Required practical 7 · Biology · Biology Paper 2, both tiers

Ecological sampling

Measure the population size of a common species in a habitat, and use sampling to investigate the effect of a factor on its distribution.

The exam trap. Throwing a quadrat is not random sampling. Random quadrats estimate population; a transect shows how numbers change.

Variables

Independent (you change): Position: random coordinates (population) or distance along a transect (distribution)
Dependent (you measure): Number of the plant species in each quadrat
Control (you keep the same): The same size of quadrat
The same way of counting plants on the edge
Sampling on the same day

Population: lay two tape measures at right angles along two sides of the area to make a grid.
Why The grid gives coordinates for random sampling.
Use a random number generator to choose coordinates and place the quadrat there. Count the plants.
Why Random sampling avoids choosing places with more or fewer plants.
Sample at least 10 quadrats and calculate the mean per m².
Why More samples give a more representative mean.
Estimate population = mean per m² × area of the whole habitat.
Why The sample is scaled up to the whole area.
Distribution: lay a tape from one habitat (for example under a tree) into another and place quadrats at regular intervals along it.
Why A transect shows how numbers change with distance.
At each quadrat on the transect, count the plants and measure an abiotic factor such as light intensity.
Why This lets you link the distribution to the factor.

Hazard	How to reduce the risk
Uneven ground and stinging plants.	Wear suitable shoes and gloves.
Working near water or roads.	Stay away from hazards and work with a partner.

Mean = 30 ÷ 10 = 3 daisies per quadrat, so 3 per m² with 1 m² quadrats. For a 50 m × 40 m field: 3 × 2000 = 6000 daisies.
Along a transect, compare counts with the abiotic factor measured at each point.

Throwing quadrats over the shoulder and calling it random.
Instead Use random coordinates on a grid.
Using random quadrats to show how numbers change with distance.
Instead Use a transect with quadrats at regular intervals for distribution.
Forgetting to scale the mean by the area of the whole habitat.
Instead Population = mean per m² × total area in m².