AQA Biology 4.7.2.1 · Required practical 7 (Combined) / 9 (Biology)
Sampling techniques
Count a few random squares, find the mean, scale up to the whole field. Then walk a line from the hedge into the open and see where a species lives and why.
Combined Science · Higher tier
Start here
Every daisy.
You are asked how many daisies there are on the school field.
If you had to guess, could you really count them all, or would you count a small patch and multiply?
Required practical 7 (Combined) / 9 (Biology) · part 1
Population size with random quadrats.
Measure the population size of a common species in a habitat. A quadrat is a square frame; a transect is a line. Step through the method.
Lay two tape measures at right angles along two edges of the habitat. They are the axes. Predict: how do you choose where each quadrat goes?
A pupil says: “I threw the quadrat over my shoulder, so it landed randomly.” Is that random sampling?
Pick one.
Mean, mode and median
Three averages from one table.
Information · made-up data for practice
Daisies counted in ten 1 m² quadrats: 1, 2, 2, 2, 3, 4, 5, 6, 7, 8.
Mean: add them all, divide by how many. 40 ÷ 10 = 4 per quadrat. Mode: the most common value. Median: the middle value when they are in order; with ten values, halfway between the 5th and 6th.
What is the mode?
Pick one.
What is the median?
Pick one.
Equation · population estimate
The count in the quadrats is a sample. To estimate the whole population, find the mean number per square metre, then multiply by the habitat area in square metres. Tap the quantity you want to find.
Equation
Population estimate
Side by side: multiply.
One over the other: divide.
Tap the quantity you want to find.
Written out
population = mean per m² × habitat area
- N
- estimated population
- individuals
- m
- mean number per m²
- per m²
- A
- habitat area
- m²
With a 1 m² quadrat, the mean per quadrat is already the mean per m². With any other quadrat, two extra lines of their own:
mean per m² = mean per quadrat ÷ quadrat area
and the quadrat side is often given in centimetres. Convert the side to metres before squaring it:
50 cm = 0.5 m, so area = 0.5 × 0.5 = 0.25 m²
Required practical 7 / 9 · part 2
Distribution along a transect.
Use sampling techniques to investigate the effect of a factor on the distribution of this species. Lay a tape from the hedge into the open field. Place a quadrat at regular intervals along it (an interrupted belt transect), count the species in each, and measure an abiotic factor at each point, here light intensity with a light meter.
At 0 m from the hedge the light meter reads 150 lux and the quadrat holds 0 daisies. In the shade of the hedge, none. Plot daisies against distance, or against light, on sensible axes.
What does the transect show that the random quadrats did not?
Pick one.
Think again · the count is not the population
“The population is the number I counted.”
Your call
ChooseA pupil counts 36 daisies in total across her twelve 1 m² quadrats on a 300 m² field. What is her estimate of the field’s daisy population?
Sort
Quadrats or transect?
Place each job.
Tap a card, then tap the box it belongs in. Tap a placed card to take it back.
Command words in this lesson
- Describe
- How to estimate a population with random quadrats, or a distribution with a transect.
- Calculate
- Mean, mode, median; the population estimate.
- Explain
- Why placement must be random and why more quadrats help.
- Plot
- Species count against distance or an abiotic factor.
Key fact
Random quadrats, placed by random coordinates, give the mean number per m². Population = mean per m² × habitat area. A transect shows how distribution changes with an abiotic factor along a line.
Key note · AQA 4.7.2.1 · RP7 (Combined) / RP9 (Biology)
Sampling techniques
- Required practical 7 (Combined) / 9 (Biology): measure the population size of a common species in a habitat, and use sampling techniques to investigate the effect of a factor on the distribution of this species.
- Random quadrats: two tapes at right angles, random coordinates from a random number generator, count, repeat many times. Never throw the quadrat.
- Mean = total ÷ number of quadrats. Mode = the most common value. Median = the middle value in order.
- Population = mean per m² × habitat area. A 50 cm quadrat is 0.25 m², so mean per m² = mean per quadrat ÷ 0.25.
- Transect: a tape across the habitat, quadrats at regular intervals (interrupted belt transect), an abiotic factor measured at each. Shows distribution against the factor. Plot the data on suitable axes.
Photograph this card. Tomorrow, cover it and say each line first.
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Forgetting to convert a 50 cm quadrat?
All counts, areas and light readings are made up for practice.