The specification says: investigate the population size of an organism in two different areas using quadrats
Aim
To compare the population size of a plant species in two different areas by sampling with quadrats.
Background
A population is all the organisms of one species living in an area. It is usually impossible to count every individual, so ecologists take a sample and use it to estimate the whole population.
A quadrat is a square frame that marks out a small, known area. To avoid bias, quadrats are placed at random, for example by using random numbers as coordinates on a grid made from two tape measures. Counting in many quadrats and calculating a mean makes the estimate more reliable.
The number of a plant in an area depends on abiotic factors such as light intensity and water, and on biotic factors such as competition. Comparing two areas that differ in one factor, such as a shaded area under a tree and an open, sunny area, shows how that factor may affect the population size.
Population size can be estimated using: population size = mean number per quadrat ÷ area of one quadrat × total area of the habitat.
Hypothesis
There will be more daisies in the open, sunny area than in the shaded area, because daisies need light for photosynthesis.
Variables
| Independent | The area sampled (shaded area under a tree or open sunny area) |
|---|---|
| Dependent | The number of daisies in each quadrat, used to calculate the mean number per quadrat |
| Control |
|
Equipment
- Quadrat, 0.5 m × 0.5 m (area 0.25 m²)
- 2 tape measures (10 m or longer)
- Random number table, or a calculator or phone random number generator
- Identification chart or key for the species
- Clipboard, pencil and recording table
- Calculator
Risk assessment
| Hazard | Risk | Precaution |
|---|---|---|
| Uneven or wet ground and tape measures | Tripping or slipping | Wear suitable footwear, look where you are walking and keep tape measures tidy. |
| Soil, plants and animal droppings | Infection or skin irritation | Do not touch plants you cannot identify, do not put hands near the mouth, and wash hands after the fieldwork. |
| Weather and sun | Sunburn or getting cold | Wear suitable clothing and sun cream if needed, and follow the teacher’s instructions. |
Method
- Choose the two areas to compare, for example a shaded area under a tree and an open, sunny area of the same lawn. Measure the size of each area (in this example each is 10 m × 10 m, or 100 m²).
- In the first area, lay one tape measure along one side and another at right angles along an adjacent side to make a grid.
- Use a random number generator or table to choose two numbers, one for each tape measure, for example 4.0 m and 7.5 m. These are your coordinates.
- Place the quadrat on the ground with its corner at that point.
- Count the number of daisies inside the quadrat. Count a plant only if more than half of it is inside the frame.
- Record the number in the results table.
- Repeat steps 3 to 6 until you have 10 quadrats in the first area.
- Repeat the whole procedure in the second area, using the same size of quadrat, the same number of quadrats and the same species.
- Add up the counts for each area and calculate the mean number of daisies per quadrat.
- Calculate the estimated population size in each area using: population size = mean per quadrat ÷ 0.25 × total area (m²).
- Compare the two areas. Wash your hands.
Results
Fill this table in as you go. Print the PDF for a copy to write on.
| Quadrat number | Number of daisies in shaded area | Number of daisies in open area |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 | ||
| 4 | ||
| 5 | ||
| 6 | ||
| 7 | ||
| 8 | ||
| 9 | ||
| 10 | ||
| Total | ||
| Mean per quadrat | ||
| Estimated population in 100 m² |
Drawing the graph
Bar chart of the mean number of daisies per quadrat (y-axis, from 0, labelled ‘Mean number of daisies per quadrat’) for each of the two areas (x-axis, categoric), with gaps between the bars.
Example results and answersPractice data, conclusion, errors and 10 exam questions (27 marks) with mark schemes
Example results
| Quadrat number | Number of daisies in shaded area | Number of daisies in open area |
|---|---|---|
| 1 | 1 | 4 |
| 2 | 0 | 6 |
| 3 | 2 | 3 |
| 4 | 1 | 5 |
| 5 | 3 | 7 |
| 6 | 0 | 4 |
| 7 | 1 | 5 |
| 8 | 2 | 6 |
| 9 | 1 | 3 |
| 10 | 1 | 5 |
| Total | 12 | 48 |
| Mean per quadrat | 1.2 | 4.8 |
| Estimated population in 100 m² | 480 | 1920 |
Conclusion
The mean number of daisies per quadrat was 1.2 in the shaded area and 4.8 in the open area, so there were four times as many daisies in the open area. Estimated population in a 100 m² area: shaded = 1.2 ÷ 0.25 × 100 = 480; open = 4.8 ÷ 0.25 × 100 = 1920. This supports the hypothesis. Daisies in the open receive more light, so they can photosynthesise faster and grow and reproduce more. However, the areas may also differ in other factors such as soil moisture, which could also affect the number of daisies, so the investigation shows a link, not proof that light is the cause.
