Sampling Techniques

Measuring biodiversity requires us to estimate the abundance and distribution of organisms across a habitat, but it is usually impossible to count every individual. Instead, ecologists take samples — small, representative portions of the habitat — and extrapolate from them to describe the whole. OCR A-Level Biology A specification 4.2.1 (c)–(d) requires you to know the main sampling techniques for plants and animals, explain the difference between random and non-random sampling, and use the mark-release-recapture method (the Lincoln index) to estimate animal populations.

Key Definitions:

• Sample — a small subset of a population used to estimate the properties of the whole.
• Random sampling — sampling in which every individual or location has an equal chance of being selected.
• Systematic sampling — sampling at regular intervals, often along a transect.
• Stratified sampling — sampling each sub-habitat in proportion to its size or importance.
• Bias — systematic error that makes a sample unrepresentative.

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Why Sample?

Sampling is necessary because:

1. Populations are too large to count in full.
2. Organisms are cryptic or mobile — many animals hide or move faster than observers.
3. Resources are limited — time, personnel and money.
4. Ethical considerations — minimising disturbance to wildlife.

A good sample is representative: it accurately reflects the composition of the whole population. Two properties make a sample representative: adequate size (large enough to reduce chance effects) and lack of bias.

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Random vs Non-Random Sampling

flowchart TD
    A[Sampling] --> B[Random]
    A --> C[Non-Random]
    B --> B1[Every point has equal chance]
    B --> B2[Avoids bias]
    B --> B3[Uses random number generator]
    C --> C1[Systematic: regular intervals]
    C --> C2[Stratified: proportional to sub-habitat]
    C --> C3[Opportunistic: easiest to sample]

Random Sampling

In random sampling, a random number generator produces coordinates within the study area and quadrats are placed at those coordinates. This avoids conscious or unconscious bias — such as placing quadrats where the flowers look prettiest. It is ideal when a habitat is relatively uniform.

Procedure:

1. Mark out the study area with a grid (tape measures at right angles give coordinates).
2. Use a random number generator to produce pairs of coordinates.
3. Place a quadrat at each coordinate and record species.
4. Repeat until a large enough sample (often ≥ 30 quadrats) is obtained.

Systematic Sampling

In systematic sampling, samples are taken at regular intervals — for example, a quadrat every 2 m along a transect. This is used when you want to investigate how communities change along an environmental gradient (e.g. distance from a path, altitude on a mountain, shore height on a beach).

Stratified Sampling

In stratified sampling, the area is divided into sub-habitats, and the number of samples from each sub-habitat is proportional to its size. For example, if a nature reserve is 70% grassland and 30% woodland, you might take 70 grassland quadrats and 30 woodland quadrats. This gives better coverage than simple random sampling when habitats are uneven.

Opportunistic Sampling

Opportunistic sampling just records what is easiest to find. It is quick but very prone to bias, and is generally avoided in scientific work.

Exam Tip: OCR often asks you to "suggest why a scientist would choose random sampling". The standard answer is: to avoid bias and to allow statistical analysis. If the question specifies a non-uniform habitat, the answer shifts to stratified or systematic sampling.

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Techniques for Plants

Quadrats

A quadrat is a square frame of known area (commonly 0.25 m² or 1 m²) used to delimit the sample. Inside each quadrat you can record:

• Species density — the number of individuals per unit area. Only works for countable plants.
• Species frequency — the proportion of quadrats containing a species.
• Percentage cover — the proportion of the quadrat surface covered by each species (useful for grasses and mats).

Point quadrats use a horizontal bar with vertical pins; each plant the pin touches is recorded. This gives an accurate measure of percentage cover without subjective estimation.

Transects

A transect is a line across the habitat along which samples are taken.

• Line transect — record only species touching the line at set intervals.
• Belt transect — place quadrats at intervals along the line; record species within each quadrat.
• Interrupted belt transect — belt transect with gaps (faster for long distances).
• Continuous belt transect — adjacent quadrats with no gaps (most thorough).