Fieldwork and sampling

Core practical: quadrats and belt transectsSpec 9.5

Field-work techniques let you investigate the relationship between organisms and their environment, for example how the number of a plant species changes with light intensity.

Quadrat

A square frame used to sample the organisms in a small, known area.

Belt transect

A line (a tape measure) laid across a habitat, with quadrats placed along it at regular intervals, to show how organisms change along an environmental gradient.

Random sampling with quadrats

1. Mark out the study area with two tape measures laid at right angles along two sides.
2. Use a random number generator to pick two coordinates, one along each tape measure.
3. Place the quadrat at that point and count the number of the chosen species inside it, or estimate the percentage cover.
4. Repeat for many quadrats and calculate the mean.

Belt transect

1. Lay a tape measure in a straight line across the habitat, for example from under a tree out into open grass.
2. Place a quadrat at regular intervals along the tape, such as every 2 m.
3. At each position count the organisms in the quadrat and measure the environmental factor, for example light intensity with a light meter or soil moisture with a moisture meter.
4. Record both in a table and plot a graph to look for a pattern.

Variables and safety

• Independent variable: the position along the transect, or the environmental factor such as light intensity.
• Dependent variable: the number or percentage cover of the species.
• Control variables: the quadrat size, the species being counted and the person counting.
• Safety: wear suitable footwear, wash hands after handling soil or plants, and do not touch plants you cannot identify.

Typical result: along a transect from shade into open ground, plants that need lots of light increase in number as light intensity increases. A scatter diagram of number against light intensity can show a correlation, but a correlation alone does not prove that one factor causes the other.

Tap to enlarge

Estimating the number of organismsSpec 9.6

You cannot count every organism in a large area, so you take samples and use them to estimate the total. A larger number of quadrats, placed randomly, gives a more reliable estimate.

Plants that are difficult to count individually, such as grass, can be measured by percentage cover: the percentage of the quadrat covered by the species.

Other points to understand about sampling:

• Take as many quadrats as possible, because a bigger sample is more representative and reduces the effect of anomalies.
• Place quadrats randomly, to avoid bias.
• Calculate a mean from the raw data. A bar chart or frequency table is useful for presenting the results.