Quadrat Sampling Virtual Lab: Estimate a Population

Updated 2026-10-06

This quadrat sampling virtual lab matches the AQA GCSE Biology required practical on field investigations. Students use random quadrats to estimate the population of a plant species, then run a belt transect from woodland shade into open sun to see how light changes where species grow. Unlike a real field, the simulation also shows the true count. Students can see how close their estimate is and how the number of quadrats and the clumping of a species affect reliability. It works on a rainy day, in winter, or as practice before real fieldwork.

Quadrat Sampling – Estimating Population Size

Curriculum links

AQA GCSE Biology 4.7.2.1 (levels of organisation) and its required practical: measure the population size of a common species in a habitat, and use sampling to investigate the effect of a factor on the distribution of a species. It is also a required practical in Combined Science: Trilogy.
NGSS HS-LS2-2: use mathematical representations to support explanations about biodiversity and populations.

Simulic is not affiliated with or endorsed by the College Board or AQA.

Before the lab (5 min)

Ask students to commit to a prediction, on paper or as question 1 of the class link:

"A line of quadrats runs from deep woodland shade into open sun. How will the number of daisies per quadrat change along it?"

Most students say "more in the sun", but some expect the most in the middle or no pattern. Ask them to name the abiotic factor that changes along the line.

Method in the simulation

The field is 20 m × 12 m (240 m²). It contains daisies, clumped dandelions, plantain, ferns and grass. All the numbers below are for the field the simulation opens with. New field builds a different one, so tell students not to press it until they finish.

Part A: estimate the population with random quadrats

Set Habitat to Woodland shade to open sun, Sampling method to Random, Species to Daisy.
Set Quadrat side to 2 m and Number of quadrats to 40. Press Sample automatically.
Record the mean per quadrat and the estimated N, calculated as N = mean per quadrat × 240 m² ÷ 4 m².
Press Clear quadrats and repeat twice. Then do the same with Dandelion.

Species	Run	Mean per quadrat	Estimated N	True count
Daisy	1 / 2 / 3
Dandelion	1 / 2 / 3

Part B: belt transect

Set Sampling method to Along a transect, Quadrat side 2 m, Number of quadrats 10, and press Sample automatically. The ten quadrats touch, so they form a belt from 0 to 20 m.
Switch Species between Daisy and Fern and record the number in each quadrat.

Distance (m)	0–2	2–4	4–6	6–8	8–10	10–12	12–14	14–16	16–18	18–20
Daisies
Ferns

Expected results

Part A. The true counts are 141 daisies and 142 dandelions. In 200 runs of 40 random 2 m quadrats, daisy estimates mostly fell between 104 and 179. Dandelion estimates ranged far more widely, from 80 to 269, because dandelions grow in clumps. A quadrat either hits a clump or misses it. With 1 m quadrats, the daisy estimate settles as the number of quadrats rises: the spread (standard deviation) was 84 plants with 5 quadrats, 53 with 10, 41 with 20 and 32 with 40.

Part B. The transect is not random, so every class gets these counts:

Distance (m)	0–2	2–4	4–6	6–8	8–10	10–12	12–14	14–16	16–18	18–20
Daisies	0	0	1	2	2	3	5	2	1	7
Ferns	9	1	4	5	3	1	1	0	0	0

The three shade quadrats hold 14 ferns and 1 daisy, and the three sun quadrats hold 0 ferns and 10 daisies. The counts jump around from quadrat to quadrat, but the trend is clear. Simpson's index from the quadrats is 0.32 under the trees and 0.66 in both part shade and open sun.

Questions for students

(Prediction, asked again after the lab) How does the number of daisies per quadrat change from shade into sun?
In the transect, what is the independent variable?
With 40 random 2 m quadrats, what is your estimate of the daisy population?
Which statement fits the transect data for daisies and ferns?
Why do dandelion estimates vary more than daisy estimates, and how could you make an estimate more reliable?

Answers for teachers: (1) It increases toward the sun. (2) Distance along the transect, which stands for light level. (3) Any value from 95 to 195 (true count 141). (4) Ferns are most common in the shade and absent in full sun; daisies show the opposite pattern. (5) Dandelions are clumped, so each quadrat either hits a clump or misses. Use more quadrats, place them at random coordinates, and repeat and average.

Common misconceptions

"The estimate should equal the true count." Sampling always has error. The aim is an estimate close enough to compare habitats.
"Place the quadrat where the plants are." Choosing spots biases the estimate. Tap the field to place quadrats by hand and compare the result with random placement.
"A transect gives the population size." A transect shows how a species is distributed. It is not random, so it should not be scaled up to the whole field.

Extension

Choose Meadow with a trampled path and run the same 2 m transect across it. Plantain peaks on the path (4 and 6 plants in the two path quadrats), while daisies fall to 0 and 2 there. Ask which species tolerates trampling, and why.
Compare quadrat size: 40 quadrats of 0.5 m versus 40 quadrats of 2 m. Which gives the steadier estimate, and what is the cost in fieldwork time?

FAQ

Why do my students get different estimates?

Random quadrats land in different places every time, so every estimate is different. That is the point: collect the class results and compare their spread with the true count.

Why does the transect give the same counts for everyone?

Transect quadrats sit at fixed distances along the line, and the field is the same until someone presses New field. That makes Part B easy to check.

Can I use this instead of real fieldwork?

Use it to teach the method and the maths. If your course requires a field visit, it is a good rehearsal: students arrive knowing why quadrats must be random.