Skills applied: CP11 ecology of a habitat

Sampling strategy

Method recap (see Core practical 11, Unit 4):

• Random sampling (random number coordinates on a grid) when an area looks uniform; avoids bias.
• Systematic sampling along a transect (line or belt) when a factor changes across the area, such as distance from a hedge or up a shore; place quadrats at regular intervals.
• Enough quadrats: plot a running mean of species number or cover against number of quadrats; stop when it levels off.

Measuring abundance and abiotic factors

• Percentage cover (estimate area covered, often using a 100-square grid quadrat), density (individuals per m²), frequency (percentage of quadrats or squares containing the species), or an abundance scale such as ACFOR.
• Abiotic factors: light (light meter), soil moisture (moisture meter or mass loss on drying), soil pH (pH probe or indicator), temperature, wind speed, salinity.
• Take abiotic readings at the same time as each quadrat and calibrate meters.
• Percentage cover is subjective: use the same observer or grid squares.

Analysis

• Kite diagrams show changing abundance along a transect.
• Spearman's rank tests correlation between an abiotic factor and abundance at the same quadrats.
• Chi-squared tests whether a species is spread evenly between zones.
• Mean density = total individuals ÷ total area sampled.
• Limitations: abiotic factors vary over the day; several factors change together; small sample; identification errors.

Worked example

12 quadrats of 0.25 m² contained 18, 22, 15, 20, 19, 25, 16, 21, 17, 23, 24, 20 daisies. Estimate the population in a 50 m² field.

1. Total = 240 daisies in 12 x 0.25 = 3 m²
2. Density = 80 per m²
3. Population = 80 x 50 = 4000

Answer: About 4000 daisies.

Check yourself

1. When should a belt transect be used rather than random quadrats?
   • When the area is uniform
   • When there is no time
   • When an abiotic factor changes across the area, such as from shade to open ground
   • When only one species is present
   Show answer

   When an abiotic factor changes across the area, such as from shade to open ground. Transects sample along a gradient.

2. How are random quadrat positions chosen?
   • Using random number coordinates on a grid laid over the area
   • Throwing the quadrat over your shoulder
   • Placing them where most plants grow
   • Placing them in a straight line
   Show answer

   Using random number coordinates on a grid laid over the area. Random coordinates avoid bias.

3. 8 quadrats, each 0.5 m², contain a total of 96 snails. Calculate the mean density per m².
   Show answer

   24 per m². Area = 4 m²; 96 / 4 = 24 per m².

4. A species was present in 14 of 40 quadrats. Calculate its percentage frequency.
   Show answer

   35 %. 14 / 40 x 100 = 35%.

5. Along a transect of 7 quadrats, Σd² for soil moisture against moss cover is 6. Calculate rₛ to 3 decimal places.
   Show answer

   0.893. 1 - 36 / 336 = 0.893.

6. For the moss data, rₛ = 0.893 with 7 pairs (critical value 0.786 at p = 0.05). What is the conclusion?
   • Accept the null hypothesis: no significant correlation
   • Reject the null hypothesis: a significant positive correlation between soil moisture and moss cover
   • There is a significant negative correlation
   • There is a significant difference between means
   Show answer

   Reject the null hypothesis: a significant positive correlation between soil moisture and moss cover. 0.893 > 0.786.

7. How can you decide whether enough quadrats have been used?
   • Stop after three quadrats
   • Use one large quadrat
   • Count until you are tired
   • Plot a running mean against number of quadrats and check it has levelled off
   Show answer

   Plot a running mean against number of quadrats and check it has levelled off. A stable running mean shows the sample is representative.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018): Unit 3, Unit 6 and Appendix 6 mathematical skills
• Pearson WBI16/01 specimen paper (2019) with Spearman's rank critical values (via Physics & Maths Tutor)

Last updated 8 November 2026. Spotted a mistake? Tell us.