Communities sampling techniques (incl. RP9) — AQA GCSE Biology

⚙ REQUIRED PRACTICAL 9

SPEC 4.7.2.1 • PAPER 2

Sampling Techniques

Quadrats, transects, and the formula for estimating populations. Required Practical 9 is built on these methods. Lesson 7.04.

You Can't Count Everything

Imagine a field with 50,000 dandelions. You can't count them all — it would take days. Instead, you sample: count organisms in a small representative area and scale up to estimate the total.

Two named field techniques

Quadrats — square frames placed at random locations to estimate population size.

Transects — lines laid across a habitat with quadrats at fixed intervals, to investigate how distribution changes with distance.

RP9 is the AQA-required field practical built on these two methods. Mark schemes are strict — get the method, the calculation, or the precision wrong and the data is rejected.

What You'll Be Able To Do

Objective 1

Describe how to use a quadrat for random sampling, and a transect for systematic sampling along a gradient.

Objective 2

Calculate population size from quadrat data using the spec-required formula — and recognise the maths terms (mean, mode, median).

Objective 3

Interpret sampling data from tables and graphs, and describe how distribution changes along a transect.

Key Terms

Quadrat

A square frame of known area used to sample organisms within a defined region. Commonly 0.5 m × 0.5 m (= 0.25 m²) or 1 m × 1 m (= 1 m²).

Transect

A line laid across a habitat along which organisms are sampled at fixed intervals. Used to investigate how distribution changes along a gradient.

Distribution

How organisms are spread across an area — i.e., where they are found.

Abundance

How many of a species are present — i.e., the population size, often per unit area.

The big distinction

Quadrats = random placement (for unbiased estimates). Transects = systematic placement (for gradient studies). Don't confuse them.

The Quadrat — Random Sampling

Procedure

Mark out the study area with two tape measures at right angles.

Use a random number generator to pick coordinates (this avoids bias).

Place the quadrat at those coordinates.

Count the organisms of the species inside, or estimate % cover for plants.

Repeat 10+ times with fresh random coordinates.

Calculate the mean count per quadrat.

The Transect — Systematic Sampling

Procedure

Lay a measuring tape in a straight line across the habitat.

Place a quadrat at fixed intervals (e.g., every 2 m or 5 m).

Count organisms (or estimate % cover) in each quadrat.

Record the count against the distance along the tape.

Plot a graph: distance on x-axis, count/cover on y-axis.

When To Use Each Method

QUADRAT (random)TRANSECT (systematic)

Goal: estimate total population sizeGoal: investigate how distribution changes

Placement: random (use random number generator)Placement: systematic (fixed intervals along a line)

Output: mean count → scale up to total populationOutput: graph of count vs distance

Use when: habitat is uniform; you want one number for the whole areaUse when: a gradient exists (e.g., distance from water, light/shade transition)

Mark scheme

"Quadrats are systematic" or "transects are random" → instant mark lost. The method must match the goal: random for population size; systematic for gradient studies.

Mean, Mode, Median — Spec-Required Maths

The spec requires you to understand the terms mean, mode, and median, and to calculate arithmetic means. You'll meet all three in sampling data.

Mean

Sum ÷ count

Add all values, divide by the number of values. The one you calculate for sampling abundance.

Median

Middle value

Sort in order; pick the middle value (or average the two middle values if even-numbered).

Mode

Most frequent

The value that appears most often in the dataset.

Worked example

Counts from 5 quadrats: 3, 5, 4, 7, 6.

Mean = (3 + 5 + 4 + 7 + 6) ÷ 5 = 25 ÷ 5 = 5

Median = sort to 3, 4, 5, 6, 7 → middle value = 5

Mode = no value repeats → no mode in this dataset

Calculating Population Size

The spec-required formula for scaling up sampling data to estimate the total population.

The formulaTotal population = mean per quadrat × (total area ÷ quadrat area)

Worked example

A student counts dandelions in 10 quadrats, each 0.25 m², in a field measuring 200 m². The mean count per quadrat is 8 dandelions. Estimate the total population.

Quadrat area = 0.25 m² (given)

Scaling factor = total area ÷ quadrat area = 200 ÷ 0.25 = 800

Total population = mean × scaling factor = 8 × 800 = 6 400 dandelions

Exam tip — show every step

Each line of working can earn a mark on its own. Even if the final number is wrong, you can still score for the method (the quadrat area, the scaling factor, the multiplication). Never just write the final answer on a calculate question.

RP9 — The Full Procedure

The Required Practical 9 task: "Measure the population size of a common species in a habitat. Use sampling techniques to investigate the effect of a factor on the distribution of this species."

Part A — Population Size

1

Define the study area (e.g., 10 m × 10 m).

2

Use a random number generator to choose quadrat coordinates.

3

Place quadrat, count organisms inside.

