Sampling Techniques: Quadrats and Transects

How many daisies are there in a field? You could try to count every one — but in a large field that is impossible, and you would trample the field doing it. Instead, ecologists sample: they count organisms in a few small, carefully chosen areas and use those results to estimate the numbers across the whole habitat. This lesson, part of Topic B4 of OCR Gateway Science A, teaches the two key sampling tools — the quadrat for estimating population size and percentage cover, and the transect for measuring how a population changes along an environmental gradient — and works through every calculation you may be asked to do. This is the focus of the B4 required practical, and the maths here is reliably examined.

By the end of this lesson you should be able to explain why and how we sample, use random sampling with quadrats to estimate a population and to find percentage cover, use a transect to measure distribution along a gradient, and carry out the population-estimate, mean and percentage-cover calculations confidently.

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Why We Sample

It is rarely possible to count every organism in a habitat. A field may contain tens of thousands of plants; counting them all would take far too long and would damage the very habitat being studied. Sampling solves this: you count the organisms in several small areas, work out the average (mean) number per area, and then scale up to estimate the total across the whole habitat. As long as the sample is large enough and chosen fairly, the estimate is reliable.

The single most important rule is that sampling must be random, to avoid bias. If you only place your sampling frames where you can see lots of plants, your estimate will be far too high; if you avoid the boggy corner, you will miss the species that live there. Random sampling means every part of the habitat has an equal chance of being chosen, so the sample fairly represents the whole.

Exam Tip: "Why use random sampling?" has one key answer: to avoid bias, so the sample is representative of the whole habitat and the estimate is valid. Saying "to make it fair" is a start, but name bias and representative for full marks.

Making sampling random

To place quadrats at random, ecologists usually lay out two tape measures at right angles along two edges of the area to make a grid of coordinates, then use a random number generator to pick the coordinates for each quadrat. This removes any human choice about where the quadrats land.

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The Quadrat

A quadrat is a square frame — commonly $0.5 \text{ m} \times 0.5 \text{ m}$0.5 m×0.5 m (an area of $0.25 \text{ m}^2$0.25 m2) or $1 \text{ m} \times 1 \text{ m}$1 m×1 m (an area of $1 \text{ m}^2$1 m2) — that you place on the ground to mark out a small sample area. Inside the quadrat you either count the number of organisms (best for plants and slow-moving animals) or estimate the percentage cover of a species. Quadrats work well for plants and for slow or stationary animals such as limpets or snails, but not for fast-moving animals, which would simply move away.

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Estimating Population Size with Quadrats

The most common quadrat calculation estimates the total population of a species in a habitat. The method has two stages: find the mean number per quadrat, then scale up to the whole area.

$\text{estimated population} = \frac{\text{total area of habitat}}{\text{area of one quadrat}} \times \text{mean number per quadrat}$estimated population=area of one quadrattotal area of habitat​×mean number per quadrat

Worked Example 1 — Estimating a population

A student places ten $0.25 \text{ m}^2$0.25 m2 quadrats at random in a $200 \text{ m}^2$200 m2 field and counts the dandelions in each. The counts are: 4, 6, 3, 5, 7, 2, 4, 6, 5, 8. Estimate the total number of dandelions in the field.

Step 1 — find the total of the counts. Add them up:

$4+6+3+5+7+2+4+6+5+8 = 50$4+6+3+5+7+2+4+6+5+8=50

Step 2 — find the mean number per quadrat. Divide the total by the number of quadrats (10):

$\text{mean} = \frac{50}{10} = 5 \text{ dandelions per quadrat}$mean=1050​=5 dandelions per quadrat

Step 3 — work out how many quadrats would fit in the whole field:

$\frac{200 \text{ m}^2}{0.25 \text{ m}^2} = 800$0.25 m2200 m2​=800

Step 4 — multiply the mean per quadrat by this number to estimate the total:

$\text{estimated population} = 800 \times 5 = 4000 \text{ dandelions}$estimated population=800×5=4000 dandelions

Answer: about 4000 dandelions in the field.

Common error: forgetting that a $0.25 \text{ m}^2$0.25 m2 quadrat is smaller than $1 \text{ m}^2$1 m2, and dividing the field area by 1 instead of 0.25. Always divide by the actual area of one quadrat.

Exam Tip: Lay the population estimate out as mean per quadrat × number of quadrats that fit in the habitat. Show the mean, show the scaling factor, then multiply — method marks are awarded for the working even if the final number slips.

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Calculating the Mean

The mean is the everyday "average": add up all the values and divide by how many values there are.

$\text{mean} = \frac{\text{sum of all the values}}{\text{number of values}}$mean=number of valuessum of all the values​

Worked Example 2 — A mean per quadrat

The numbers of buttercups in six quadrats are 7, 9, 8, 6, 10 and 8. Find the mean number of buttercups per quadrat.

Step 1 — add the values:

$7+9+8+6+10+8 = 48$7+9+8+6+10+8=48

Step 2 — divide by the number of values (6):

$\text{mean} = \frac{48}{6} = 8$mean=648​=8

Answer: a mean of 8 buttercups per quadrat.

A mean is more reliable than a single quadrat because it smooths out the variation between quadrats. The more quadrats you sample, the more reliable the mean — which is why ecologists take many readings rather than one.

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Percentage Cover

For plants that are hard to count as individuals — grasses, mosses, clover spreading in mats — it is easier to estimate the percentage cover: the percentage of the quadrat's area that is covered by that species. With a gridded quadrat (say, a $10 \times 10$10×10 grid of 100 small squares), you count how many small squares the species covers and read that directly as a percentage.

$\text{percentage cover} = \frac{\text{number of squares covered by the species}}{\text{total number of squares}} \times 100\%$percentage cover=total number of squaresnumber of squares covered by the species​×100%

Worked Example 3 — Percentage cover

A quadrat is divided into a $10 \times 10$10×10 grid, giving 100 small squares. Moss covers 37 of the squares. What is the percentage cover of moss?

Step 1 — count the total number of squares: $10 \times 10 = 100$10×10=100.

Step 2 — divide the covered squares by the total and multiply by 100:

$\text{percentage cover} = \frac{37}{100} \times 100\% = 37\%$percentage cover=10037​×100%=37%

Answer: the moss covers 37% of the quadrat.

For a grid of exactly 100 squares the percentage is simply the number of squares covered, which is why 100-square quadrats are popular. If a square is only partly covered, the usual rule is to count it if the species covers more than half of that square.

Worked Example 4 — Percentage cover with a different grid

A quadrat has a $5 \times 5$5×5 grid of 25 squares. A species of grass covers 15 of them. What is the percentage cover?

Step 1 — total squares: $5 \times 5 = 25$5×5=25.

Step 2 — divide and multiply by 100:

$\frac{15}{25} \times 100\% = 60\%$2515​×100%=60%

Answer: the grass covers 60% of the quadrat.