllabus formula $D = 1 - \sum (n/N)^2$ .

$D = 1 - \sum (n/N)^2$ .
Range: 0 (no diversity) to ~1 (max diversity).
Captures both richness AND evenness.
Two communities with same species count can have very different D.
Use to compare biodiversity between sites or over time.

See the full worked example for biodiversity →

Statistical tests for ecological data

Spearman's rank for correlations; t-test for comparing means; chi-squared for categorical data.

Cambridge 9700 expects you to recognise when each of the following tests applies — full calculation is mainly examined on Paper 5 (Planning, Analysis & Evaluation), but Paper 4 may ask which test is appropriate.

(1) Spearman's rank correlation coefficient ( $r_s$ ).

What it tests: Strength and direction of a monotonic relationship between two ranked variables.
When to use: When you have two variables measured for several samples, and you want to know if they increase/decrease together. Example: soil moisture content vs species abundance across transect quadrats.
Result: $r_s$ ranges from −1 (perfect negative correlation) through 0 (no correlation) to +1 (perfect positive correlation). Compared against a critical value table to decide significance.
Non-parametric — does not require the data to be normally distributed.

(2) Student's t-test.

What it tests: Whether the means of two samples differ significantly.
When to use: Comparing two means with continuous, approximately normally-distributed data. Example: mean number of dandelion plants per quadrat in two different grasslands.
Two main forms:
- Unpaired t-test — two independent samples (e.g. two different fields).
- Paired t-test — two measurements of the same units (e.g. before vs after a treatment in the same quadrats).
Result: A t value compared to a critical value at the chosen significance level (typically p < 0.05); if t exceeds critical, the means differ significantly.
Parametric — assumes normally distributed data with similar variances.

(3) Chi-squared test ( $\chi^2$ ).

What it tests: Whether observed frequencies of categorical data match expected frequencies.
When to use: When data are counts in categories (not measurements on a scale). Examples:
- Test whether observed offspring ratios in a genetic cross match expected Mendelian ratios (e.g. 3:1).
- Test whether the number of plants in different habitats matches a hypothesis of even distribution.
- Test whether species presence is associated with a habitat factor.
Formula: $\chi^2 = \sum \dfrac{(O - E)^2}{E}$ , where $O$ = observed and $E$ = expected count.
Result: Compared to critical $\chi^2$ value at $(k-1)$ degrees of freedom. If $\chi^2$ exceeds critical, observed differs significantly from expected.

Choosing the right test — flowchart.

Are your data counts in categories (not measurements)? → Chi-squared.
Do you have two means to compare? → t-test (if normally distributed) or non-parametric alternative.
Do you have two variables measured per sample and want to know if they correlate? → Spearman's rank.

The null hypothesis (H₀). All three tests use the same logic:

State a null hypothesis (no difference / no correlation / no association).
Calculate the test statistic.
Compare with the critical value at p < 0.05.
If test statistic exceeds critical → reject H₀ (significant result).
If not → fail to reject H₀ (not significant).

Note that "fail to reject H₀" does not mean H₀ is true — it means we don't have enough evidence to reject it.

Spearman's rank: correlation between two variables (e.g. moisture vs abundance).
t-test: compare two means (continuous, normally distributed data).
Chi-squared: compare observed vs expected frequencies (categorical data).
Null hypothesis tested at p < 0.05.
Test statistic > critical value → reject H₀ → significant.

Why biodiversity matters

Ecosystem services + genetic resources + resilience + ethical/aesthetic value.

The biodiversity crisis — the sixth mass extinction, driven by human activity — is one of the most serious issues facing humanity. Cambridge 9700 expects you to articulate why we should care, in four broad categories.

(1) Ecosystem services. Biodiverse ecosystems provide free services that human society depends on:

Pollination — about one third of food crops (apples, almonds, oilseed rape, tomatoes, coffee, cocoa) depend on insect pollinators. Loss of bee diversity would cost agriculture tens of billions of dollars annually.
Water filtration — wetlands and forested watersheds filter and store fresh water. New York City pays to protect its Catskill watershed because it provides clean water at a fraction of the cost of a treatment plant.
Nutrient cycling and decomposition — soil bacteria, fungi and invertebrates break down dead matter and recycle nutrients. Without them, soils become infertile.
Climate regulation — forests are major carbon sinks; deforestation accelerates climate change. Marine plankton produce half of atmospheric oxygen.
Pest control — diverse food webs regulate pest populations naturally; loss of natural predators leads to outbreaks (e.g. the locust upsurges seen when desert grassland is overgrazed).
Disease regulation — biodiverse ecosystems may dilute pathogen transmission (the "dilution effect" hypothesis).

Replacing these services with technology is enormously expensive — often impossible.

(2) Genetic resources for agriculture and medicine.

Crop wild relatives are reservoirs of useful alleles for disease resistance, drought tolerance and yield. Most major crops (wheat, rice, maize, potato) have been improved by crossing with wild relatives — e.g. the 1970s southern corn leaf blight in the USA was overcome by crossing maize with a wild Mexican relative carrying a resistance allele.
About 25% of modern medicines originate from plants and microbes:
- Taxol (anti-cancer) from Pacific yew.
- Aspirin (analgesic) originally from willow bark.
- Penicillin from Penicillium mould.
- Quinine (anti-malarial) from cinchona bark.
- Digoxin (heart drug) from foxglove.
- Artemisinin (anti-malarial) from sweet wormwood — discovered from traditional Chinese medicine.

