Random spots, count, scale up: For non-mobile organisms, count them in quadrats placed at random, then scale up.
The points to remember
- Mark the area out as a grid and pick coordinates with random numbers.
- Place a quadrat at each point and count the individuals inside it.
- Take enough quadrats (often 10 or more) and find the mean per quadrat.
- Population density = mean per quadrat ÷ area of one quadrat (individuals per m²).
- Population size = density × total area of the habitat.
- Choose a quadrat size to suit the organism: 0.25 m² for small plants, bigger for shrubs.
Worked example: Ten 1 m² quadrats on a 2000 m² lawn hold 62 daisies. Mean = 62 ÷ 10 = 6.2 per m². Population ≈ 6.2 × 2000 = 12 400 daisies.
Real example: in the UK's National Plant Monitoring Scheme, volunteers record the plants in the same small square plots every year, so changes can be tracked.
Remember it as: Random spot, count, find the mean, multiply by the area.
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Counting works for separate plants. For grass, moss or seaweed, measure how much there is instead.
Three ways to measure abundance
- Density: the number of individuals per unit area, e.g. 16 limpets per m².
- Percentage cover: the % of the quadrat's area covered by the species; used when plants cannot be counted.
- Percentage frequency: occurrences ÷ possible occurrences × 100: in 5 of 100 squares = 5%.
- Cover and frequency give abundance, but not the actual population size.
Remember it as: Cover: how much space. Frequency: how many squares.
In the grid, 30 of the 100 squares are more than half covered: cover = 30%. Clover is present in 30 + 12 = 42 squares: frequency = 42%.
Not population size: One big clover plant can cover half a quadrat, so cover and frequency show abundance, not the number of plants.
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Measuring change along a transect: To see how plants change across a habitat, combine a transect with quadrats.
The points to remember
- Lay a transect in a straight line across the gradient, e.g. outwards from a glacier's end.
- Sample at regular intervals along it (systematic sampling), e.g. every 50 m.
- At each point, place a quadrat, at random close to the point, to avoid bias.
- Estimate percentage cover (count the squares covered) or frequency in each quadrat.
- Repeat with several parallel transects and find the mean at each distance.
Remember it as: Line, regular points, a quadrat at each, cover, repeat.
Real example: at Studland Bay in Dorset, quadrats along a transect from the beach inland show bare sand, then marram grass, then heather as the dunes get older.
Describe the whole method: 'Systematic sampling' on its own is not a method: say where the transect goes, how the quadrats are placed, and what you record.
Quadrats are simple and cheap, but they have limits.
The points to remember
- Works only for non-mobile or very slow organisms: plants, barnacles, limpets.
- Cover estimates are subjective: two people may give different values.
- Too few or too small quadrats miss rare or clumped species.
- Plants that spread, such as grass, cannot be counted: use cover instead.
- Results are estimates: repeat and average to make them more reliable.
Remember it as: Still, subjective, enough, cover for grass, repeat.
Real example: a daisy survey on a school field works well; a survey of rabbits on the same field would not, because rabbits move.
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How this comes up: Paper 1 or Section A: describe how percentage cover data were collected [3], or calculate density, a population estimate or percentage frequency from quadrat counts [1-2].
A student measured how the cover of plants changes with distance inland across a sand dune, as shown in the graph above.
Describe a method used to collect the data on percentage cover shown in the graph.
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