Required practical 9: Field investigations: quadrats and transects

Aim

To estimate the population size of a common plant in a habitat using random quadrats, and to find how a factor such as light affects its distribution along a transect.

Background

An ecologist cannot usually count every organism in a habitat. Instead, a sample is taken and used to estimate the population size (the total number of a species) and the distribution (where the species is found).

A quadrat is a square frame, usually 0.25 m² (50 cm × 50 cm) or 1 m². The number of organisms inside it is counted. Placing quadrats at random positions avoids bias and gives a sample that represents the whole habitat.

A transect is a line, such as a tape measure, laid across a habitat where a factor changes, for example from shade into open ground. Quadrats are placed at regular intervals along the line, so you can see how the number of organisms changes with the factor.

Environmental factors that may affect distribution include light intensity, soil moisture, soil pH, temperature, and trampling. The species you choose must be easy to identify and count.

Hypothesis

The number of plants per quadrat will increase with distance from the hedge, because light intensity increases with distance from the shade of the hedge, and the plant needs light for photosynthesis.

Variables

Equipment

• Quadrat (0.25 m², 50 cm × 50 cm)
• Two 30 m measuring tapes
• Random number generator or calculator (or random number table)
• Light meter
• Identification key or guide for the chosen species
• Clipboard, results table and pencil
• Gloves
• Stakes or markers (optional)

Risk assessment

Method

1. Choose a habitat (for example a field) and a common species that is easy to identify, such as daisies or dandelions, in an area you can measure.
2. Lay two tape measures at right angles along two sides of the sampling area to make a pair of axes.
3. Part A, population size: use a random number generator to pick two numbers, one for each tape measure, to give coordinates.
4. Place the quadrat with its corner at that coordinate and count the plants of the species inside the quadrat. Record the number in a separate Part A table with the headings Quadrat number and Number of plants in quadrat.
5. Repeat at least 10 times with new random coordinates. Using many quadrats makes the sample more representative.
6. Calculate the mean number of plants per quadrat. Divide by the area of the quadrat to find the mean per m².
7. Measure the total area of the habitat in m². Estimate the population: mean number per m² × total area of the habitat in m².
8. Part B, distribution: lay a tape measure across the habitat, in a straight line from the shade of a hedge into open ground. This is the transect.
9. Place a quadrat on the line at 0 m, touching the hedge side of the tape, then every 2 m along the transect up to 10 m.
10. At each position, count the plants of the species inside the quadrat.
11. At each position, measure the light intensity at ground level with a light meter, held at the same angle each time.
12. Record the results in a table. Repeat on one or two other transects if time allows, and calculate the mean for each distance.
13. Plot a graph of number of plants against distance from the hedge, and describe the pattern in distribution.

Results

Fill this table in as you go. Print the PDF for a copy to write on.

Drawing the graph

Line graph of number of plants per quadrat (y-axis) against distance from the hedge in m (x-axis). The independent variable goes on the x-axis. Plot a point for each distance and join the points with straight lines, because the data are continuous along a transect. Use more than half the grid and equal scale steps.