Core Practical: Investigating Population Size Using Quadrats and Transects

This lesson covers the core practical on investigating population size using quadrats and transects as required by the Edexcel GCSE Combined Science specification (1SC0). You need to understand how to use random sampling and systematic sampling to estimate population size, calculate population estimates and evaluate the reliability of your results.

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Why Sample?

It is usually impossible to count every organism in a habitat. Instead, scientists take samples — small representative portions of the habitat — and use the data to estimate the total population.

For sampling to be valid:

• It must be representative of the whole area
• It must avoid bias (personal choice about where to sample)
• Multiple samples must be taken to ensure reliability

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Equipment

Equipment	Purpose
Quadrat	A square frame (usually 0.25 m² or 1 m²) placed on the ground to define a sample area
Measuring tape	Used to set up transect lines or measure distances in the sampling area
Random number generator	Produces random coordinates to decide where quadrats are placed (avoids bias)
Identification key	Used to identify the species found in each quadrat
Recording sheet	To record species and numbers found in each sample

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Method 1: Random Sampling with Quadrats

Random sampling is used to estimate the population size of a species across a whole area.

Procedure

Step	Action
1	Lay two measuring tapes at right angles along two edges of the study area to create an x-axis and y-axis
2	Use a random number generator to produce pairs of coordinates (x, y)
3	Place the quadrat at each pair of random coordinates
4	Count the number of the target species inside the quadrat (or estimate percentage cover for plants)
5	Repeat for at least 10 quadrats (more is better for reliability)
6	Calculate the mean number of organisms per quadrat
7	Scale up to estimate the total population for the whole area

graph TD
    A["Set up x and y axes with measuring tapes"] --> B["Generate random coordinates"]
    B --> C["Place quadrat at coordinates"]
    C --> D["Count target species in quadrat"]
    D --> E["Record results"]
    E --> F{"More quadrats needed?"}
    F -->|"Yes"| B
    F -->|"No"| G["Calculate mean per quadrat"]
    G --> H["Estimate total population"]

Calculating Population Estimate

Step 1: Calculate the mean number of organisms per quadrat.

Mean = total number of organisms counted ÷ number of quadrats

Step 2: Scale up to the total area.

Estimated population = mean per quadrat × (total area ÷ area of one quadrat)

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Worked Example

A student uses a 0.25 m² quadrat to sample daisies in a field measuring 200 m². They place 10 quadrats and count the following:

Quadrat	Number of daisies
1	3
2	5
3	2
4	4
5	6
6	3
7	4
8	5
9	2
10	6

Step 1: Mean

Total daisies = 3 + 5 + 2 + 4 + 6 + 3 + 4 + 5 + 2 + 6 = 40

Mean = 40 ÷ 10 = 4 daisies per quadrat

Step 2: Estimate total population

Number of quadrats that fit in the field = 200 ÷ 0.25 = 800

Estimated population = 4 × 800 = 3,200 daisies

Exam Tip: Always show your working in population estimate calculations. Write the mean clearly, then show the scaling-up calculation. Common mistakes include forgetting to divide by the quadrat area.

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Percentage Cover

For species that are hard to count individually (e.g. grass, moss), use percentage cover — estimate what percentage of the quadrat is covered by that species.

Species	Quadrat 1	Quadrat 2	Quadrat 3	Mean
Grass	70%	65%	75%	70%
Moss	10%	15%	5%	10%
Bare soil	20%	20%	20%	20%

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Method 2: Belt Transect

A belt transect is a systematic sampling method used to investigate how species distribution changes across a habitat (e.g. from a pond edge to open grassland, or from the bottom to the top of a rocky shore).

Procedure

Step	Action
1	Lay a measuring tape in a straight line across the area of interest (the transect line)
2	Place a quadrat at regular intervals along the tape (e.g. every 2 metres)
3	At each position, count the species or estimate percentage cover
4	Record the results and the distance along the transect
5	Look for patterns or trends in species distribution related to distance

When to Use a Transect

Use a transect when you want to investigate the effect of a changing environmental factor on species distribution. Examples:

Scenario	Environmental gradient
Shore to inland	Distance from the sea; exposure to salt spray
Pond edge to dry grassland	Soil moisture, light levels
Path edge to woodland interior	Trampling, light intensity
Base to top of a hill	Altitude, temperature, wind exposure

graph LR
    A["Start of transect (e.g. pond edge)"] -->|"Quadrat every 2m"| B["0m"]
    B --> C["2m"]
    C --> D["4m"]
    D --> E["6m"]
    E --> F["8m"]
    F --> G["End of transect (e.g. dry grassland)"]

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Variables

Variable type	In this practical
Independent	Position of the quadrat (random coordinates or distance along transect)
Dependent	Number of organisms (or percentage cover) found in each quadrat
Control	Same quadrat size, same method of counting, same time of day, same habitat area

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Improving Reliability and Validity

Strategy	How it helps
Use random sampling	Avoids bias — no personal choice about where to place quadrats
Take more samples	Larger sample size gives a more reliable mean and a better estimate
Repeat the investigation	Check for consistency; calculate a mean across repeats
Use the same quadrat size	Ensures fair comparison between samples
Sample at the same time of day	Some organisms are more active at certain times; keeps conditions consistent
Identify species correctly	Use an identification key to avoid miscounting

Exam Tip: If asked how to improve the investigation, the best answer is usually "increase the number of quadrats" or "use more random coordinates". This increases the sample size and makes the estimate more reliable.

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Sources of Error