Many species likely contain undiscovered useful compounds. Each species lost is a potential medicine lost forever.

(3) Resilience and adaptation.

Diverse ecosystems are more stable and resilient — better able to recover from disturbance (fire, drought, disease, climate change).
Genetic diversity within a species provides the raw material for natural selection to respond to new selection pressures. Inbred populations and monocultures are vulnerable to single pathogens:
- The Irish potato famine (1845-1852) killed a million people because the dominant 'Lumper' potato cultivar was uniformly susceptible to Phytophthora infestans.
- The Gros Michel banana, the dominant export variety of the early 20th century, was wiped out by Panama disease (a Fusarium fungus). Today's dominant Cavendish banana is now threatened by a new strain of the same pathogen.

(4) Ethical, cultural and aesthetic value.

Intrinsic right. Many ethical traditions hold that other species have a right to exist independent of usefulness to humans.
Aesthetic and recreational value. Wildlife tourism is worth over US$120 billion globally annually.
Cultural identity. Indigenous and local communities depend on local biodiversity for food, fuel, medicine and cultural identity.
Inter-generational equity. Future generations have a right to inherit a biologically rich planet — this is the precautionary principle.

A useful summary phrase: biodiversity is the insurance policy for life on Earth, including human life. Lost species cannot be replaced.

Ecosystem services: pollination, water filtration, decomposition, carbon storage.
Genetic resources: crop disease resistance, medicines (25% of drugs).
Resilience: diverse systems recover from disturbance; monocultures vulnerable.
Examples: Irish potato famine, Gros Michel banana.
Ethical / cultural / aesthetic value: intrinsic right, tourism, indigenous communities.
Lost biodiversity is essentially irreplaceable.

See the full worked example for biodiversity →

How it’s examined

Cambridge 9700 examines this on Paper 4 (6-mark calculation + assumptions, almost guaranteed each session) and Paper 5 (Planning, Analysis & Evaluation — designing sampling investigations, applying statistical tests, evaluating evidence). Frequent questions: (a) calculate Simpson's diversity index (6 marks — appears most sessions); (b) describe mark-release-recapture and state assumptions (5-6 marks); (c) compare random and systematic sampling (4-5 marks); (d) explain why biodiversity is important (4-6 marks 'discuss' question); (e) distinguish the three levels of biodiversity (3-4 marks). Always show full working for Simpson's: N, all (n/N), all squared, sum, then 1 − sum.

Sources: Cambridge International AS & A Level Biology 9700 syllabus (2025-2027); 9700 Examiner Reports 2022-2024; 9700/42 May/Jun 2024 question paper and mark scheme; 9700/42 Oct/Nov 2024 question paper and mark scheme; Simpson, E.H. (1949) — Measurement of diversity; IPBES (2019) — Global Assessment of Biodiversity and Ecosystem Services. Last reviewed 2026-05-12.

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Step-by-step worked examples — Biodiversity

Step-by-step solutions to past-paper-style questions on biodiversity , written exactly the way a tutor would explain them at the board.

