Cambridge International AS & A Level Biology 9700: Classification, biodiversity and conservation – Study Guide

This guide teaches topic 18, Classification, biodiversity and conservation, of the Cambridge International AS & A Level Biology 9700 syllabus for examination in 2025, 2026 and 2027. It covers every learning outcome in sections 18.1 to 18.3. This is A Level content, examined on Paper 4 (A Level Structured Questions), not on the AS papers. The syllabus says Paper 4 and Paper 5 may include questions using its statistical tests, so the correlation work also matters for Paper 5.

When you have worked through it, use the revision notes for recall and the practice questions to test yourself. The 9700 Biology hub lists every topic, the printable checklist lets you tick off outcomes, and the A Level diagnostic shows where your gaps are.

What this topic covers

Section	What you must be able to do
18.1 Classification	Discuss three species concepts; describe the three domains; state three differences between Archaea and Bacteria; use the taxonomic hierarchy; outline the four eukaryotic kingdoms; classify viruses by nucleic acid
18.2 Biodiversity	Define ecosystem and niche; explain the three levels of biodiversity; explain random sampling; use quadrats, transects and the Lincoln index; use Spearman’s and Pearson’s correlation; calculate Simpson’s index
18.3 Conservation	Explain four causes of extinction; outline why biodiversity matters; outline zoos, botanic gardens, conserved areas, frozen zoos and seed banks; describe IVF, embryo transfer and surrogacy; explain control of invasive alien species; outline IUCN and CITES

The formulae for the Lincoln index, Simpson’s index, Spearman’s rank and Pearson’s linear correlation are provided in the exam. You must know how to use them and interpret the results. Calculators are allowed.

18.1 Classification

Three species concepts

The syllabus limits you to three definitions.

• Biological species concept: a group of organisms that can interbreed to produce fertile offspring and are reproductively isolated from other such groups. It fails for organisms that reproduce asexually, for fossils, and for populations that never meet.
• Morphological species concept: organisms are placed in the same species because they look alike in structure. It can mislead when males and females look very different, when larvae differ from adults, or when two species look almost identical.
• Ecological species concept: a species is a group of organisms that share the same ecological niche. Niches can overlap, and one species can occupy different niches in different places, so boundaries are not always clear.

Three domains

All cellular life is placed in three domains: Archaea, Bacteria and Eukarya. Archaea and Bacteria are both prokaryotes (no nucleus), but they differ in three ways the syllabus names:

Feature	Archaea	Bacteria
Membrane lipids	Glycerol joined to branched hydrocarbon chains by ether bonds	Glycerol joined to unbranched fatty acids by ester bonds
Ribosomal RNA	Base sequences differ from bacterial rRNA	Base sequences differ from archaeal rRNA
Cell wall	No peptidoglycan	Contains peptidoglycan (murein)

Taxonomic hierarchy

Eukarya are classified into ranks: kingdom, phylum, class, order, family, genus, species. For humans: Animalia, Chordata, Mammalia, Primates, Hominidae, Homo, sapiens. The binomial name uses the genus and species, is written in italics (or underlined by hand), and the genus has a capital letter: Homo sapiens.

The four eukaryotic kingdoms

Kingdom	Key features
Protoctista	Eukaryotes that do not fit the other kingdoms; mostly unicellular or simple multicellular; some photosynthesise (algae), some are heterotrophic; very varied
Fungi	No chlorophyll; heterotrophic, feeding saprotrophically or parasitically by extracellular digestion and absorption; cell walls of chitin; usually hyphae forming a mycelium (yeasts are unicellular); reproduce by spores; store glycogen
Plantae	Multicellular; cellulose cell walls; chloroplasts, autotrophic by photosynthesis; often a large permanent vacuole; store starch
Animalia	Multicellular; no cell walls; heterotrophic, usually ingesting food; nervous coordination; store glycogen

Viruses

Viruses are not cells, so they sit outside the three domains. You only need to classify them by their nucleic acid: DNA or RNA, and single-stranded or double-stranded. HIV and influenza viruses have single-stranded RNA; herpes viruses have double-stranded DNA.

