Sampling and Fieldwork Techniques

To study the distribution and abundance of organisms in the real world, ecologists use sampling techniques. You cannot count every single organism in an ecosystem, so you take a sample and use it to estimate the total population. This lesson covers the key sampling methods you need for AQA GCSE Biology, including the Required Practical on using quadrats and transects.

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Why Do We Sample?

It is usually impossible to count every organism in a habitat because:

• The habitat may be too large
• Organisms may move around (animals)
• Counting every individual would take too long and be too expensive
• The process of counting might disturb the habitat

Instead, ecologists take representative samples and use them to make estimates about the whole population. For results to be reliable, samples must be:

Requirement	What It Means
Representative	The sample must reflect the conditions of the whole area
Random	Every part of the area must have an equal chance of being sampled
Sufficient	Enough samples must be taken to give reliable data (reducing the effect of anomalies)
Repeatable	Others should be able to follow the same method and get similar results

Exam Tip: If asked why random sampling is important, say: "So that the results are representative and not biased towards areas chosen by the investigator." This is a very common 1- or 2-mark question.

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Required Practical: Quadrats and Transects

What Is a Quadrat?

A quadrat is a square frame (usually 0.5 m x 0.5 m = 0.25 m2, or 1 m x 1 m = 1 m2) that is placed on the ground to sample the organisms within it. Quadrats are most commonly used to study plants and slow-moving organisms such as limpets or barnacles.

Using Quadrats for Random Sampling

To carry out random sampling with quadrats:

1. Lay out two long tape measures at right angles along two edges of the study area, creating an x-axis and a y-axis
2. Generate random coordinates using a random number generator (or random number table)
3. Place the quadrat at each set of random coordinates
4. Count or estimate the organisms within the quadrat
5. Repeat at least 10 times (more samples = more reliable data)
6. Calculate the mean number of organisms per quadrat
7. Scale up to estimate the total population in the whole area

Estimating Percentage Cover

For organisms that are difficult to count individually (like grass or moss), you estimate the percentage cover — the proportion of the quadrat area covered by that species.

Method	How It Works
Visual estimation	Look at the quadrat and estimate the percentage covered by each species
Point frame quadrat	A frame with pins that are lowered onto the quadrat; the species touching each pin is recorded

Calculating Population Size

To estimate the total population of a species in a study area:

Estimated population = mean number per quadrat x (total area / area of one quadrat)

Worked Example

A student uses a 0.5 m x 0.5 m quadrat (area = 0.25 m2) and counts dandelions in 10 random quadrats. The results are:

Quadrat	1	2	3	4	5	6	7	8	9	10
Count	3	5	2	4	6	3	4	5	3	5

• Mean count per quadrat = (3+5+2+4+6+3+4+5+3+5) / 10 = 40 / 10 = 4 dandelions per quadrat
• Total study area = 2,000 m2
• Estimated population = 4 x (2,000 / 0.25) = 4 x 8,000 = 32,000 dandelions

Exam Tip: Always show your working when calculating population estimates. Write out the formula, substitute the values, and calculate step by step. You can gain method marks even if your arithmetic is wrong.

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Transects

A transect is a line along which organisms are sampled. Transects are used when you want to study how the distribution of organisms changes across an area — for example, from a pond edge to dry grassland, or from the bottom of a seashore to the top.

Types of Transect

Type	Method
Line transect	A tape measure is laid out and organisms touching or crossing the line are recorded
Belt transect	A tape measure is laid out and quadrats are placed at regular intervals along the line
Interrupted belt transect	Quadrats are placed at set intervals (e.g. every 2 m) along the transect line
Continuous belt transect	Quadrats are placed continuously, one after another, along the entire transect length

graph LR
    A["Start of transect<br/>(e.g. pond edge)"] --> B["Quadrat at 0 m"]
    B --> C["Quadrat at 2 m"]
    C --> D["Quadrat at 4 m"]
    D --> E["Quadrat at 6 m"]
    E --> F["Quadrat at 8 m"]
    F --> G["End of transect<br/>(e.g. dry grassland)"]
    style A fill:#bbdefb,stroke:#1565c0
    style G fill:#fff9c4,stroke:#f9a825

When to Use a Transect Instead of Random Quadrats

Use a transect when you expect the distribution of organisms to change in response to an environmental gradient — a gradual change in an abiotic factor. Examples include:

• Distance from a path — trampling affects plant distribution
• Distance from a pond or stream — moisture levels change
• Height up a seashore — exposure time changes with tidal zone
• Distance into a woodland — light intensity decreases
• Altitude up a hillside — temperature and wind exposure change

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Measuring Abiotic Factors During Fieldwork

When sampling organisms, it is important to also measure the abiotic factors in each sampling location. This allows you to correlate changes in species distribution with changes in the environment.

Abiotic Factor	Equipment Used	How to Measure
Light intensity	Light meter (lux meter)	Hold sensor at ground level, record reading in lux
Temperature	Thermometer or temperature probe	Place at soil surface or required height
Soil pH	pH probe or universal indicator with soil sample	Mix soil with distilled water, test with probe/indicator
Soil moisture	Moisture meter	Push probe into soil, record reading
Wind speed	Anemometer	Hold in the air, record reading in m/s
Humidity	Hygrometer	Hold in the air, record percentage humidity

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Sampling Animals: Capture-Recapture Method

For mobile animals that cannot be sampled with quadrats, ecologists use the capture-recapture (or mark-recapture) method to estimate population size.

Method

1. Capture a sample of animals from the habitat
2. Count them and mark them in a harmless way (e.g. a small dot of non-toxic paint)
3. Release them back into the habitat
4. Wait a suitable period for the marked animals to mix back into the population
5. Recapture a second sample from the same habitat
6. Count the total number in the second sample and how many are marked

The Lincoln Index Formula

Estimated population = (number in first sample x number in second sample) / number of marked individuals in second sample

Or written more concisely:

N = (M x S) / R

Where:

• N = estimated total population
• M = number marked and released in the first sample
• S = total number caught in the second sample
• R = number of marked (recaptured) individuals in the second sample

Worked Example

• First sample: 40 woodlice captured, marked and released
• Second sample: 50 woodlice captured, of which 10 are marked