1Calculating Simpson's diversity index (6 marks)
Extended• Adapted from 9700/42 May/Jun 2024• Simpson's, diversity index, calculation, Paper 4
Question
A pond was sampled and the following invertebrates collected: 24 water beetles, 16 dragonfly nymphs, 8 mayfly nymphs, 2 freshwater shrimp. Calculate Simpson's diversity index (D) for the community, showing all working. (6 marks)
Step-by-step solution
1. Step 1
  Find the total number of individuals (N). $N = 24 + 16 + 8 + 2 = 50$ .
2. Step 2
  Calculate $(n/N)$ for each species. Water beetles: $24/50 = 0.48$ . Dragonfly nymphs: $16/50 = 0.32$ . Mayfly nymphs: $8/50 = 0.16$ . Freshwater shrimp: $2/50 = 0.04$ .
3. Step 3
  Square each value: $(n/N)^2$ . Water beetles: $0.48^2 = 0.2304$ . Dragonfly: $0.32^2 = 0.1024$ . Mayfly: $0.16^2 = 0.0256$ . Shrimp: $0.04^2 = 0.0016$ .
4. Step 4
  Sum the squared values: $\sum (n/N)^2$ . $0.2304 + 0.1024 + 0.0256 + 0.0016 = 0.3600$ .
5. Step 5
  Apply Simpson's formula. $D = 1 - \sum (n/N)^2 = 1 - 0.36 = 0.64$ .
6. Step 6
  Interpret. D = 0.64 indicates moderately high biodiversity. A value near 1 = very diverse; near 0 = dominated by one species. With four species but dominance by water beetles (48%), the community is reasonably diverse but not maximally even.
Answer
N = 50. (n/N): 0.48, 0.32, 0.16, 0.04. (n/N)²: 0.2304, 0.1024, 0.0256, 0.0016. Sum = 0.36. D = 1 − 0.36 = 0.64. Moderately high biodiversity.
Examiner tip
Mark scheme: (1) N correct; (2) all (n/N) ratios calculated; (3) all squared; (4) sum correct; (5) D = 1 − sum; (6) final value with interpretation. Examiner Reports flag candidates who forget to subtract from 1, or who miscalculate one (n/N)² and propagate the error.
2Mark-release-recapture (Lincoln index) — calculation and assumptions (6 marks)
Extended• Adapted from 9700/42 Oct/Nov 2024• Lincoln index, mark-release-recapture, population, Paper 4
Question
A student studying woodlice in a leaf litter habitat captured and marked 60 woodlice with a small dot of waterproof ink, then released them. One week later, she recaptured 50 woodlice, of which 12 were marked. (a) Use the Lincoln index to estimate the total population. (b) State three assumptions of this method. (6 marks)
Step-by-step solution
1. Step 1
  Lincoln index formula. $N = \dfrac{n_1 \times n_2}{m_2}$ where $n_1$ = number marked in first capture, $n_2$ = total number in second capture, $m_2$ = marked individuals recaptured.
2. Step 2
  Identify the values. $n_1 = 60$ (marked + released), $n_2 = 50$ (total second capture), $m_2 = 12$ (marked recaptured).
3. Step 3
  Substitute and calculate. $N = \dfrac{60 \times 50}{12} = \dfrac{3000}{12} = 250$ woodlice.
4. Step 4
  Assumption 1 — Marks don't affect survival or behaviour. The ink must not make woodlice more visible to predators or alter their behaviour. Marks should be permanent between captures but harmless.
5. Step 5
  Assumption 2 — No births, deaths or migration between samples. A 'closed' population. If individuals die or new ones immigrate, the marked-to-unmarked ratio is distorted.
6. Step 6
  Assumption 3 — Sufficient time for full mixing. Marked individuals must redistribute randomly through the population before the second capture, so the recapture sample is representative. Too short an interval = marked individuals still concentrated near release point.
Answer
(a) N = (60 × 50) / 12 = 250 woodlice. (b) Three assumptions: (1) marks do not affect survival/behaviour; (2) closed population — no births, deaths, immigration or emigration; (3) marked individuals have time to mix randomly with the unmarked population before recapture.
Examiner tip
Mark scheme: (a) formula stated (1) + correct substitution (1) + answer 250 (1). (b) 1 mark per assumption, max 3 — must be clearly stated. Examiner Reports note candidates often forget the 'sufficient mixing' assumption, or write vague answers like 'population is constant' without specifying no births/deaths/migration.
3Genetic, species and ecosystem diversity (4 marks)
Core• biodiversity, genetic diversity, species diversity, Paper 2/4
Question
Distinguish between genetic diversity, species diversity and ecosystem diversity. (4 marks)
Step-by-step solution
1. Step 1
  Genetic diversity = the variety of alleles within a species. Measured as the number of alleles per locus, heterozygosity, or DNA sequence diversity. Important because high genetic diversity allows a species to adapt to environmental change and resist disease.
2. Step 2
  Species diversity = the variety of different species in a community, including both the species richness (number of species) and species evenness (relative abundance of each). Captured by indices like Simpson's D. Two communities can have the same number of species but very different diversity if one is dominated by a single species.
3. Step 3
  Ecosystem diversity = the variety of different habitats / ecosystems in a region. A tropical rainforest landscape with rivers, hilltops, forest interior and forest edge has higher ecosystem diversity than a uniform agricultural plain.
4. Step 4
  Summary table. Genetic (within species) < Species (within community) < Ecosystem (across region). Each level contributes to overall biodiversity and each must be conserved.
Answer
Three levels of biodiversity: (1) Genetic = variety of alleles within a species; (2) Species = number + relative abundance of species in a community (richness + evenness); (3) Ecosystem = variety of habitats / ecosystems in a region. All three contribute to total biodiversity.
Examiner tip
Mark scheme: 1 mark each for clearly defining genetic, species (must mention both richness and evenness), and ecosystem diversity; 4th mark for overarching idea that all three together = biodiversity. Examiner Reports note candidates often define only species diversity.
4Random vs systematic sampling (5 marks)
Extended• random sampling, systematic sampling, transects, Paper 4
Question
Compare random and systematic sampling techniques, stating when each is appropriate. (5 marks)
Step-by-step solution
1. Step 1
  Random sampling. Quadrats placed at coordinates chosen from a random number table or random number generator. Every position has an equal chance of being sampled. Eliminates conscious or unconscious bias.
2. Step 2
  When to use random sampling. When the habitat is uniform — e.g. an apparently homogeneous grass field, the canopy of a single forest type. The aim is to estimate population density or species composition of the whole area without bias.
3. Step 3
  Systematic sampling. Quadrats placed at fixed intervals along a transect line — a line transect records what touches the line, a belt transect records what falls within a strip of fixed width. The line is laid out across a gradient.
4. Step 4
  When to use systematic sampling. When there is a clear environmental gradient — e.g. seashore (high tide to low tide), pond edge to centre, forest interior to forest edge, soil moisture gradient. Reveals how species distribution changes with the gradient.
5. Step 5
  Statistical implications. Random sampling allows standard parametric statistics (means, t-tests) because the samples are independent. Systematic sampling samples are not independent (adjacent quadrats are correlated), so analyses focus on visualising change along the gradient or correlating species cover with a measured environmental variable (e.g. soil moisture).
Answer
Random: quadrats at random coordinates (number table). Use when habitat is uniform; estimates mean density/cover without bias. Systematic: quadrats at fixed intervals along a transect across an environmental gradient. Use to study how distribution changes with the gradient (e.g. shore zonation, soil moisture gradient). Random allows independent-sample statistics; systematic visualises gradient change.
Examiner tip
Mark scheme: (1) random = random coordinates / equal probability; (2) when to use random (uniform habitat); (3) systematic = fixed intervals along transect; (4) when to use systematic (environmental gradient); (5) example or statistical implication. Examiner Reports note candidates confuse 'systematic' with 'biased' — systematic just means evenly spaced.
5Why biodiversity is important (4 marks)
Core• biodiversity importance, ecosystem services, Paper 4
Question
Explain four reasons why biodiversity is important. (4 marks)
Step-by-step solution
1. Step 1
  Ecosystem services. Biodiverse ecosystems provide essential services: pollination by insects (one third of food crops), water filtration by wetlands, decomposition and nutrient cycling by soil organisms, climate regulation by forests (carbon storage). Loss of species disrupts these services.
2. Step 2
  Genetic resources for agriculture and medicine. Wild relatives of crops are sources of disease-resistance genes and traits for crop improvement. About 25% of modern medicines originate from plants and microbes (e.g. taxol from yew, aspirin from willow, penicillin from Penicillium). Lost species = lost potential medicines.
3. Step 3
  Resilience to environmental change. Diverse ecosystems and gene pools are more resilient — better able to recover from disturbance (fire, disease, climate change). Genetic diversity within a species provides raw material for natural selection to respond to new pressures.
4. Step 4
  Ethical, cultural and aesthetic value. Many people hold that other species have intrinsic right to exist. Biodiversity has aesthetic and cultural value (tourism, recreation, art, religion). Indigenous communities depend on local biodiversity for livelihoods and cultural identity.
Answer
(1) Ecosystem services — pollination, water filtration, carbon storage, nutrient cycling. (2) Genetic resources for agriculture (disease-resistance) and medicine (25% of drugs from plants). (3) Resilience — diverse systems recover better from disturbance and climate change. (4) Ethical / aesthetic / cultural / economic (tourism) value.
Examiner tip
Mark scheme: 1 mark per clear reason, max 4. Cambridge accepts ecological, economic, ethical and 'insurance' (genetic resource) reasons. Examiner Reports note candidates who give only one or two reasons cap at 1-2 marks; spread across categories.

