Required Practical 12: Population Sampling

Section 1 of 6Part 1: population size by random sampling

1

Mark out the survey area and measure its total area in square metres.

2

Use a grid with random number tables or a random number generator to select coordinates.

3

Place a quadrat of known area at each coordinate.

4

Count the number of the chosen plant species inside every quadrat.

5

Calculate the mean number per quadrat from at least ten quadrats.

6

Estimate the population: mean per quadrat × (total area ÷ area of one quadrat).

Section 2 of 6Part 2: effect of a factor with a transect

1Pick the factor

• Pick the factor For example distance from a tree, a river or a path, or change in altitude.

2Lay the line

• Lay the line Stretch a tape measure across the area in the direction of change.

3Quadrat at intervals

• Quadrat at intervals Place a quadrat every metre, or a set distance, along the line.

4Record and measure

• Record and measure Count the plants and measure the factor at each point.

5Analyse

• Analyse Plot abundance against distance to see any trend.

Section 3 of 6Making results valid

Random placement

• Random placement Prevents bias, such as only sampling the nearest or easiest ground.

Large sample

• Large sample More quadrats give a mean closer to the true value.

Same quadrat size

• Same quadrat size Keeps counts comparable between positions.

Safety

• Safety Wear sensible footwear, watch for hazards on slopes or near water, and wash hands after handling soil and plants.

Section 4 of 6Results and conclusions

Observation	Possible conclusion
Plant count falls steadily with distance from a tree	Shade or competition from the tree may reduce growth
More plants on the moist side of the slope	Water availability may limit the species
Great variation between quadrats	Take more samples before drawing conclusions
Data show a trend only along the transect	Link may be correlation, so test other factors

Section 5 of 6Key terms

Term	Meaning
Random sampling	Choosing sample positions by chance so every place is equally likely.
Bias	A systematic error that pushes results in one direction.
Population estimate	A calculated approximation of the total number of organisms.
Transect	A line along which samples are taken at set intervals.
Mean	The total of the values divided by the number of values.

Section 6 of 6Exam technique

1

Throwing a quadrat over the shoulder and calling it random without a method.

2

Forgetting to scale the mean up by the ratio of total area to quadrat area.

3

Changing the quadrat size part-way through the investigation.

4

Claiming the factor causes the pattern when only a correlation was shown.