Cambridge International AS & A Level Biology 9700: Classification, biodiversity and conservation – Revision Notes

These notes condense topic 18, Classification, biodiversity and conservation, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027, sections 18.1 to 18.3. This is A Level content, examined on Paper 4; the statistics can also appear in Paper 5. For full explanations and longer worked examples, read the study guide first.

Then test yourself with the practice questions and the A Level diagnostic. The 9700 Biology hub and printable checklist cover the rest of the course.

18.1 Classification

Species concepts (definition + one limitation each)

Concept	Definition	Limitation
Biological	Can interbreed to produce fertile offspring; reproductively isolated from other species	Cannot test asexual organisms, fossils or separated populations
Morphological	Share the same structural features	Males/females or larvae/adults may look different; similar-looking species may be separate
Ecological	Occupy the same ecological niche	Niches overlap and can vary from place to place

Domains

• Three domains: Archaea, Bacteria, Eukarya.
• Archaea and Bacteria are both prokaryotes. Differences (only these three are required):
  • Membrane lipids: Archaea have ether bonds and branched chains; Bacteria have ester bonds and unbranched fatty acids.
  • rRNA: different base sequences.
  • Cell walls: Bacteria have peptidoglycan; Archaea do not.

Hierarchy

Kingdom → phylum → class → order → family → genus → species. Memory aid: “King Philip Came Over For Good Soup”.

Kingdoms at a glance

	Protoctista	Fungi	Plantae	Animalia
Cells	Mostly unicellular or simple	Hyphae/mycelium (yeasts unicellular)	Multicellular	Multicellular
Cell wall	Some	Chitin	Cellulose	None
Nutrition	Autotrophic or heterotrophic	Heterotrophic: saprotrophic or parasitic, absorb after extracellular digestion	Photosynthesis	Heterotrophic, ingest
Other	Very varied group	Spores; store glycogen	Chloroplasts; starch store; large vacuole	Nervous coordination; glycogen store

Viruses

Not in any domain. Classified by nucleic acid (DNA or RNA) and strands (single or double). Four possible groups: ssDNA, dsDNA, ssRNA, dsRNA.

18.2 Biodiversity

Definitions

• Ecosystem: a relatively self-contained, interacting community of organisms and their environment.
• Niche: the role of an organism in its ecosystem, including where it lives, what it eats and its interactions with biotic and abiotic factors.
• Biodiversity is assessed at three levels: range of ecosystems and habitats; number of species and their relative abundance; genetic variation within species.

Formulae (all provided in the exam)

Quantity	Formula	Meaning of symbols
Lincoln index	N = (n₁ × n₂) / m₂	n₁ first sample (marked); n₂ second sample (all); m₂ marked in second sample
Simpson’s index	D = 1 − Σ(n/N)²	n individuals of each type; N total
Spearman’s rank	rₛ = 1 − (6 × ΣD²) / (n³ − n)	D difference in rank; n number of pairs
Pearson’s linear	r = (Σxy − n x̄ ȳ) / ((n − 1) sₓ sᵧ)	x̄, ȳ means; s standard deviations; n pairs

Method in steps: sampling a site

1. Decide: random quadrats for a uniform area; belt or line transect for a gradient.
2. For random: grid the area with tapes, generate random coordinates, place quadrats.
3. Record presence, number of individuals or percentage cover.
4. Take enough quadrats that the running mean levels off.
5. Calculate frequency, density or mean cover.

Method in steps: Simpson’s index

1. Total all individuals to get N.
2. For each type work out n/N and square it.
3. Add the squares.
4. Subtract the total from 1.

Method in steps: Spearman’s rank

1. State the null hypothesis: there is no correlation between the two variables.
2. Rank each variable separately (smallest = 1). Tied values share the mean rank.
3. Find D for each pair and square it; add to get ΣD².
4. Substitute into the formula.
5. Compare the size of rₛ with the critical value at p = 0.05. If larger, the correlation is significant; reject the null hypothesis.

Must-know distinctions

• Spearman’s for non-normal or ordinal data, over five pairs; Pearson’s for normal, continuous data with a linear pattern, at least five pairs.
• Correlation vs causation: a significant correlation does not prove one factor causes the other.
• Species richness (number of species) vs relative abundance (evenness).
• Line transect (species touching the line) vs belt transect (quadrats along the line).
• D near 1 = high diversity, stable; D near 0 = one species dominates.

Lincoln index assumptions

No births, deaths, immigration or emigration between samples; marks not lost and not harmful; marked individuals mix randomly back into the population; marking does not change the chance of recapture.

