SPEC 3.7.4 - GENETICS, POPULATIONS, EVOLUTION & ECOSYSTEMS
Population Sampling Methods
Quadrats, belt transects, percentage cover, frequency, mark-release-recapture & Required Practical 12
Why Do We Sample?
It is rarely possible to count every individual in a population. Sampling allows us to estimate population size and distribution from a manageable subset of the habitat.
Slow-moving / Non-motile Organisms
Use quadrats — placed randomly, or at regular intervals along a transect.
Examples: plants, lichens, limpets, barnacles.
Motile Organisms
Use mark-release-recapture (MRC) — quadrats cannot reliably sample mobile animals.
Examples: insects, small mammals, fish.
Key Principle — Random Sampling
Sampling must be random to avoid bias. Use a random number generator or random number tables to produce coordinates; place the quadrat at each coordinate. This ensures every point in the habitat has an equal chance of being sampled, making the sample representative of the whole habitat.
Examiner Alert
Do not write "random sampling ensures accuracy." This is vague and will not score marks. The correct statement is: "random placement ensures every point has an equal probability of selection, preventing systematic bias and making the sample representative of the habitat."
Quadrats
Definition
Quadrat — a square frame of known area placed on the ground to sample the abundance and distribution of organisms within it.
Two measures of abundance can be recorded using quadrats:
Percentage Cover
The proportion of the quadrat's area covered by a species, expressed as a percentage.
Use when: plants overlap, or individuals are too numerous to count (e.g. grass, moss).
Note: total percentage cover across all species in a quadrat can exceed 100% due to overlapping layers.
Frequency
The number of quadrats (out of the total sampled) in which a species is present, expressed as a proportion or percentage.
Use when: species are difficult to count individually; gives a measure of how widespread a species is across the habitat.
Estimating Total Population from Quadrats
Mean number per quadrat × (total habitat area ÷ quadrat area)
Example: Mean = 6 plants per 0.25 m² quadrat; field area = 500 m²
Estimated total = 6 × (500 ÷ 0.25) = 6 × 2000 = 12,000 plants
Belt Transects
Definition
Belt transect — a strip of habitat sampled systematically along an environmental gradient, using quadrats placed at regular intervals along a tape measure.
When to Use a Belt Transect
Use a belt transect when you want to investigate how species distribution changes along an environmental gradient, for example:
Shore zonation — from low tide to high tide mark
Altitude gradient — from valley floor to mountaintop
Light gradient — from open field into woodland edge
Distance from a pollution source — river, road or industrial site
Method
Lay a tape measure along the environmental gradient
Place a quadrat at regular intervals (e.g. every 2 m) along the tape
Record species present and their abundance (percentage cover or frequency) in each quadrat
Measure the relevant abiotic variable (e.g. light intensity, soil pH, soil moisture) at each quadrat position
Plot species distribution against the abiotic factor to reveal correlations
Exam Tip
A belt transect is not random — it is systematic. This is deliberate: we want to capture the change across a gradient, so quadrats must be placed at defined intervals along it. Random placement would defeat the purpose.
Required Practical 12
Required Practical 12 — Use of a Transect to Investigate Distribution
Aim: To investigate how the distribution of organisms varies along an environmental gradient using a belt transect.
VariableDetail
Independent variablePosition along the transect (distance in metres) / abiotic factor (e.g. light intensity, soil moisture, distance from sea)
Dependent variableAbundance of species in each quadrat (percentage cover or frequency)
Control variablesQuadrat size (same area at each position); time of sampling (same day/season to avoid seasonal variation); same observer (reduces recording bias); same transect line used throughout
Expected results: Species composition and abundance changes along the gradient, reflecting species' adaptations to different abiotic conditions — e.g. shade-tolerant species increase in frequency as light intensity decreases into woodland.
Exam Tip — Control Variables
Always state WHAT is controlled AND WHY. Example: "Quadrat size is kept constant (0.5 × 0.5 m) so that percentage cover values are comparable between sampling positions." Stating only what is controlled, without the reason, will not gain full marks.
Mark-Release-Recapture: Method
Used to estimate the population size of motile organisms that cannot be reliably sampled with quadrats.
1
First capture: Capture a sample of the organism (n₁). Count them. Mark each individual with a method that is harmless, non-toxic, and does not affect survival, behaviour, or visibility to predators (e.g. small paint spot, leg ring, ear tag). Record n₁.
2
Release and allow time for mixing: Release all marked individuals back into the habitat. Allow sufficient time for marked individuals to disperse randomly and mix back into the population before the second sample is taken.
3
Second capture: Capture a second sample (n₂). This sample will contain both marked and unmarked individuals. Count the total in the second sample (n₂) and count how many are marked (m).
4
Calculate: Use the mark-release-recapture formula to estimate total population size N.
Examiner Alert
The second sample is NOT the "recaptured sample." It is a fresh sample that happens to contain some previously marked individuals alongside new, unmarked ones. Students who call it the "recaptured sample" lose marks.
Mark-Release-Recapture: Formula
N = (n₁ × n₂) / m N = estimated population size | n₁ = first sample size | n₂ = second sample size | m = marked individuals in second sample
Worked Example
A student catches and marks 35 great crested newts (n₁ = 35). After releasing them and waiting 48 hours, a second sample of 50 newts is captured (n₂ = 50). Of these, 7 are marked (m = 7).
N = (n₁ × n₂) / m = (35 × 50) / 7 = 1750 / 7 = 250
Estimated population size = 250 newts.
What happens to N if m is smaller?
If fewer marked individuals are found in the second sample (smaller m), N increases. This makes sense: if very few of your marked animals turn up in the second sample, the population must be very large.
