Belt transects

A belt transect is a way of sampling how the plants or slow-moving animals in a habitat change from one place to another. You lay a tape measure in a straight line across the habitat and place quadrats along it, either touching end to end (a continuous belt transect) or at regular intervals (an interrupted belt transect). Recording what is in each quadrat, and measuring a factor such as light at each one, shows how the distribution of a species changes along the line.

What is a belt transect?

A transect is a line across a habitat, usually a tape measure. On its own, the line only tells you what is at certain points along it. A belt transect adds quadrats along the line, so you can record how much of each species there is in each quadrat. AQA A-level Biology names quadrats along a belt transect as one way to investigate the distribution of organisms, and OCR's A-level practical PAG 3.2 uses an interrupted belt transect to measure the abundance of a range of plant species.

When to use a belt transect

Use a belt transect whenever conditions change across a habitat in one direction, so that the species are likely to change with them. Examples include going from shade under a tree out into sunlight, from the edge of a path into untrodden grass, or from the sea up a rocky shore or sand dune. Because the quadrats follow the change, you can link the abundance of a species to a measured abiotic factor.

How to do a belt transect

This is the method from AQA's GCSE Biology required practical handbook, which runs a transect from a tree to open ground:

1. Lay a 30 m tape measure in a straight line from the base of a tree to an open area of ground.
2. Put a quadrat against the line, with one corner at the 0 m mark.
3. Count the plants of your chosen species inside the quadrat, or estimate their percentage cover.
4. Measure the abiotic factor at that position, for example light intensity with a light meter (a phone app can work).
5. Move the quadrat 5 m up the line and repeat, continuing every 5 m to the end.
6. Record the results in a table, then plot graphs of the number of plants against distance and against light intensity.

If space is short, AQA suggests a shorter line with the quadrats closer together. OCR notes that quadrat size and transect length can be reduced, and that smaller quadrats are acceptable. Repeating the transect at several places across the habitat makes the results more representative.

Measuring abundance in each quadrat

• Counting individuals (density) works when the plants are separate and easy to count, such as dandelions or plantains.
• Percentage cover is the percentage of the quadrat's area covered by a species. It suits grasses and mosses, where you cannot tell one plant from the next. Dividing the quadrat into a grid of small squares helps you estimate it.
• Frequency is the percentage of quadrats, or of the small squares in a quadrat, in which the species is present. It is quick, but it does not show how much of the species is there.

AQA A-level Biology asks you to use both percentage cover and frequency as measures of the abundance of a sessile (non-moving) species.

Worked example: analysing belt transect results

A student runs an interrupted belt transect from the base of a tree into an open field, with a 0.5 m × 0.5 m quadrat every 5 m, counting dandelions and measuring light intensity.

1. Describe the pattern: the number of dandelions increases with distance from the tree, from 0 at the base to 12 at 30 m.
2. Link it to the factor: light intensity also increases with distance, so more dandelions grow where there is more light. This is a positive correlation.
3. Explain it: more light means more photosynthesis, so dandelions grow better in the open. Shade under the tree limits them.
4. Be careful with the conclusion: other factors change with distance too, such as soil moisture and competition with tree roots, so the data show a correlation, not proof that light is the cause.

The Ecology Calculator works out Simpson's diversity index from quadrat counts if you record several species; see the index of diversity guide.

Worked example: estimating a population with random quadrats

Transects show distribution. To estimate the size of a population across an even area, AQA uses random quadrats and this equation:

estimated population size = total area × number counted ÷ area sampled

1. A field is 25 m × 12 m, so the total area is 300 m².
2. Ten random 1 m² quadrats contain 38 plantains in total, so the area sampled is 10 m².
3. Estimated population = 300 × 38 ÷ 10 = 1,140 plantains.

For animals that move about, use mark release recapture instead.

Common mistakes

• Using a transect to estimate population size. Transects show distribution; random quadrats give a population estimate.
• Placing quadrats by eye. Keep them at the same regular interval and on the same side of the line.
• Measuring the factor somewhere else. Measure light, moisture or pH at each quadrat, at about the same time.
• Doing only one transect. One line may be unusual; repeat it in other places and calculate means.
• Claiming a factor causes the change. Field data show correlation; other factors change along the line too.

Practise with a short timed paper

Transects and quadrats are in AQA GCSE Biology (required practical 9) and AQA A-level Biology (3.7.4). The daily GCSE Biology Higher and A-level Biology: Year 2 papers are short timed papers of 8 questions, with a new one every day.

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