EnglandEnvironmental ScienceSyllabus dot point

How do environmental scientists sample populations and monitor the environment reliably?

Methods of sampling populations and habitats including quadrats, transects and capture techniques, the importance of random sampling and replication, and the abiotic and biotic factors that are monitored.

A focused answer to AQA A-Level Environmental Science 3.6 research methods, covering sampling techniques such as quadrats, transects and mark-release-recapture, the importance of random sampling and replication, and the abiotic and biotic factors monitored.

Generated by Claude Opus 4.812 min answerUpdated 2026-06-02

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

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What this dot point is asking
Why we sample
Sampling techniques
Random sampling and replication
Abiotic and biotic factors monitored

What this dot point is asking

AQA wants you to describe methods of sampling populations and habitats (quadrats, transects and capture techniques), explain why random sampling and replication matter, and identify the abiotic and biotic factors that are monitored. Command words are Describe, Explain and Calculate, so expect a mark-release-recapture calculation using the Lincoln index.

Why we sample

Sampling techniques

Key fact

Quadrats are frames placed on the ground to measure the abundance, density or percentage cover of plants and slow-moving organisms. Placed at random coordinates, the mean count is scaled up to estimate the population of the whole area.
Transects are lines along which quadrats are placed at intervals, used to record how a community changes along a gradient, for example from the edge to the centre of a wood or up a rocky shore (a belt transect records abundance, a line transect records presence).
Mark-release-recapture estimates mobile animal populations: a first sample is captured, marked and released; a second sample is taken later, and the proportion marked gives the population size via the Lincoln index.
Traps such as pitfall traps, light traps and nets capture mobile animals for counting.

Random sampling and replication

Sampling at random positions (for example using random-number grid coordinates) removes the bias of choosing where organisms look most abundant, which would overestimate the population. Taking many replicate samples and calculating a mean gives a more reliable estimate, smooths out the effect of anomalies, and allows the spread (standard deviation) to be reported so the precision of the estimate is known.

Abiotic and biotic factors monitored

Abiotic factors (non-living physical and chemical conditions): temperature, light intensity, pH, dissolved oxygen, soil moisture, salinity and wind, measured with instruments such as thermometers, light meters, pH probes and oxygen meters.
Biotic factors (living): the species present, their abundance and distribution, and the use of indicator species to assess pollution.

Monitoring these over time reveals how a habitat is changing, for example in response to pollution, succession or climate change, which is the basis of environmental impact assessment.

Estimating a population by mark-release-recapture

A first sample of $50$ beetles is marked and released. A later sample of $70$ beetles contains $14$ marked individuals. Estimate the total population using the Lincoln index.

step 1: Write the Lincoln index formula

Estimated population $N = \frac{n_1 \times n_2}{m}$ , where $n_1$ is the number first marked, $n_2$ is the second sample size, and $m$ is the number of marked individuals recaptured.

step 2: Substitute the values

$N = \frac{50 \times 70}{14}$ .

step 3: Calculate and interpret

$50 \times 70 = 3500$ , then $\frac{3500}{14} = 250$ beetles. The estimate assumes marked beetles mixed randomly, none were lost, and the population did not change between samples. Markers reward the formula, correct substitution, the answer of $250$ , and noting the assumptions; the common slip is dividing by the wrong sample.

Exam-style practice questions

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

AQA 20185 marksDescribe how mark-release-recapture is used to estimate the size of an animal population, and state two assumptions the method depends on.

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Markers split this between the method (3) and the assumptions (2).

Method: capture a sample of the animals, count and mark them harmlessly, then release them and allow time for them to mix back into the population. Later capture a second sample, count how many of those are marked, and use the Lincoln index: population equals the first sample multiplied by the second sample, divided by the number marked in the second sample.

Assumptions: marked individuals mix randomly and evenly with the population; the marking does not harm them or make them more visible to predators; no significant births, deaths, immigration or emigration occur between the two samples; and marks are not lost. Any two valid assumptions score.

AQA 20215 marksIn a mark-release-recapture study, 60 woodlice were captured and marked. A later sample of 80 contained 24 marked individuals. Calculate the estimated population size and comment on its reliability.

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A full worked calculation is required.

Lincoln index: estimated population equals $\frac{\text{first sample} \times \text{second sample}}{\text{marked recaptured}} = \frac{60 \times 80}{24}$ .

Calculation: $60 \times 80 = 4800$ , then $\frac{4800}{24} = 200$ woodlice.

Comment: the estimate of $200$ is only as reliable as the assumptions hold; a small second sample or poor mixing would reduce reliability, so repeating the study and taking larger samples would improve confidence. Award the correct formula, the substituted values, the answer of $200$ , and a reliability comment.

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Sources & how we know this

AQA A-level Environmental Science (7447) specification — AQA (2017)