7.1.3Practical Skills
Practical: Sampling Techniques: Quadrats — OCR Gateway GCSE Combined Science A J250 - Biology Revision Notes & Topical Questions
5 min read 8 parts Lesson 4 of 11
Part 1 of 6What a quadrat is for
The equipment
- The equipment A quadrat is a square frame of wood or wire, for example 0.25 m2 or 1 m2, laid on the ground.
Ecology
- Ecology It lets ecologists estimate how many organisms live in an area (abundance) and how they are spread out (distribution).
Three measurements
- Three measurements Number of individuals of one species.
- Species richnesshow many different species are present.
- Percentage coverthe share of the quadrat covered, used for plants such as grass or moss that are too hard to count.
Part 2 of 6Method for comparing two areas
1
Mark out each study area with two tape measures laid at right angles to make a grid.
2
Use a random number generator to pick coordinates on the grid so that sampling is not biased.
3
Place the quadrat at each coordinate and count the individuals of the chosen species, using a key to identify them.
4
Repeat for many quadrats in the area, because more samples give more reliable results.
5
Calculate the mean number per quadrat for each area.
6
Estimate total population = mean per quadrat × (total area ÷ quadrat area).
Part 3 of 6Why the areas differ
| Factor type | Examples |
|---|---|
| Biotic | Competition, predation, food-web interactions |
| Abiotic | Light intensity, mineral availability, water, pH, temperature, salinity |
Part 4 of 6Problems and solutions
Limitations
Missed plants
- Missed plants Small plants can hide under leaves; move leaves aside gently with a pencil.
Identification
- Identification Use a species identification key.
Too many to count
- Too many to count Record as more than 100 (>100) or approximately (~30).
Bias
- Bias Random sampling removes bias from placing the quadrat where plants look dense.
Part 5 of 6Worked example: estimating a population
1Results
- Results Ten 0.25 m2 quadrats in a field contain a total of 45 daisies.
2Mean per quadrat
- Mean per quadrat Mean = 45 ÷ 10 = 4.5 daisies per quadrat.
3Scale up
- Scale up The field has an area of 800 m2, which is 800 ÷ 0.25 = 3200 quadrat areas.
4Estimate
- Estimate Population = 4.5 × 3200 = 14 400 daisies.
Part 6 of 6Transects for a changing habitat
When to use
- When to use Random quadrats suit an even habitat;
- if conditions change in a line, such as from a hedge into a field, use a transect.
How it works
- How it works Lay a tape measure along the line and place a quadrat at regular intervals, such as every 2 m.
What it shows
- What it shows Plotting abundance against distance shows how the distribution of species changes with an abiotic factor such as light or soil moisture.
- Quadrat results should be summarised with a mean, and a larger number of samples reduces the effect of any unusual quadrat.
- If species cannot be counted, estimate the percentage cover by counting the squares of a gridded quadrat that are more than half covered.
Key terms
| Term | Meaning |
|---|---|
| Quadrat | A square frame used to sample a small area of habitat. |
| Abundance | How many individuals of a species are present. |
| Percentage cover | The proportion of the quadrat area covered by a species. |
| Species richness | The number of different species in an area. |
| Random sampling | Choosing sample sites by chance so that every position has an equal chance. |
| Transect | A line across a habitat along which samples are taken at regular intervals. |
Avoid these mistakes
1
Placing quadrats by hand where plants look interesting - this introduces bias.
2
Using too few quadrats and generalising the result to the whole area.
3
Counting species richness as the total number of plants.
4
Forgetting the abiotic factors that could explain differences between the two areas.
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