Field investigations required practical

Aim: To estimate the population size of a common plant species using random quadrat sampling and to investigate how its distribution changes along a transect.

Variables

• Independent: For random sampling, the area sampled; for a transect, the distance along the line (and the abiotic factor that changes with it, such as light intensity).
• Dependent: Number of individual plants (or percentage cover) in each quadrat.
• Control: Size of quadrat used; Number of quadrats sampled; Method of counting (e.g. counting a plant only if more than half is inside the quadrat); Time of day and weather when sampling

Equipment

• Quadrat (e.g. 0.5 m × 0.5 m or 1 m × 1 m)
• Two long tape measures
• Random number generator or random number table
• Light meter or other abiotic measuring equipment
• Clipboard and recording sheet
• Plant identification key
• Gloves

Method

1. Random sampling: lay two tape measures at right angles along two edges of the area to form axes.
2. Use a random number generator to pick a pair of coordinates and place the quadrat at that point.
3. Count the number of the chosen plant species (for example daisies) inside the quadrat.
4. Repeat for at least ten randomly chosen positions and calculate the mean number per quadrat.
5. Measure the total area of the field so that the population size can be estimated.
6. Transect: stretch a tape measure in a straight line across a habitat where conditions change, such as from under a tree into open grass.
7. Place the quadrat at regular intervals along the tape (for example every 2 m) and count the plants or estimate percentage cover.
8. At each quadrat, measure an abiotic factor such as light intensity or soil moisture and record it with the count.

Safety

• Wear gloves and wash hands afterwards, as soil and plants may carry bacteria or animal waste.
• Watch out for stinging or poisonous plants and uneven ground.
• Stay in the agreed area, work with a partner, and check for hazards such as water, roads or tides.

Results and calculations

Calculate the mean number per quadrat and estimate the population using estimated population = (total area ÷ area of one quadrat) × mean number per quadrat. For a transect, plot the number (or percentage cover) and the abiotic factor against distance along the line, and look for a correlation.

Common mistakes and improvements

• Choosing quadrat positions by eye, which introduces bias; always use random coordinates.
• Too few quadrats giving an unrepresentative sample; use more quadrats and check that the mean stops changing.
• Inconsistent counting of plants on the edge of the quadrat; agree a rule, such as counting only if more than half is inside.
• Using different quadrat sizes or areas in calculations; check units and convert everything to m².

Exam tips

• Explain why random sampling is used: to avoid bias so the sample is representative of the whole area.
• Practise the population estimate calculation and calculations of mean, median, mode and range from quadrat data.
• Explain when a transect is used instead: to show how the distribution of a species changes across a gradient of an abiotic factor.
• Be careful with conclusions: a correlation with light intensity does not prove light causes the change; other factors may vary too.
• Suggest ways to improve validity, such as sampling more quadrats or repeating transects in different places.