Biology required practicals

Population size and distribution (sampling)

AQA 4.7 · RP9

GCSE Biology (8461) · Required practical 9 — method, variables, the marks examiners report students losing.

Verified against AQA 8461 (2026 spec)

Measure the population size of a common species using random sampling with quadrats, and investigate how a factor affects the distribution of a species using a transect.

Apparatus

  • Quadrats (e.g. 0.25 m2)
  • Two long tape measures to lay out a grid for random sampling
  • A tape measure for the transect line
  • A random-number source (e.g. calculator) and a results table

Method

  1. 1Random sampling: lay two tape measures at right angles to make a grid, then use random numbers to choose coordinates for placing the quadrat.
  2. 2Count the number of the chosen species in each quadrat, and repeat for many quadrats.
  3. 3Calculate the mean number per quadrat, then scale up to estimate the population of the whole area.
  4. 4Transect (distribution): lay a tape in a straight line across a changing area (e.g. from shade to open ground).
  5. 5Place a quadrat at regular intervals along the line and record the abundance of the species and the factor (e.g. light) at each point.

Variables

Independent

Distance along the transect (or the factor that changes, e.g. light intensity)

Dependent

Number / abundance of the species per quadrat

Control

  • Quadrat size and counting method
  • Same species being counted
  • Same time of day / conditions

Results & processing

  • Estimated population = (mean number per quadrat / area of one quadrat) x total area of the habitat.
  • Random sampling gives a fair estimate of population size; a transect shows how abundance changes across the area.
  • Plotting abundance against the factor (e.g. light level) along the transect shows any correlation.

Where students lose marks

Choosing where to put the quadrat by eye for population size.

Fix: Population size needs RANDOM sampling (random coordinates) to avoid bias; only the transect is placed deliberately.

Using too few quadrats.

Fix: Take many samples and a mean, so the estimate is reliable and less affected by uneven distribution.

Forgetting to scale up by area.

Fix: Divide the mean count by the quadrat area, then multiply by the total habitat area to estimate the whole population.

Improve the method

  • Increase the number of quadrats and take a mean to improve reliability.
  • Use the same size quadrat and counting rule throughout for a fair comparison.
  • Repeat the transect and average, and measure the factor with a meter (e.g. a light meter) for objective data.

Try it — exam-style

Medium
3 marks
ORIGINAL

A field is 200 m2. A student counts a mean of 3 daisies per 0.25 m2 quadrat. Estimate the total number of daisies in the field.

Medium
2 marks
ORIGINAL

Explain why quadrats must be placed randomly when estimating the population size of a species.

Questions are written in the style of past AQA papers — never copied from them.

Drill it properly

Ecology flashcards

Sampling, food chains and biodiversity, drilled.

Stuck on population size and distribution (sampling)?

The population estimate maths and the random-vs-transect distinction are the marks students lose — I drill both, and your first lesson is free.

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