4

Repeat for ≥10 quadrats.

5

Calculate the mean count per quadrat.

6

Scale up to total population using the formula.

Part B — Effect of a Factor

1

Identify the factor (e.g., distance from water, light intensity, trampling).

2

Lay a transect across the gradient.

3

Place quadrats at fixed intervals along the transect.

4

Count organisms in each quadrat.

5

Also measure the factor at each interval (if possible).

6

Plot count (or % cover) against distance / factor value.

Validity controls

Same quadrat size throughout · same time of day · same time of year · at least 10 samples · measure the abiotic factor (if testing its effect) at each quadrat position.

Worked Example — Reading Sampling Data

Question: A student investigated the distribution of a plant species along a 20-metre transect from a pond into a field. Quadrats were placed at 5-metre intervals. The results are shown below. Describe the distribution and suggest a reason. 4 marks

1

Describe: The plant is most abundant at 10 m from the pond (5 plants). The count decreases on either side — 0 plants at the pond edge (0 m) and 2 plants at 20 m.

2

Suggest a reason: At 0 m, conditions are likely too wet / waterlogged for the plant (roots cannot respire). At 20 m, conditions are likely too dry (water becomes limiting). At 10 m, soil moisture is at an optimal level for the species.

Mark scheme map

Four creditable points: (1) identify the peak with data (10 m, 5 plants); (2) describe the trend on either side; (3) name a specific abiotic factor (moisture / water); (4) link both extremes to the factor (too wet / too dry). Quote the numbers from the graph — "5 plants at 10 m" not just "more plants".

Examiner Alerts

Alert 1

Random vs systematic — get the right one. Quadrats placed using random numbers = random sampling (for population estimates). Transects with fixed intervals = systematic sampling (for gradient studies). Examiner reports flag confusion between these as the most common RP9 error.

Alert 2

Match the units. In the population formula, quadrat area and total area must be in the same units (both m², or both cm²). If the quadrat is given as 50 cm × 50 cm and the field as 200 m², convert first.

Alert 3

Quote the data when describing graphs. "There were more plants in the middle" earns zero. "There were 5 plants at 10 m, falling to 2 plants at 20 m" earns the mark. Mark schemes reward specific values, not vague trends.

Common Errors

Wrong

"Place the quadrats where you see the most plants."

Right

Place quadrats at random coordinates using a random number generator — this avoids sampling bias.

Wrong

"Take 1 or 2 samples; that's enough."

Right

Take at least 10 samples to give a reliable mean.

Wrong

"Total population = mean × total area" (forgetting to divide by quadrat area).

Right

Total population = mean × (total area ÷ quadrat area). The scaling factor accounts for how many quadrats fit in the whole area.

Wrong

"Transects are placed randomly across the habitat."

Right

Transects are systematic — a single line with quadrats at fixed intervals.

Your Turn

Question:

500 m²

10 quadrats

0.5 m × 0.5 m

4, 6, 5, 7, 3, 6, 8, 5, 4, 7

(a)

1 mark

(b)

3 marks

(a)

Mean count:

Sum = 4 + 6 + 5 + 7 + 3 + 6 + 8 + 5 + 4 + 7 = 55

Mean = 55 ÷ 10 = 5.5 plants per quadrat ✓ (1 mark)

(b)

Total population:

Quadrat area = 0.5 × 0.5 = 0.25 m² ✓ (1 mark)

Scaling factor = total area ÷ quadrat area = 500 ÷ 0.25 = 2 000 ✓ (1 mark)

Total population = mean × scaling factor = 5.5 × 2 000 = 11 000 clover plants ✓ (1 mark)

✓

Total: 4/4 marks. Each step is creditable — show all your working.

Recap — Three Takeaways

Two named methods.

Quadrats = random placement, for population estimates. Transects = systematic intervals along a line, for gradient studies.

The formula.

Total population = mean per quadrat × (total area ÷ quadrat area). Show every step; matching units; use the mean.

Quote the data.

For graph questions, name specific numbers from the data — "5 plants at 10 m" not "more in the middle".

Next lesson

7.05 — Levels of Organisation

Producers, primary consumers, secondary and tertiary consumers, predator-prey cycles, food chains. Spec 4.7.2.1, Paper 2.

Lesson Summary Card

Key Definitions

Quadrat = square frame of known area; used for random sampling .

Transect = line across a habitat; used for systematic sampling along a gradient.

Distribution = where organisms are found across an area.

Abundance = how many organisms (population size).

The Population Formula

Total = mean per quadrat × (total area ÷ quadrat area)

Maths Skills (Spec Required)

Mean = sum ÷ count (the one you calculate for sampling abundance)

Median = middle value when sorted

Mode = most frequent value

Top exam tips

For RP9 questions: random for population, systematic for gradient. Match the units. Quote specific numbers from graphs and data tables. Show every step in calculations.