18.2 Biodiversity

Definitions

• Ecosystem: a relatively self-contained, interacting community of organisms and the environment in which they live and with which they interact.
• Niche: the role of an organism in its ecosystem: where it lives, what it feeds on, and how it interacts with the biotic and abiotic factors around it.

Three levels of biodiversity

1. The number and range of different ecosystems and habitats.
2. The number of species (species richness) and their relative abundance (how evenly individuals are spread among species).
3. The genetic variation within each species. A species with low genetic diversity is less able to adapt to change.

Random sampling

Samples must be random so that every part of the area has an equal chance of being chosen. This removes bias from the person sampling. Lay two tapes at right angles, generate random coordinates, and place a quadrat at each point. Take enough samples to be representative.

Sampling methods

• Frame quadrat: a square frame (for example 0.5 m × 0.5 m). Record species present, the number of individuals, or percentage cover. From these you get species frequency (percentage of quadrats containing the species), density (individuals per m²) or mean percentage cover.
• Line transect: a tape laid across an area where conditions change (for example from a path into woodland). Record the species touching the line at regular intervals.
• Belt transect: quadrats placed along the line, either continuously or at regular intervals, recording abundance in each. It gives more data than a line transect.

Transects are systematic, not random. Use them to study how distribution changes along an environmental gradient.

Mark-release-recapture

For mobile animals, capture a sample, mark them harmlessly, release them, then capture a second sample later. The Lincoln index estimates population size:

N = (n₁ × n₂) / m₂

n₁ = number caught and marked in the first sample; n₂ = total caught in the second sample; m₂ = number of marked animals in the second sample.

Worked example. A student catches 48 beetles in pitfall traps, marks each with a dot of non-toxic paint and releases them. Two days later she catches 60 beetles, of which 12 are marked.

N = (48 × 60) / 12 = 2880 / 12 = 240

Estimated population: 240 beetles.

The method assumes that marks do not harm the animal or make it easier to catch, marks are not lost, marked animals mix fully with the population, and there are no births, deaths or migration between samples.

Correlation: Spearman’s and Pearson’s

Both tests give a value between −1 (perfect negative correlation) and +1 (perfect positive correlation), with 0 meaning no correlation. A correlation does not show that one variable causes the other.

• Spearman’s rank correlation (rₛ) is used when the data are not normally distributed, or are ordinal. It needs more than five pairs, ideally 10 to 30, with a scatter graph suggesting an increasing or decreasing relationship.
• Pearson’s linear correlation (r) is used for continuous, normally distributed data where a scatter graph suggests a linear relationship. It needs at least five pairs, ideally ten or more.

You may be given partly completed calculations to finish.

Worked example: Spearman’s. Along a woodland edge, eight quadrats give light intensity (arbitrary units) and percentage cover of a shade-tolerant moss.

Quadrat	Light	Cover / %	Rank light	Rank cover	D	D²
1	2	70	1	7	−6	36
2	5	74	2	8	−6	36
3	9	55	3	6	−3	9
4	14	48	4	4	0	0
5	20	50	5	5	0	0
6	26	30	6	3	3	9
7	33	22	7	2	5	25
8	41	15	8	1	7	49

ΣD² = 164, n = 8.

rₛ = 1 − (6 × ΣD²) / (n³ − n)
   = 1 − (6 × 164) / (512 − 8)
   = 1 − 984 / 504
   = −0.952 (3 s.f.)

With a critical value of 0.738 for n = 8 at p = 0.05, the size of rₛ (0.952) is greater than the critical value. There is a significant negative correlation: moss cover falls as light intensity rises. Reject the null hypothesis of no correlation.