Key Formulae — Biodiversity

The formulae you need to memorise for biodiversity on the Cambridge International A Level 9700 paper, with every variable defined in plain English and a note on when to use it.

Simpson's index of diversity (D)
$D = 1 - \sum \left(\frac{n}{N}\right)^2$
$D$
Simpson's index of diversity. Ranges from 0 (no diversity — single species) to nearly 1 (maximum diversity — many evenly distributed species). Higher D = greater biodiversity.
$n$
Number of individuals of one species found in the sample.
$N$
Total number of individuals of all species in the sample.
$\sum (n/N)^2$
Sum of the squared proportions of each species in the community. Calculated separately for every species and then added.
When to use
When comparing species diversity between communities. Two communities with the same number of species can have very different D values: a community dominated by one abundant species has low D (low evenness), while a community where all species are equally abundant has high D (high evenness). Simpson's index therefore captures both richness and evenness. Always calculate to 2 decimal places.
Example
Pond sample: 24 water beetles, 16 dragonfly nymphs, 8 mayfly nymphs, 2 freshwater shrimp. N = 50. $\sum (n/N)^2 = (24/50)^2 + (16/50)^2 + (8/50)^2 + (2/50)^2 = 0.2304 + 0.1024 + 0.0256 + 0.0016 = 0.36$ . Therefore $D = 1 - 0.36 = 0.64$ — moderately high biodiversity.
Lincoln index (mark-release-recapture)
$N = \frac{n_1 \times n_2}{m_2}$
$N$
Estimated total population size in the habitat.
$n_1$
Number of individuals captured, marked and released in the first sample.
$n_2$
Total number of individuals captured in the second sample (marked + unmarked).
$m_2$
Number of marked individuals in the second sample (i.e. the recaptures).
When to use
When estimating populations of mobile animals (woodlice, fish, mice, butterflies) that move freely and cannot easily be counted directly. Assumes: (1) marks don't affect survival or behaviour; (2) closed population — no births, deaths, immigration or emigration between samples; (3) sufficient time for marked individuals to mix randomly through the population before recapture.
Example
60 woodlice captured, marked, released. One week later, 50 woodlice recaptured, of which 12 were marked. $N = \dfrac{60 \times 50}{12} = 250$ woodlice in the habitat.

