Organisation of an Ecosystem » Required Practical: Measuring Population Size

The aim of the practical

This required practical has two jobs. First, you measure the population size of a common species in a habitat. Second, you use sampling to investigate the effect of a factor on the distribution of that species.

Your teacher will pick a common plant that is easy to spot and count, such as dandelions on a school field. You already know how quadrats and transects work from the lesson Sampling with Quadrats and Transects. Here you put them to use as a full investigation.

Key terms:

• Line transect: a tape measure laid in a straight line, with samples taken at points along it.
• Belt transect: a strip of a set width along the line, sampled with a quadrat placed edge to edge or at set intervals.
• Light meter: a device that measures light intensity, used here to measure an abiotic factor.

Part 1: Measuring the population size

The aim is to estimate how many dandelions live on the whole field by counting them in small samples.

1. Mark out the area to be studied with two tape measures laid at right angles. Measure its length and width to work out its area in m².
2. Use a random number generator to choose two numbers. Each number is a distance along one tape measure. These are your coordinates.
3. Place a 1 m² quadrat where the two distances meet. Do not move it to a nicer spot.
4. Count the dandelions inside the quadrat and record the number in a table.
5. Repeat with new random coordinates until you have at least 10 quadrats. More quadrats give a better estimate.
6. Calculate the mean number per quadrat, then estimate the population.

Use the right area units. If your quadrat is 0.25 m² and the mean count is 3, then there are 3 ÷ 0.25 = 12 per m². Multiply by the field area after that.

Part 2: Investigating the effect of a factor

Now ask a question about distribution. Dandelions need light to photosynthesise, so you might predict they grow better in the open than in the shade of a tree. Your hypothesis could be: the further from the tree, the more dandelions there will be.

A random sample cannot test this, because conditions change gradually across the ground. A transect is better.

A belt transect does the same job but samples a whole strip. You place the quadrat edge to edge along the line, so nothing along the strip is missed. It takes longer, but it gives a fuller picture.

Take more than one quadrat at each distance, for example three, and use the mean. This lowers the effect of one odd patch of ground.

Variables

• Independent variable: the factor you change or compare, here the light intensity (or the distance from the tree).
• Dependent variable: what you measure, here the number of dandelions per quadrat.
• Control variables: the size of the quadrat, the species counted, the number of quadrats at each point, the time of day for light readings, and the person counting.

Soil type and moisture are hard to control in a field. This is why you must say that they could affect the results.

Recording and using your results

Draw a results table before you go outside. Include units in the headings.

Plot the distance or light intensity (independent variable) on the x-axis and the mean number of dandelions on the y-axis. Choose scales that fill most of the grid.

Safety and ethics

Evaluating the investigation

• Is the sample big enough? A small number of quadrats may not represent the whole habitat.
• Was it random? Placing quadrats by choice makes the estimate biased.
• Were other factors changing? Moisture or soil pH might change along the transect too, so light may not be the only cause.
• Were results repeated? Repeating the transect on another day shows whether the pattern is reliable.

A pattern in the data shows a link, but it does not prove that light caused the change on its own.

Exam-style question

A student investigated how the distribution of moss on a wall changes with the amount of shade.

(a) Describe how the student should use a quadrat to measure the number of moss patches along a line from the shady side to the sunny side. [3 marks]

(b) Name the independent variable and the dependent variable. [2 marks]

(c) The student recorded a mean of 6 patches per m² over a total wall area of 30 m². Calculate the estimated population. [1 mark]