Quadrat measures

• Species frequency = percentage of quadrats in which the species is present. Quick, but ignores how many individuals there are.
• Density = number of individuals per m². Divide the mean count per quadrat by the quadrat area.
• Percentage cover = the proportion of the quadrat area covered by the species. Use it when individuals are hard to count, such as grasses and mosses.

Worked reminder: Pearson’s from a partly completed calculation

Given n = 6, Σxy = 420, x̄ = 5, ȳ = 12, sₓ = 2, sᵧ = 6.5: r = (420 − 6 × 5 × 12) / (5 × 2 × 6.5) = 60 / 65 = 0.923 (3 s.f.), a strong positive correlation. Always check that r lies between −1 and +1; a value outside that range means a slip.

18.3 Conservation

Causes of extinction

Climate change; competition (often from introduced species); hunting by humans; degradation and loss of habitats.

Reasons to maintain biodiversity

Ethical; ecosystem services (pollination, nutrient cycling, clean water, soil); food and medicines; genes for crops; food web stability; tourism and economic value; aesthetic value.

Conservation methods

• Zoos: captive breeding with breeding records to avoid inbreeding; release; research; education.
• Botanic gardens: grow and propagate rare plants; reintroduce; research; education.
• National parks and marine parks: protect whole habitats in place; restrict harmful activities.
• Frozen zoos: gametes, embryos and tissue in liquid nitrogen.
• Seed banks: seeds dried and stored cold; germination tested and stocks regrown.

Assisted reproduction (mammals)

IVF (fertilisation outside the body) → embryo transfer (embryo placed in a uterus) → surrogacy (carried by another female, possibly of a related, more common species).

Invasive alien species

Control because they compete with, prey on, spread disease to or hybridise with natives; they often lack natural predators; they change habitats and cause economic damage.

IUCN vs CITES

• IUCN: assesses status; publishes the Red List; advises governments.
• CITES: agreement between governments; controls international trade in endangered species and their products.

Quick self-test

1. State the biological species concept.
2. Give one difference in membrane lipids between Archaea and Bacteria.
3. Put these ranks in order, largest first: family, class, genus, kingdom, order, phylum.
4. How are viruses classified in this syllabus?
5. Define niche.
6. A student marks 25 snails, releases them, and later catches 40, of which 8 are marked. Estimate the population.
7. A sample has two species, with 5 individuals each. Calculate Simpson’s index.
8. A second sample has 18 of one species and 1 each of two others. Calculate D and compare with question 7.
9. For six pairs, ΣD² = 12. Calculate rₛ.
10. When should you use Pearson’s rather than Spearman’s?
11. State two roles of a frozen zoo.
12. What is the main role of CITES?

Answers

1. Organisms that can interbreed to produce fertile offspring and are reproductively isolated from other groups.
2. Archaea: ether bonds (branched chains). Bacteria: ester bonds (unbranched fatty acids).
3. Kingdom, phylum, class, order, family, genus.
4. By type of nucleic acid (DNA or RNA) and whether it is single- or double-stranded.
5. The role of an organism in its ecosystem, including where it lives, what it feeds on and how it interacts with biotic and abiotic factors.
6. N = (25 × 40) / 8 = 125.
7. D = 1 − (0.5² + 0.5²) = 1 − 0.50 = 0.5.
8. N = 20. Σ(n/N)² = 0.81 + 0.0025 + 0.0025 = 0.815; D = 0.185. This is lower: three species but one dominates, so diversity is lower than in question 7.
9. rₛ = 1 − (6 × 12) / (216 − 6) = 1 − 72/210 = 0.657 (3 s.f.).
10. When the data are continuous, normally distributed, and a scatter graph suggests a linear relationship.
11. Store gametes, embryos or tissue samples in liquid nitrogen; keep genetic diversity for future IVF or embryo transfer.
12. It controls international trade in endangered species and their products.

Where marks are usually lost

• Leaving out “fertile” or “reproductively isolated” in the biological species concept.
• Listing features of Archaea beyond the three the syllabus names, and missing the three it does.
• Giving the hierarchy in the wrong order, or capitalising the species name in a binomial.
• Describing a transect when the question asks for a random method, or the reverse.
• Swapping n₂ and m₂ in the Lincoln index, giving a far too small population.
• Stating what D shows without linking it to both species number and evenness.
• Ranking one variable high-to-low and the other low-to-high in Spearman’s.
• Concluding “significant” without comparing with the critical value and stating the null hypothesis decision.
• Describing zoos only as “keeping animals safe” without breeding, reintroduction, research or education.
• Mixing up IUCN (assesses status, Red List) and CITES (controls trade).

Official syllabus

Cambridge International AS & A Level Biology 9700 syllabus, for examination in 2025, 2026 and 2027 (version 1), published by Cambridge University Press & Assessment. Topic 18, sections 18.1 to 18.3, and the Mathematical requirements.