What you'll learn
This required practical is about measuring how living things are spread out in a habitat and how many there are. For AQA GCSE Biology you need to know how to use a quadrat to estimate the number of organisms in an area, how to use a transect to investigate how a species changes across a habitat, and how to turn sample data into an estimate for a whole field. This guide explains the equipment, the method for random and systematic sampling, how to calculate a mean and scale up to a population estimate, and the common errors that lose marks. By the end you should be able to describe the method precisely and carry out the calculations from a set of quadrat readings.
Key terms and definitions
Population — All the organisms of one species living in a particular habitat.
Community — All the populations of different species living together in a habitat.
Quadrat — A square frame, often 0.5 m by 0.5 m, placed on the ground to sample the organisms inside it.
Transect — A line across a habitat along which samples are taken to study how a species changes with distance.
Random sampling — Taking samples at randomly chosen positions so the results are not biased and fairly represent the whole area.
Systematic sampling — Taking samples at regular intervals, for example along a transect, to study a gradual change.
Abundance — How common an organism is in a habitat, measured as a count or as percentage cover.
Percentage cover — The proportion of a quadrat covered by a particular species, estimated as a percentage.
Core concepts
Why we sample
It is impossible to count every organism in a large habitat, so we count the organisms in small sample areas and use these to estimate the total. The key idea is that a fair sample represents the whole area, so the method of choosing where to place the quadrats matters as much as the counting itself.
Using a quadrat for random sampling
To estimate how many of a species live in a field, you use random sampling to avoid bias:
- Lay out two long tape measures at right angles along two edges of the field to make a grid with coordinates.
- Use a random number generator to produce pairs of coordinates.
- Place the quadrat at each coordinate and count the number of the chosen organism inside it (or estimate its percentage cover).
- Repeat for many quadrats — the more samples, the more reliable the estimate.
- Calculate the mean number per quadrat, then scale up to the whole area.
Choosing positions randomly is essential: if you placed quadrats only where the plant looked common, you would overestimate the population.
Scaling up to a population estimate
Once you have the mean number of organisms per quadrat, you scale up using the ratio of the total area to the quadrat area:
Estimated population = mean number per quadrat × (total area ÷ area of one quadrat)
For example, if the mean is 5 daisies per 0.25 m² quadrat and the field is 1000 m², the estimate is 5 × (1000 ÷ 0.25) = 5 × 4000 = 20 000 daisies.
Using a transect for distribution
A transect investigates how a species changes across a habitat, for example from the edge of a pond to dry land, where conditions change gradually. You lay a tape measure in a straight line across the area and place a quadrat at regular intervals — this is systematic, not random, because you deliberately want to sample the whole gradient. At each point you record the abundance or percentage cover of the species and often an abiotic factor such as light or moisture. Plotting abundance against distance shows how the species is distributed and lets you link changes to the environment.
Percentage cover
For plants that are hard to count as individuals, such as grass or moss, you estimate percentage cover instead — the fraction of the quadrat filled by that species, judged to the nearest 5% or 10%. This is quicker and avoids the problem of deciding where one plant ends and another begins. A quadrat divided into a grid of 100 small squares makes this easier: you count how many small squares the plant covers, and that number is roughly the percentage cover.
Biotic and abiotic factors
The way organisms are distributed in a habitat is affected by two kinds of factor. Abiotic factors are non-living conditions such as light intensity, temperature, soil moisture, and mineral content. Biotic factors are living influences such as competition for food, predators, and disease. When you use a transect and record an abiotic factor alongside the abundance of a species, you can look for a link between the two — for example, a plant that grows more in the light and less in the shade of a hedge. Being able to explain a distribution in terms of a named abiotic factor is exactly what the higher-mark questions reward.
Making the investigation valid
An investigation is only trustworthy if it is fair. To make the sampling valid, you should keep the size of the quadrat the same throughout, sample enough quadrats to reduce the effect of chance, and use random positions when estimating population size. When comparing two areas, such as a mown lawn and an unmown field, only the area being studied should differ — the quadrat size and method must stay the same. These points about validity and reliability are frequently examined, so it helps to be able to state clearly what makes the method fair.
Worked examples
Example 1: Calculating a mean
A student places ten quadrats and records these numbers of dandelions: 4, 6, 5, 3, 7, 5, 4, 6, 5, 5. What is the mean per quadrat? Add them: 4+6+5+3+7+5+4+6+5+5 = 50. Divide by the number of quadrats: 50 ÷ 10 = 5 dandelions per quadrat.
Example 2: Estimating a total population
Using the mean of 5 dandelions per 0.25 m² quadrat from Example 1, estimate the number of dandelions in a 200 m² field. Estimated population = 5 × (200 ÷ 0.25) = 5 × 800 = 4000 dandelions. Always show the scaling ratio clearly.
Example 3: Choosing a sampling method
A student wants to know whether a plant grows more in the shade of a hedge than in the open. Which method should they use and why? They should use a transect (systematic sampling), because they want to study how the plant changes across a gradient — from shaded to open ground. Random sampling would not show the pattern with distance from the hedge.
Example 4: Improving reliability
A student takes only three quadrat readings and gets very different numbers each time. How could they make their estimate more reliable? They should take many more quadrats. A larger number of random samples reduces the effect of chance variation between quadrats and gives a mean that better represents the whole field.
Common mistakes and how to avoid them
The most common error is not sampling randomly. If you choose where to put the quadrat, you bias the result. Always describe using random coordinates from a random number generator when the question is about estimating a population.
Students often forget to divide by the correct quadrat area when scaling up. Check the units: if the quadrat is 0.5 m by 0.5 m, its area is 0.25 m², not 0.5 m². Using the wrong area gives an answer that is out by a factor of two or more.
Another mistake is confusing when to use random and systematic sampling. Use random sampling to estimate how many there are across a uniform area; use a transect (systematic) to study how a species changes across a gradient. Picking the wrong one loses method marks.
Be careful with reliability versus accuracy. Taking more quadrats improves reliability (reduces the effect of chance). It is not the same as making the counting more accurate.
Finally, do not write that a quadrat "counts all the plants in the field". A quadrat only samples a small area; the whole-field number is an estimate based on scaling up.
Exam technique for "Required practical: population sampling and distribution"
When asked to describe how to estimate a population size, give an ordered method: set up a grid with tape measures, generate random coordinates, place quadrats, count the organisms, repeat many times, find the mean, and scale up. Naming "random number generator" and "mean" earns marks that vague answers miss.
Calculation questions are common and are usually worth several marks. Always show your working: the mean first, then the scaling ratio (total area ÷ quadrat area), then the final estimate. Include units.
If a question describes a gradient, such as distance from a path or a pond, recognise that a transect is the right method and mention recording an abiotic factor alongside abundance so you can explain the pattern. Practise both the calculation and the written method, because either can come up.
Quick revision summary
- Use a quadrat to sample organisms in a small area and estimate the total by scaling up.
- Use random sampling (random coordinates) to estimate population size fairly across a uniform area.
- Estimated population = mean per quadrat × (total area ÷ quadrat area); check the quadrat area in m².
- Use a transect (systematic sampling) to study how a species is distributed across a gradient.
- Estimate percentage cover for plants that are hard to count individually.
- Take many samples to improve reliability; the whole-habitat figure is always an estimate.