Model Answers — Biodiversity

High-scoring sample answers for biodiversity on the Cambridge International A Level 9700 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question
9700/42 May/Jun 2024 Q6 (adapted)6 marks
Q (6 marks). Define Simpson's index of diversity and calculate D for a freshwater community containing 24 water beetles, 16 dragonfly nymphs, 8 mayfly nymphs and 2 freshwater shrimp. Show all working and comment on the value obtained.
Model answer
Simpson's index of diversity (D) is a quantitative measure of species diversity that takes into account both the number of species (species richness) and the relative abundance of each species (species evenness). The formula is:

$D = 1 - \sum \left(\frac{n}{N}\right)^2$

where $n$ is the number of individuals of one species and $N$ is the total number of individuals of all species in the sample.

D ranges from 0 (no diversity) when one species completely dominates, up to a maximum approaching 1 (very high diversity) when many species are present in roughly equal numbers.

Step 1 — Calculate N (total individuals). $N = 24 + 16 + 8 + 2 = 50$

Step 2 — Calculate $n/N$ for each species.

Water beetles: $24/50 = 0.48$

Dragonfly nymphs: $16/50 = 0.32$

Mayfly nymphs: $8/50 = 0.16$

Freshwater shrimp: $2/50 = 0.04$

(Check: $0.48 + 0.32 + 0.16 + 0.04 = 1.00$ ✓)

Step 3 — Square each proportion.

Water beetles: $0.48^2 = 0.2304$

Dragonfly nymphs: $0.32^2 = 0.1024$

Mayfly nymphs: $0.16^2 = 0.0256$

Freshwater shrimp: $0.04^2 = 0.0016$

Step 4 — Sum the squared proportions. $\sum (n/N)^2 = 0.2304 + 0.1024 + 0.0256 + 0.0016 = 0.3600$

Step 5 — Apply Simpson's formula. $D = 1 - 0.3600 = 0.64$

Comment. A value of D = 0.64 indicates moderately high biodiversity. There are four species present (reasonable richness), but water beetles dominate (48% of individuals), which lowers evenness and prevents D approaching the maximum of 1. A more even distribution (e.g. 12, 12, 13, 13) with the same N would give a higher D — about 0.74 — illustrating how Simpson's index responds to evenness as well as richness.

Comparing D values between sites is the standard use of the index: a community with higher D is more diverse. This makes Simpson's index particularly useful for monitoring habitat change — for example, a sudden drop in D after pollution or habitat disturbance flags a loss of biodiversity.
Why this scores
Why this scores 6/6. Mark scheme: (1) definition/formula stated; (2) N = 50; (3) all n/N values calculated; (4) all squared; (5) sum and D = 0.64; (6) interpretation linking value to evenness/richness. Examiner Reports flag candidates who give D as the sum (0.36) instead of (1 − sum) = 0.64, losing 1-2 marks.
Question
9700/42 Oct/Nov 2024 Q6 (adapted)6 marks
Q (6 marks). Describe how the mark-release-recapture (Lincoln index) method is used to estimate the population of a mobile animal species. State three assumptions of the method.
Model answer
The mark-release-recapture method (also called the Lincoln index) is used to estimate the population size of mobile animals that cannot easily be counted directly — woodlice, fish, mice, butterflies, snails and many others.

The procedure:

Step 1 — First capture and marking. A sample of the species is captured, ideally using a method appropriate to the organism (pitfall traps for insects, sweep nets for butterflies, Longworth traps for small mammals, nets for fish). The number captured is $n_1$ .

Each individual is then marked harmlessly — a dot of waterproof ink for woodlice, a fin clip for fish, a numbered ring for birds, a microchip for mammals. The mark must:

Last at least until the recapture.

Not affect the animal's survival, behaviour or visibility to predators.

The marked individuals are then released back at the point of capture.

Step 2 — Wait for full mixing. Allow sufficient time (typically several days to a week, depending on species) for the marked individuals to redistribute randomly through the population. Too short an interval and marked animals are still concentrated near the release point.

Step 3 — Second capture. Capture another sample of the same species using the same method. Let $n_2$ = total number captured this time, and $m_2$ = number of marked individuals among them.

Step 4 — Apply the Lincoln index formula.

$N = \frac{n_1 \times n_2}{m_2}$

The logic: if marked animals have mixed randomly through the population, the proportion of marked animals in the second sample ( $m_2 / n_2$ ) equals the proportion of marked animals in the whole population ( $n_1 / N$ ). Rearranging gives the formula.

Worked example. If 60 woodlice are marked and released, then 50 are captured a week later and 12 are marked: $N = (60 × 50) / 12 = 250$ woodlice.

Three assumptions of the method:

(1) Marks do not affect the animals. The mark must be permanent for the duration of the study but harmless: it must not make the animals more visible to predators, hamper their movement or feeding, or alter their behaviour. Ink, paint and waterproof markers are usually fine; clipping fins or attaching radio collars can affect some species.

(2) Closed population — no births, deaths, immigration or emigration between the two samples. If individuals die, the marked proportion declines; if new individuals arrive, the marked proportion is diluted; either way, the estimate is biased. This is why the interval between captures should be short relative to the species' lifespan and migration.

(3) Sufficient time for marked individuals to mix randomly through the population before the second sample. If marked animals are still clustered near the release point, the second sample (taken elsewhere) will catch too few marked individuals and overestimate the population.

