How do you do the quadrat and transect required practical?

What does the field investigations practical investigate?

It has two parts. First, you estimate the population size of a common species in a habitat, such as daisies on a school field, by counting it in a sample of quadrats. Then you use sampling to see how a factor, such as light intensity, affects the distribution of that species: where it is found, and how many there are in each place.

A quadrat is a square frame, often 50 cm by 50 cm, that marks out a small area to count in. A transect is a line, usually a long tape measure, that you sample along.

It is one of AQA's required practicals in both GCSE Biology and Combined Science: Trilogy, in the ecology topic.

What equipment do you need?

The basics are a quadrat and a way to put it down at random: two tape measures to make a grid, and random numbers to pick coordinates. For the distribution part, add a meter for the factor you are measuring, such as a light meter.

• A quadrat, for example 50 cm by 50 cm
• Two long tape measures, to mark out a grid along two edges of the area
• A random number generator, such as a calculator or an app
• Another long tape measure for the transect
• A light meter, or a meter for whichever factor you are testing
• A picture or key to identify the species
• A clipboard and a results table

What is the method?

For the population estimate, count the species in quadrats placed at random, then scale up to the whole area. For distribution, sample at regular points along a transect and measure the factor at each point.

• Choose a common species that is easy to spot and count, such as daisies.
• Lay two tape measures at right angles along two edges of the area, to make a grid.
• Use a random number generator to pick a pair of coordinates, and place the quadrat there.
• Count the individuals inside the quadrat. Set a rule for ones on the edge, such as counting them only if more than half is inside, and stick to it.
• Repeat for plenty of quadrats, at least 10.
• Measure the length and width of the whole area, so you can work out its size.
• For the distribution part, lay a tape measure across the habitat where conditions change, for example from under a tree out into open grass.
• Place the quadrat at regular points along the tape, say every 2 m. At each point, count the species (or estimate how much of the quadrat it covers) and measure the light intensity.

What are the variables?

The population estimate is a measurement rather than a test, so it has no independent variable. These variables are for the distribution part, where you look at how a factor affects where the species is found.

How do you work out and present the results?

To estimate the population, find the mean number per quadrat, then scale it up: estimated population = (total area ÷ area of one quadrat) × mean number per quadrat.

Worked example: a field is 40 m by 50 m, so its area is 2000 m². Ten quadrats of 0.25 m² hold 3, 5, 0, 2, 4, 6, 1, 3, 2 and 4 daisies. That is 30 daisies in total, a mean of 3 per quadrat, with a range from 0 to 6. The field holds 2000 ÷ 0.25 = 8000 quadrat-sized squares, so the estimate is 8000 × 3 = 24,000 daisies.

For the transect, plot the number of plants (or the percentage cover) against distance along the line, and plot light intensity against the same distances so you can compare the patterns. A scatter graph of number against light intensity shows whether the two are correlated. A correlation does not prove that light is the cause, because other things, such as how damp the soil is, change along the line too.

How do you make it accurate and safe?

Random coordinates stop you choosing spots that look interesting, which would bias the estimate. More quadrats bring the estimate closer to the real population, because one or two unusual squares have less effect on the mean. Stick to the same edge rule throughout. Take light readings at the same height, and as close together in time as you can.

Most fieldwork hazards come from the site itself. Watch out for stinging nettles and animal droppings, and stay inside the area your teacher sets. Near water, take extra care. Wash your hands afterwards, and cover any cuts before you handle soil or plants.

What do exam questions ask about it?

Expect the population calculation, sometimes with a quadrat size in cm that you have to turn into m² first. Questions also ask why quadrats go at random coordinates, and how a transect can show the effect of a factor. You may be asked what a graph of transect results shows, or why it can't prove a cause. An anomaly might be a quadrat that landed on a path. The 6-mark version asks you to describe how to estimate a population, or how to investigate the distribution of a named plant.

An exam-style question we wrote: "A student placed ten 1 m² quadrats at random in a garden with an area of 60 m². They counted 45 dandelions altogether. Estimate the number of dandelions in the garden." The mean is 45 ÷ 10 = 4.5 per m², so the estimate is 4.5 × 60 = 270.

Common mistakes

• Mixing up the quadrat area. A 50 cm by 50 cm quadrat is 0.5 m × 0.5 m = 0.25 m², not 0.5 m².
• Placing quadrats where it looks interesting. Random coordinates keep the sample fair.
• Scaling up the total count instead of the mean. Divide by the number of quadrats first.
• Saying a factor "causes" the pattern from transect data alone. It shows a correlation, and other factors change along the line too.
• Counting plants on the edge differently each time. Set a rule and keep it.