Quadrats, transects and abiotic measurements

What you’ll learn

• Estimate density, frequency and abundance using appropriate sampling.
• Distinguish random sampling from a transect investigation.
• Evaluate environmental correlations and sampling bias.

Understand the idea

Core practical 11 studies distribution and abundance in a habitat. Randomly selected quadrat positions can estimate population density without deliberately choosing favourable patches. The quadrat area must be known, and boundary rules must be consistent. Frequency is the proportion of quadrats in which a species occurs; it differs from mean number per quadrat and from percentage cover. Choose the measure that suits the organism and the question.

A transect samples change along a stated environmental gradient. Regularly spaced positions provide systematic data, and parallel transects can help assess variation beyond one line. Measure relevant abiotic variables with calibrated instruments using a consistent position and timing. A single midday temperature reading may not represent the conditions that determined a plant's distribution over the season.

Estimating total abundance from sample density assumes the sampled area represents the habitat to which the estimate is applied. A correlation between abundance and an abiotic factor does not by itself establish causation because other factors may covary. Field plans should respect access, weather and terrain risks, minimise disturbance and avoid unnecessary removal of organisms. Report limits rather than extrapolating confidently beyond the sampled habitat.

Key formulae

$$\mathrm{density}=\frac{\mathrm{total\ individuals\ counted}}{\mathrm{total\ area\ sampled}}$$

$$\mathrm{frequency}\ (\%)=\frac{\mathrm{quadrats\ containing\ species}}{\mathrm{total\ quadrats}}\times100$$

$$\widehat N=\mathrm{estimated\ density}\times\mathrm{habitat\ area}$$

Worked example

Five randomly placed $0.25\ \mathrm{m^2}$ quadrats contain 3, 5, 2, 6 and 4 individuals of a plant species. Estimate its density and the total abundance in a representative $50\ \mathrm{m^2}$ habitat.

1. The total count is $20$ and the total sampled area is $5\times0.25=1.25\ \mathrm{m^2}$.
2. Estimated density is $20/1.25=16\ \mathrm{individuals\,m^{-2}}$.
3. Estimated abundance is $16\times50=800$, assuming these quadrats represent the whole habitat.

Answer: $16\ \mathrm{individuals\,m^{-2}}$ and an estimated total of $800$.