Other assumptions sometimes mentioned: the catching method is equally efficient for marked and unmarked individuals (no learning effect — "trap-shy" or "trap-happy" individuals can bias results), and individuals are equally likely to be caught regardless of age, sex or behaviour.
Why this scores
Why this scores 6/6. Mark scheme: (1) capture sample $n_1$ , mark and release; (2) wait, then recapture $n_2$ counting marked $m_2$ ; (3) formula $N = (n_1 × n_2) / m_2$ ; (4) assumption — marks don't affect survival/behaviour; (5) assumption — closed population; (6) assumption — sufficient mixing time. Cambridge typically awards 3 marks for procedure + 3 for assumptions.
Question
9700/42 May/Jun 2024 Q6(c) (adapted)4 marks
Q (4 marks). Explain why biodiversity is important.
Model answer
Biodiversity is important for four distinct reasons — ecological, economic, scientific and ethical.

(1) Ecosystem services. Biodiverse ecosystems provide essential free services on which human society depends:

Pollination by insects, especially bees — about a third of global food crops (apples, almonds, tomatoes, oilseed rape) depend on insect pollination.

Water filtration by wetlands and forests, providing clean water at minimal cost.

Nutrient cycling and decomposition by soil organisms (bacteria, fungi, earthworms) — without which dead matter would not be recycled and soils would be infertile.

Climate regulation — forests are major carbon sinks; loss of forest cover accelerates climate change.

Pest control — diverse food webs regulate pest populations naturally; loss of natural predators leads to pest outbreaks.

Loss of biodiversity disrupts these services and they can rarely be replaced by technology at reasonable cost.

(2) Economic and genetic resources for agriculture and medicine.

Wild relatives of crops are reservoirs of useful alleles for disease resistance, drought tolerance and yield improvement. Modern wheat, rice, maize and potato varieties have been improved by crossing with wild relatives.

About 25% of modern medicines originate from plants and microbes — taxol (anti-cancer, from Pacific yew), aspirin (originally from willow bark), penicillin (from Penicillium mould), quinine (anti-malarial, from cinchona bark), digoxin (heart drug, from foxglove).

Lost species = lost potential medicines and crop genes, often before they are even discovered.

(3) Resilience and adaptation. Diverse ecosystems are more stable and resilient — better able to recover from disturbance (fire, drought, disease, climate change). Genetic diversity within a species provides the raw material for natural selection to respond to new selection pressures. Low-diversity monocultures and inbred populations are vulnerable to single pathogens (e.g. the Irish potato famine, banana monoculture disease).

(4) Ethical, cultural and aesthetic value.

Many ethical traditions hold that other species have an intrinsic right to exist, independent of any usefulness to humans.

Biodiversity has aesthetic and recreational value (wildlife tourism is a major industry — over US$120 billion annually).

Indigenous and local communities depend on local biodiversity for food, fuel, medicine and cultural identity.

Future generations have a right to inherit a biologically rich planet (the "precautionary principle").

In summary, biodiversity is the insurance policy for life on Earth, including human life. Once lost, species cannot be recreated.
Why this scores
Why this scores 4/4. Mark scheme: 1 mark per reason, max 4. Cambridge accepts ecological / economic / scientific / ethical reasons — must spread across categories, not 4 versions of the same argument. Examiner Reports note candidates who write only about "saving cute animals" cap at 1-2 marks.

Question

9700/42 Oct/Nov 2024 Q6(c) (adapted)5 marks

Q (5 marks). Compare random and systematic sampling techniques. State when each is appropriate.

Model answer

Random and systematic sampling are two contrasting ways to deploy quadrats when sampling vegetation or sedentary animals. They suit different ecological contexts.

Random sampling.

In random sampling, quadrats are placed at positions chosen using a random number table, a calculator's random number function, or a coin-toss method. A grid of coordinates is laid over the study area, and the coordinates of each quadrat are picked at random. Every position has an equal probability of being sampled.

Strengths. Random sampling eliminates bias — both conscious bias (the researcher choosing "interesting" spots) and unconscious bias (placing quadrats where it's easier to walk). It yields independent samples that can be analysed with standard statistics (means, standard errors, t-tests, ANOVA).

When to use it. Random sampling is appropriate when the habitat is approximately uniform — a flat grassland field, the interior of a tropical forest, the middle of a sandy beach. The aim is to estimate the mean population density or species composition of the whole area.

Systematic sampling.

In systematic sampling, quadrats are placed at fixed intervals along a transect line — typically a string or measuring tape laid across the habitat.

A line transect records only what touches the line at each interval.
A belt transect records all individuals within a strip of fixed width (typically a quadrat) at each interval.

The transect is laid across an environmental gradient so that consecutive quadrats sample progressively different conditions.

Strengths. Systematic sampling reveals how species composition changes with the gradient — for example, how seaweed zonation changes from high tide to low tide on a rocky shore, or how plant species change with soil moisture from a pond edge to dry pasture. It is the only practical way to study zonation and succession.

When to use it. Systematic sampling is appropriate when there is a clear environmental gradient — e.g.

Seashore (high tide to low tide).
Pond edge to centre.
Forest interior to forest edge.
Up a hill (changing altitude).
Across a soil moisture or pH gradient.

