Topic Explainer Video

Check out this @lauradoesGCSEbiology video that explains the required practical - sampling to measure population size and the effect of a factor on distribution, then read the study notes. Once you’ve gone through them, don’t forget to try the practice questions!

Purpose of the Required Practical

This practical teaches you how to:

• Estimate the population size of an organism (e.g. daisy plants).
• Investigate how an abiotic factor (like light) affects where organisms are found.

Method 1: Using Quadrats to Estimate Population Size – Re-cap

Key Steps:

1. Place quadrats randomly in your study area (e.g. use a random number generator to select co-ordinates).
2. Count the number of the target organism in each quadrat.
3. Use a large number of quadrats.
4. Calculate the mean number per quadrat.
5. Multiply the mean number per metre² by the total area to estimate the population.

Example:
If the mean number of daisies per m² is 3, and the field is 100 m², then estimated total daisies = 300.

Method 2: Using Transects to Measure Distribution- Re-cap

Key Steps:

1. Stretch a transect line (like a tape measure) across your study area.
2. Place a quadrat at regular intervals along the line (e.g. every 1 m).
3. Count or estimate percentage cover of the organism in each quadrat.
4. Measure the abiotic factor (e.g. light using a light metre) at each point.
5. Look for patterns - does the distribution change as the abiotic factor changes?

Example:
More plants may grow in an open field than under trees as there’s more light for photosynthesis and less competition for water and minerals.

Step-by-Step Method: Investigating Distribution Across a Pollution Gradient

Aim: To find out how the number of a particular plant species (e.g. daisies) changes with distance from a road, and how pollution (e.g. exhaust fumes) might affect its distribution.

Equipment:

• 50 m measuring tape (transect line)
• Quadrat (e.g. 0.5 m × 0.5 m)
• Notebook and pen
• Calculator

Method:
Choose the site:

• Select an area running at a right angle from the road edge into a field.

Lay out the transect:

• Stretch a measuring tape from the edge of the road to 50 m into the field.

Decide sampling points:

• Mark points at regular intervals along the transect, e.g. every 5 m: 0 m, 5 m, 10 m … up to 50 m.

Count the plants:

• At each sampling point, place the quadrat on the ground and count the number of target plant species (e.g. daisies) inside the quadrat at each point.
• If the plants are too numerous or hard to count, estimate percentage cover using the gridded squares method.

Record other abiotic observations (optional):

• Measure light intensity if relevant (though in this case the main factor is pollution).

Repeat:

• If possible, repeat the transect at least 2 more times at different places along the road to get more representative data. Remember to use many quadrats alongside each transect.

Record results:

• Record your data in a table:

Analyse your data:

• Calculate the mean number of daisies at each distance using your repeats.
• Plot a line graph of distance from road on the x-axis and mean number of daisies on the y-axis.

Interpret your results:

• Look for a pattern - for example, are there fewer plants near the road due to higher pollution?
• Link your findings to abiotic factors: exhaust fumes and salt in winter can damage plants and reduce plant growth near roads.

Key idea:

• If the number of plants increases as you move away from the road, you can suggest that pollution close to the road affects their distribution.

Practice Question