All subjects · Biology · WBI14 (Unit 4)

Core practical 11: ecology of a habitat

Core practical 11 uses quadrats, transects and abiotic measurements to study how organisms are distributed in a habitat.

Sampling methods

Random sampling with quadrats compares two areas. Lay out a grid, generate random coordinates, and place a quadrat (often 0.25 m² gridded) at each. Random placement avoids bias.
Belt transect studies change along an environmental gradient (for example from shade to open ground). Lay a tape and place quadrats at regular intervals, such as every 2 m. A line transect records only organisms touching the line.

Abundance can be measured as:

density: number of individuals per m²
percentage cover: the percentage of the quadrat area covered (useful for plants that are hard to count)
frequency: the percentage of quadrats in which the species occurs

Abiotic measurements

At each sampling point measure abiotic factors that might explain the pattern: light intensity (light meter or sensor), soil moisture, soil pH, temperature, wind speed, salinity on a shore.

Take enough samples (for example 20 quadrats per area) to give a representative mean.

Statistics

To compare mean abundance in two areas, use a Student's t-test.
To test for a correlation between an abiotic factor and abundance along a transect, use Spearman's rank correlation coefficient:

rs = 1 - (6Σd²) / (n(n² - 1)), where d is the difference between ranks and n the number of pairs.

Compare rs (or t) with the critical value at p = 0.05. A significant correlation does not prove the abiotic factor causes the pattern.

Worked example

Along a transect, 6 pairs of light and plant cover readings give Σd² = 8. Calculate Spearman's rs.

rs = 1 - (6 x 8) / (6 x (36 - 1))
= 1 - 48 / 210
= 1 - 0.229 = 0.771

Answer: rs = 0.77 (a positive correlation).

Key idea: Random quadrats compare areas (t-test); belt transects study gradients (Spearman's rank). Measure density, percentage cover or frequency, plus abiotic factors at each point.

Check yourself

36 daisies are counted in 10 quadrats, each 0.25 m². Calculate the mean density in plants per m².
Show answer
14.4 per m². Total area = 2.5 m²; 36 / 2.5 = 14.4 per m².
A species is found in 12 of 20 quadrats. What is its percentage frequency?
Show answer
60 %. 12 / 20 x 100 = 60%.
For 8 pairs of readings, Σd² = 20. Calculate Spearman's rank correlation coefficient to 2 decimal places.
Show answer
0.76. rs = 1 - 120 / (8 x 63) = 1 - 120/504 = 0.76.
Why are quadrats placed using random coordinates?
- To make sure every quadrat contains the species
- To sample only the most interesting areas
- To avoid bias in choosing where to sample
- To reduce the number of quadrats needed to one
Show answer
To avoid bias in choosing where to sample. Random sampling gives every point an equal chance, so results are representative.
Which method is most suitable for studying how plant species change from the edge of a wood into an open field?
- A belt transect with quadrats at regular intervals
- Random quadrats in the middle of the field
- A single quadrat at the woodland edge
- Counting all plants in the field
Show answer
A belt transect with quadrats at regular intervals. A transect follows an environmental gradient.
When is percentage cover more useful than counting individuals?
- When animals are moving quickly
- When there are only two plants
- When plants such as grasses are hard to count as separate individuals
- When measuring light intensity
Show answer
When plants such as grasses are hard to count as separate individuals. Grasses and mosses grow as mats, so individuals cannot be counted.
Which statistical test compares the mean number of a species in two different fields?
- Student's t-test
- Spearman's rank correlation
- Chi-squared test
- Simpson's index of diversity
Show answer
Student's t-test. A t-test compares two means.

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Last updated 7 November 2026. Spotted a mistake? Tell us.