Key differences:

Feature	Random	Systematic
Position	Random coordinates	Fixed intervals along transect
Habitat	Uniform	Has a gradient
Bias	Eliminated by randomisation	Eliminated by even spacing
Sample independence	Yes (parametric statistics OK)	Adjacent quadrats correlated
Reveals	Mean density/cover	Change along gradient

Choosing between them. If the habitat is uniform → random. If there is an obvious environmental change across the area → systematic. Some studies use stratified random sampling, dividing the area into different habitats and sampling randomly within each — a compromise.

Why this scores

Why this scores 5/5. Mark scheme: (1) random = random coordinates / equal probability; (2) systematic = fixed intervals / transect; (3) random eliminates bias / for uniform habitats; (4) systematic samples gradient / zonation; (5) named example (shore zonation, soil gradient) or appropriate context for each. Examiner Reports note candidates who say systematic = "ordered = biased" misunderstand the method.

Question
Practice question — recurrent Paper 4 style4 marks
Q (4 marks). Distinguish between genetic diversity, species diversity and ecosystem diversity. Give one example of each.
Model answer
Biodiversity is conventionally measured at three nested levels — within species, between species and between ecosystems. Each level captures a different facet of biological variety and each must be considered separately for conservation.

(1) Genetic diversity — the variety of alleles within a species. Measured as:

Number of alleles per locus across the population.

Heterozygosity (proportion of loci at which individuals are heterozygous).

DNA sequence diversity (single nucleotide variants).

High genetic diversity gives a species adaptive flexibility — the raw material for natural selection to respond to disease, climate change or other pressures. Low genetic diversity (e.g. cheetahs after their Pleistocene bottleneck; northern elephant seals after near-extinction) leaves a species vulnerable.

Example: Different dog breeds show very high genetic diversity within the species Canis familiaris — coat colour, body size, temperament alleles. By contrast, the cheetah (Acinonyx jubatus) has so little genetic diversity that tissue grafts between unrelated individuals are accepted without rejection.

(2) Species diversity — the variety of different species in a community or area. Captured by two components:

Species richness — simply the number of species present.

Species evenness — the relative abundance of each species.

Two communities can have the same richness but very different evenness. Simpson's diversity index combines both: $D = 1 - \sum (n/N)^2$ .

Example: A tropical rainforest in Borneo may contain 200+ tree species per hectare with no single species dominant — very high species diversity. A plantation forest of one species (e.g. oil palm monoculture) has species richness = 1 and species diversity ≈ 0.

(3) Ecosystem diversity — the variety of different ecosystems / habitats in a region. Different habitats support different communities, so high ecosystem diversity supports higher overall species and genetic diversity.

Example: A region with rivers, wetlands, forests, grasslands and coastal habitats has high ecosystem diversity. A uniform agricultural plain of cropland has low ecosystem diversity even if individual fields contain some biodiversity.

Why all three matter. Conserving biodiversity requires attention to all three levels simultaneously — a strategy that protects only one (e.g. saving a single species in a zoo, with no attention to genetic diversity within the captive population, and no attention to the habitats it came from) is incomplete.
Why this scores
Why this scores 4/4. Mark scheme: 1 mark each for clearly defining genetic / species / ecosystem diversity, plus 1 mark for valid example. Cambridge will award up to 4 marks for clear distinctions with examples. Examiner Reports note candidates who define only species diversity cap at 1-2.

Key Definitions and Keywords — Biodiversity

Definitions to memorise and the exact keywords mark schemes credit for biodiversity answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9700 sitting.

Biodiversity
Examiner keyword
The variety of life — measured at three levels: genetic diversity within species, species diversity within communities, and ecosystem diversity across regions.
Genetic diversity
Examiner keyword
The variety of alleles within a species. Provides the raw material for adaptation and resilience to environmental change.
Species diversity
Examiner keyword
The variety of species in a community, captured by species richness (number of species) and species evenness (relative abundance). Measured by Simpson's index of diversity.
Species richness
The number of different species present in a community. A simple count, not weighted by abundance.
Species evenness
How equally individuals are distributed across the species in a community. Higher evenness means no single species dominates.
Ecosystem diversity
Examiner keyword
The variety of different habitats and ecosystems within a region. Drives both species and genetic diversity by supporting different communities.
Simpson's index of diversity (D)
Examiner keyword
Quantitative measure of species diversity: $D = 1 - \sum (n/N)^2$ . Ranges from 0 (no diversity) to nearly 1 (very diverse). Captures both richness and evenness.
Quadrat
Examiner keyword
A square sampling frame (often 0.25 m × 0.25 m or 1 m × 1 m) placed on the ground to sample plants or sedentary animals. Used for percentage cover, frequency or counts.
Random sampling
Examiner keyword
Sampling method in which quadrat positions are chosen at random (e.g. using random number tables), giving every position an equal chance of being sampled. Used in uniform habitats.
Systematic sampling
Examiner keyword
Sampling method in which quadrats are placed at fixed intervals along a transect line, usually across an environmental gradient. Reveals how species distribution changes with the gradient.
Line transect
A line laid across a habitat; species touching the line are recorded at fixed intervals. Best for studying zonation across a gradient.
Belt transect
A strip of fixed width along a transect line, divided into quadrats at fixed intervals. Records all individuals within the strip; more detailed than a line transect.
Mark-release-recapture
Examiner keyword
Method to estimate population size of mobile animals: capture, mark and release a sample; later recapture another sample and count marked individuals. Population estimated using the Lincoln index $N = (n_1 \times n_2) / m_2$ .
Lincoln index
Examiner keyword
The formula used in mark-release-recapture: $N = \dfrac{n_1 \times n_2}{m_2}$ . Estimates total population from numbers marked and recaptured.
Spearman's rank correlation
Non-parametric statistical test for the strength of a monotonic relationship between two ranked variables (e.g. species abundance vs soil pH).
t-test
Parametric statistical test that compares the means of two samples to decide whether they differ significantly.
Chi-squared test ( $\chi^2$ )
Statistical test comparing observed and expected frequencies of categorical data (e.g. testing genetic ratios or quadrat counts against an expected distribution).