If the mark falls off some animals before the second sample, m is artificially low → N is overestimated.
Exam Tip
A common error is to subtract rather than multiply n₁ and n₂. The formula uses multiplication: N = (n₁ × n₂) / m. Show every step of working and include correct notation.
MRC: Assumptions & Their Consequences
The mark-release-recapture method only gives a valid estimate if the following assumptions are met. Examiners require you to state the assumption AND explain what happens if it is violated.
AssumptionIf violated → effect on N
Marked individuals mix randomly back into the population before the second sampleIf insufficient time is allowed for dispersal, marked animals are not randomly distributed → marked individuals are over- or under-represented in the second sample → N is inaccurate
Marking does not affect survival or behaviourIf marking makes animals more conspicuous to predators → increased death rate of marked animals → m is lower than expected → N is overestimated
Marks are not lost between the first and second captureIf marks fall off → m is artificially low → N is overestimated
The population is closed (no births, deaths, immigration or emigration between samples)If the population changes in size between captures → N is inaccurate; immigration increases apparent population; deaths of marked animals reduce m and overestimate N
Marking does not make animals more or less likely to be recapturedIf marked animals avoid traps after first capture (trap shyness) → m is lower → N is overestimated
Examiner Intelligence
Examiner Alert — Top Errors in This Topic
MRC formula error: Many students subtract n₁ and n₂ instead of multiplying. The formula is N = (n₁ × n₂) / m. Learn it and show working.
Assumptions without explanation: Stating "marking must not affect survival" scores 1 mark. Stating "if marking makes individuals more conspicuous to predators, m will be lower than expected and N will be overestimated" scores full marks.
"Recaptured sample" error: The second sample is NOT a recapture sample — it contains both previously marked and new, unmarked individuals.
Dispersal time: Students say "allow time for re-integration" but must add "so that marked individuals are randomly distributed throughout the population" — not just that they have returned to the habitat.
Random sampling justification: Never say "to ensure accuracy." Say "to ensure every point has an equal probability of selection, preventing bias."
Estimating total from quadrats: Must use: mean per quadrat × (habitat area ÷ quadrat area). Students who omit quadrat area from the calculation cannot reach the total.
DEEP Chain — Evaluating MRC Validity
D — Define: Mark-release-recapture estimates population size using N = (n₁ × n₂) / m, where n₁ is the first sample, n₂ the second sample, and m the number of marked individuals in the second sample.
E — Explain mechanism: The method assumes that the ratio of marked to unmarked individuals in the second sample reflects their ratio in the whole population. If marked individuals have mixed randomly, this ratio gives an unbiased estimate of N.
E — Evidence/Example: If marks increase predation risk (e.g. bright paint spot on a cryptic insect), death rate of marked individuals increases → m is lower than expected → N is overestimated. This is a key limitation of any mark that changes an animal's appearance.
P — Practical/Quantitative: n₁ = 40, n₂ = 60, m = 8. N = (40 × 60) / 8 = 300. If two marks fell off (m effectively = 6): N = (40 × 60) / 6 = 400. Loss of marks causes N to be overestimated by 100 individuals — a 33% error.
Practice Question
A student is investigating the distribution of marram grass (Ammophila arenaria) along a sand dune system. She places a 0.5 m × 0.5 m quadrat at 5 m intervals along a 50 m belt transect running from the shoreline inland. At each position she records the percentage cover of marram grass and measures the soil moisture content.
(a) Explain why the student uses a belt transect rather than randomly placed quadrats for this investigation. [2 marks]
(b) State one control variable the student should maintain and explain why it must be controlled. [2 marks]
(c) A separate study uses mark-release-recapture to estimate the population of sand lizards on the same dunes. 24 lizards are caught and marked; after 3 days, a second sample of 30 lizards is caught, of which 6 are marked. Calculate the estimated population size. Show your working. [2 marks]
(d) Suggest one reason why this estimate might not be valid, and explain the effect on N. [2 marks]
(a) A belt transect is used to investigate how species distribution changes along an environmental gradient (1) — in this case, the gradient in soil moisture/wind exposure/salinity from shoreline to inland; randomly placed quadrats would not capture this systematic change (1).
(b) E.g. Quadrat size must be kept constant (0.5 × 0.5 m at every position) (1) so that percentage cover values are directly comparable between sampling positions (1).
(c) N = (n₁ × n₂) / m = (24 × 30) / 6 = 720 / 6 = 120 lizards (2).
(d) E.g. If the marks (paint) fade or rub off over 3 days, some marked lizards would not be identified in the second sample (1). m would be lower than the true value → N would be overestimated (1).
Synoptic Links & Summary
Synoptic Links
Lesson 7.05 — Sampling methods are used to monitor changes in community composition during succession and to evaluate conservation management outcomes
Lesson 4.07 (Biodiversity) — Quadrat data feeds into calculation of Simpson's diversity index D; understanding how sampling affects the reliability of diversity estimates
Paper 3 practical skills — This is a high-frequency practical question topic: expect data tables of quadrat results, MRC calculations, evaluation of sampling validity, and graph interpretation
Key Points Checklist
Quadrats for non-motile/slow-moving organisms; MRC for motile organisms
Random placement: ensures equal probability of selection; prevents bias
Two measures of abundance: percentage cover and frequency
Belt transect: systematic sampling along an environmental gradient
MRC formula: N = (n₁ × n₂) / m — use multiplication, not subtraction
5 assumptions of MRC: random mixing; marking harmless; marks not lost; population closed; no trap shyness (marked animals not more or less likely to be recaptured)
Each assumption: state it AND explain the effect on N if violated
RP12: belt transect; measure abiotic factor at each quadrat; compare species distribution with environmental gradient
Next: Why Do We Sample?