3.2.4 Practical: Ecological Sampling

3.2.4 Practical: Ecological Sampling: Describe how to use random quadrat sampling to estimate abundance and to investigate how an abiotic factor affects distribution, including line transects.

Lay two measuring tapes along the edges of the habitat to make a grid, then use random numbers to choose coordinates.

Place the quadrat where the coordinates meet and count the individuals of the species or estimate the percentage cover.

Repeat with a running mean, starting with about five quadrats and adding more until the mean stops changing.

Calculate the area of one quadrat, then work out how many quadrats would cover the whole site.

Multiply the mean number of individuals per quadrat by the number of quadrats to estimate the total.

Soil Moisture and Plant Abundance

Design

Choose a wetter and a drier area that are as similar as possible in light, soil type and climate.

Measurements

In each of 10 quadrats per area record the abundance of one species and the soil moisture reading.

Analysis

Calculate the mean abundance and mean soil moisture for each area and compare them in a bar chart.

Expected result

Areas with more moisture generally support more plants, but species adapted to dry soil may be commoner in drier places.

Layout

Stretch a tape in a straight line across the study area and mark sampling points at regular intervals.

Sampling

Place the quadrat at each point and record the abundance of the chosen species.

Abiotic readings

Measure factors such as light, temperature, soil moisture and pH at each point, using the same technique each time.

Kite diagram

Plot abundance above and below a line representing distance, then join the points to form a kite-shaped outline.

Replicates

Run two more parallel transects to improve reliability.

Limitations and Improvements

Other differences

The two areas may differ in more than moisture, so choose areas close together and note any differences.

Small sample

Use more quadrats per area, guided by a running mean.

Inconsistent counting

Agree a counting rule, for example more than half the plant inside the frame.

Disturbance

Walk beside the tape, not on it, and place the quadrat before stepping near it.

Question: Eight 0.5 m × 0.5 m quadrats placed at random in a 400 m² field contained 6, 9, 4, 8, 10, 5, 7 and 11 daisies. Estimate the number of daisies in the field.

Total = 6 + 9 + 4 + 8 + 10 + 5 + 7 + 11 = 60, so the mean = 60 ÷ 8 = 7.5 daisies per quadrat.
Area of one quadrat = 0.5 × 0.5 = 0.25 m².
Number of quadrats covering the field = 400 ÷ 0.25 = 1600.
Estimated total = 1600 × 7.5.

Answer: There are approximately 12 000 daisies in the field.

3.2.4 – Key Points for the Exam

Abundance: The number of individuals of a species in an area.
Random sampling: Choosing sample sites by chance to avoid bias.
Transect: A line along which samples are taken to show changes across a habitat.
Kite diagram: A graph showing the abundance of a species along a transect.

Common Mistakes

Choosing quadrat positions by eye instead of using random coordinates.
Forgetting to scale the mean up by the number of quadrats that fit in the site.
Using the area of the whole site instead of the area of one quadrat when working out how many quadrats are needed.
Comparing two areas that differ in several abiotic factors and blaming only moisture.