Common Mistakes and Misconceptions — Biodiversity

The traps other students keep falling into on biodiversity questions — taken from recent Cambridge International A Level 9700 examiner reports and mark schemes — and how to avoid them.

✕Reporting Simpson's index as the sum $\sum (n/N)^2$ instead of $1 - \sum (n/N)^2$
9700 Examiner Reports 2024
Why it happens
Candidates calculate the sum correctly but forget the final '1 −' step.
How to avoid it
Always write the full formula at the start: $D = 1 - \sum (n/N)^2$ . After calculating the sum, the last step is always to subtract from 1. If your D > 1 or D < 0, you have made a mistake.
✕Writing Lincoln formula upside down ( $N = m_2 / (n_1 \times n_2)$ ) or forgetting which sample is which
9700 Examiner Reports 2024
Why it happens
The formula has three variables and three places.
How to avoid it
Remember: N (whole population) is on the top. The two captures multiply on the top; marked recaptures divide on the bottom. Sanity check: $N$ should be larger than either sample, so multiplying must dominate.
✕Giving vague mark-release-recapture assumptions like 'population is constant'
9700 Examiner Reports 2023-2024
Why it happens
Candidates know an assumption exists but can't name it precisely.
How to avoid it
Three precise assumptions: (1) marks don't affect survival or behaviour; (2) closed population — no births, deaths, immigration or emigration between samples; (3) sufficient time for full mixing of marked individuals before recapture.
✕Defining species diversity as just the number of species (only richness)
9700 Examiner Reports 2024
Why it happens
Common sense suggests 'more species = more diverse', but it's incomplete.
How to avoid it
Species diversity = richness AND evenness. A community with 10 species, 9 of which are rare, is less diverse than one with 10 species each accounting for ~10% of individuals. Simpson's index responds to both.
✕Placing quadrats 'randomly' by throwing them over your shoulder
9700 Examiner Reports 2024
Why it happens
Looks random; quick.
How to avoid it
True random sampling requires random number coordinates — never throw or guess. Examiners expect: lay out a grid, generate random coordinates from a random number table / calculator function, place quadrat at coordinate. Throwing is a textbook example of biased sampling.
✕Confusing 'systematic' with 'biased' / 'researcher's choice'
9700 Examiner Reports 2023
Why it happens
The word 'systematic' sounds like 'system' / 'pre-chosen'.
How to avoid it
Systematic sampling means evenly spaced along a transect — not researcher's choice. It is unbiased for studying gradients. Researcher's-choice sampling is subjective sampling and is a different (poor) method.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Biodiversity — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Biodiversity

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1Calculating Simpson's diversity index (6 marks)

2Mark-release-recapture (Lincoln index) — calculation and assumptions (6 marks)

3Genetic, species and ecosystem diversity (4 marks)

4Random vs systematic sampling (5 marks)

5Why biodiversity is important (4 marks)

Simpson's index of diversity (D)

Lincoln index (mark-release-recapture)

Biodiversity

Genetic diversity

Species diversity

Species richness

Species evenness

Ecosystem diversity

Simpson's index of diversity (D)

Quadrat

Random sampling

Systematic sampling

Line transect

Belt transect

Mark-release-recapture

Lincoln index

Spearman's rank correlation

t-test

Chi-squared test (χ2\chi^2χ2)

✕Reporting Simpson's index as the sum ∑(n/N)2\sum (n/N)^2∑(n/N)2 instead of 1−∑(n/N)21 - \sum (n/N)^21−∑(n/N)2

✕Writing Lincoln formula upside down (N=m2/(n1×n2)N = m_2 / (n_1 \times n_2)N=m2​/(n1​×n2​)) or forgetting which sample is which

✕Giving vague mark-release-recapture assumptions like 'population is constant'

✕Defining species diversity as just the number of species (only richness)

✕Placing quadrats 'randomly' by throwing them over your shoulder

✕Confusing 'systematic' with 'biased' / 'researcher's choice'

Practice questions

Past paper style quiz

Assess your understanding

Biodiversity — frequently asked questions

Chi-squared test ( $\chi^2$ )

✕Reporting Simpson's index as the sum $\sum (n/N)^2$ instead of $1 - \sum (n/N)^2$

✕Writing Lincoln formula upside down ( $N = m_2 / (n_1 \times n_2)$ ) or forgetting which sample is which