Errors and improvements
| Error | Effect on the results | Improvement |
|---|---|---|
| Only 10 quadrats are used, so by chance they may land on unusually dense or sparse patches (random error) | The mean may not represent the whole area and the population estimate may be inaccurate | Use more quadrats and calculate a mean, or use a larger sample area. |
| Quadrats are placed by hand without using random numbers, for example on areas that look interesting (bias, systematic error) | The sample is not representative, so the results are biased | Use random numbers as coordinates on the grid for every quadrat. |
| Plants are hard to count or identify, for example small daisies hidden by grass or plants on the edge of the quadrat | Counts may be too low or inconsistent between people | Use the same person and a rule for plants on the edge, use an identification chart, and for dense plants estimate the percentage cover instead. |
| The two areas differ in more than one way, for example soil moisture and trampling as well as light | The difference in population size cannot be put down only to light, so the conclusion is less valid | Choose areas that are as similar as possible apart from the factor studied, and measure other factors such as light and soil moisture in each. |
Exam questions
10 questions, 27 marks. Write your answers on paper, then open each mark scheme.
Question 1
Explain why the quadrats were placed at random.
Show mark scheme for question 1
- to avoid bias (1)
- so that the sample is representative of the whole area (1)
Question 2
The numbers of dandelions in five quadrats were 3, 5, 4, 6 and 2. Calculate the mean number of dandelions per quadrat.
Show mark scheme for question 2
- 3 + 5 + 4 + 6 + 2 = 20 (1)
- 20 ÷ 5 = 4 (1)
Question 3
A field has an area of 400 m². Each quadrat has an area of 0.25 m². The mean number of dandelions per quadrat was 4. Estimate the number of dandelions in the field.
Show mark scheme for question 3
- 4 ÷ 0.25 = 16 per m² (1)
- 16 × 400 (1)
- 6400 (1)
Question 4
Explain why 10 quadrats were used in each area instead of one.
Show mark scheme for question 4
- to reduce the effect of chance / of one unusual quadrat (1)
- so that the mean is more reliable / representative of the area (1)
Question 5
Give three variables that must be kept the same when comparing the number of daisies in two areas.
Show mark scheme for question 5
- size of quadrat (1)
- number of quadrats (1)
- the species counted (1)
- person counting / rule for plants on the edge / time of year (1)
- Max 3
Question 6
The shaded area had fewer daisies than the open area. Suggest an explanation for this.
Show mark scheme for question 6
- less light in the shade (1)
- so less photosynthesis (1)
- so less glucose for growth and reproduction / fewer seeds (1)
Question 7
Suggest one abiotic factor, other than light intensity, that could differ between the two areas and describe how you would measure it.
Show mark scheme for question 7
- soil moisture / soil pH / temperature (1)
- a suitable method, for example a moisture meter, pH probe or thermometer used in the same way in each area (1)
Question 8
Where plants are very small and close together they are difficult to count. Suggest another way of measuring the abundance of the plants in a quadrat.
Show mark scheme for question 8
- estimate the percentage cover of the species in the quadrat (1)
- for example by counting the squares of a gridded quadrat that the plant covers (1)
Question 9
The mean number of daisies per quadrat was 1.2 in the shaded area and 4.8 in the open area. Calculate how many times greater the mean was in the open area.
Show mark scheme for question 9
- 4.8 ÷ 1.2 (1)
- 4 (times) (1)
Question 10
A student wants to compare the number of dandelions in two fields. Describe how the student could use quadrats to do this.
Show mark scheme for question 10
| Level | Marks | What the answer does |
|---|---|---|
| 3 | 5–6 | A clear, ordered method including random placement of quadrats using coordinates, a suitable number of quadrats, counting, calculating a mean for each field and comparing them, with control of variables |
| 2 | 3–4 | A method with random sampling and counting in both fields but with some steps, detail or ordering missing |
| 1 | 1–2 | A simple or partial method, for example placing quadrats and counting with little detail |
Indicative content
- mark out a grid with two tape measures in each field
- use random numbers as coordinates to place the quadrat
- count the number of dandelions in each quadrat
- use the same size of quadrat and the same number of quadrats (at least 10) in each field
- calculate the mean number per quadrat for each field
- compare the means, or estimate the population using mean ÷ area of quadrat × area of field
Exam tips
Written and checked against the Edexcel IGCSE Biology (4BI1) specification · Updated October 2026