Worked example: Pearson’s, partly completed. For eight paired readings you are given Σxy = 2202, x̄ = 9.625, ȳ = 25.25, sₓ = 3.889, sᵧ = 9.528.

r = (Σxy − n x̄ ȳ) / ((n − 1) sₓ sᵧ)
  = (2202 − 8 × 9.625 × 25.25) / (7 × 3.889 × 9.528)
  = (2202 − 1944.25) / 259.38
  = 257.75 / 259.38
  = 0.994 (3 s.f.)

A strong positive linear correlation.

Simpson’s index of diversity

D = 1 − Σ(n/N)²

n = number of individuals of each type; N = total individuals of all types.

Worked example. A pond net sample contains five species with 20, 12, 8, 6 and 4 individuals. N = 50.

Species	n	n/N	(n/N)²
A	20	0.40	0.1600
B	12	0.24	0.0576
C	8	0.16	0.0256
D	6	0.12	0.0144
E	4	0.08	0.0064

Σ(n/N)² = 0.2640, so D = 1 − 0.2640 = 0.736.

D ranges from 0 to just below 1. A value near 1 means high diversity: many species, with individuals spread evenly. A value near 0 means low diversity, with one species dominating. High-D communities are usually more stable.

18.3 Conservation

Why species become extinct

• Climate change: temperature and rainfall shift faster than species can adapt or move; habitats such as coral reefs and polar ice shrink.
• Competition: introduced or more successful species out-compete native species for food, space or light.
• Hunting by humans: for food, trophies, skins or traditional medicines. Populations fall below the size needed to recover.
• Degradation and loss of habitats: deforestation, farming, drainage, building and pollution remove or fragment habitats, leaving small, isolated populations with low genetic diversity.

Why maintain biodiversity

Ethical reasons; ecosystem services such as pollination, nutrient cycling and water purification; sources of food, medicines and genes for crop improvement; stable food webs; economic value, including ecotourism; aesthetic value.

Conservation of endangered species

Approach	Role
Zoos	Captive breeding with records that avoid inbreeding; reintroduction to the wild; research; education
Botanic gardens	Grow and propagate rare plants; reintroduction; research; education
Conserved areas (national parks, marine parks)	Protect whole habitats in place; restrict hunting, fishing, logging and building
Frozen zoos	Store gametes, embryos and tissue samples in liquid nitrogen for future use in assisted reproduction
Seed banks	Store dried seeds at low temperature; test germination and regrow stock; keep genetic diversity of plants

Assisted reproduction in mammals

• IVF (in vitro fertilisation): eggs are collected from a female (often after hormone treatment) and fertilised with sperm in a dish.
• Embryo transfer: the resulting embryos are placed in the uterus of a female. One valuable female’s eggs can give many embryos.
• Surrogacy: the embryo is carried by a surrogate mother, which may be a female of a more common, closely related species. The endangered female does not have to carry every pregnancy.

Using stored sperm or embryos from distant populations increases genetic diversity.

Invasive alien species

An invasive alien species is one introduced outside its natural range that spreads. Control is needed because they may compete with native species, eat them, carry disease, hybridise with them, or change the habitat, and they often have no natural predators in the new area. They can also cause economic harm to farming and fisheries.

IUCN and CITES

• IUCN (International Union for Conservation of Nature): assesses the conservation status of species and publishes the Red List, using categories from Least Concern to Extinct. It gives governments evidence for conservation priorities.
• CITES (Convention on International Trade in Endangered Species of Wild Fauna and Flora): an international agreement between governments that controls trade in endangered species and their products, banning trade for the most threatened and requiring permits for others.

Common errors

• Writing “species can breed” instead of “produce fertile offspring”.
• Saying Archaea have “no cell wall”; they have walls, just without peptidoglycan.
• Putting viruses in a kingdom or domain.
• Swapping n₂ and m₂ in the Lincoln index.
• Forgetting to square n/N, or forgetting the “1 −” in Simpson’s index.
• Concluding “light causes low moss cover” from a correlation.

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.