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7.4.3Genetics, Populations, Evolution and Ecosystems (A Level only)

Estimating the Size of a Population — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 9 parts Lesson 17 of 25 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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7.4.3 Estimating the Size of a Population — AQA A Level Biology 7402 notes: Describe and evaluate methods of estimating population size, including random sampling with quadrats, transects and mark-release-recapture.It is almost never possible to count every individual in a popula…

  • representative
  • Systematic sampling
  • Species frequency
  • Percentage cover
  • Estimated population = (n1 x n2) / m
  • Exam-style question 1

Section 1 of 9Why estimate and how to sample

It is almost never possible to count every individual in a population, so we count a sample and use it to estimate the whole. The sample must be representative and large enough to reduce the effect of chance. Sampling methods are:

Why estimate and how to sample
Random sampling
Random sampling used when the area is fairly uniform. Divide the area into a grid by laying two long tape measures at right angles; use random numbers (from a table or calculator) as coordinates for each quadrat. This avoids bias in choosing sites.
Systematic sampling
Systematic sampling sampling at regular intervals along a line (a transect), used where there is an environmental gradient, such as from the sea inland or across a footpath. Samples may be taken continuously (belt transect) or at intervals (interrupted transect).

Section 2 of 9Quadrats

A quadrat is a frame of known area (commonly 0.25 m2, or 1 m2), and placed at each sample point. The data collected may be:

Species frequency
Species frequency the proportion of quadrats (or of squares in a grid quadrat) in which the species occurs.
Percentage cover
Percentage cover estimate of the area covered by a species; useful for plants that are hard to count as individuals, such as grass or moss, and quick to measure.
Density
Density number of individuals per unit area.

Mean number per quadrat x (total area / quadrat area) estimates the population size. For example, a mean of 6 plants per 0.25 m2 quadrat in a field of 2000 m2 gives 6 x (2000 / 0.25) = 48 000 plants.

To improve reliability, use a large number of quadrats, a suitable size, and calculate the mean. A running mean can show when enough samples have been taken: when the mean stops changing significantly, the sample is large enough.

Section 3 of 9Mark-release-recapture

Used for motile animals, such as woodlice, beetles and small mammals.

1
Capture as many individuals as possible in the area (n1).
2
Mark each one in a harmless, non-toxic way that does not make it more visible to predators (e.g. a small dot of paint or a clip on a hair) and release them.
3
Allow time for the marked animals to mix with the population.
4
Capture a second sample (n2) and count how many are marked (m).

Estimated population = (n1 x n2) / m. The method assumes: the marked animals mix randomly; no births, deaths, immigration or emigration in the period; the mark does not affect survival; and the mark does not wear off.

Section 4 of 9Choosing and evaluating methods

Choose pitfall traps for ground invertebrates, pooters for small insects, sweep nets for insects in vegetation, kick sampling for stream invertebrates (timed to standardise the effort), and quadrats for sessile organisms. Evaluation points include: the accuracy of random sampling depends on a sufficient number of samples; the effects of the habitat on the distribution; observer error in estimating percentage cover; harm to the habitat from trampling; and ethical considerations - handle animals with care and return them to where they were caught.

Differences between methods should be considered. Quadrat counting is direct and cheap but unsuitable for fast-moving animals. Mark-recapture is suitable for mobile organisms but needs the assumptions above, and trapping may change animals' behaviour (trap-happy or trap-shy animals).

Section 5 of 9Using the data

MethodUsed forMeasure
Random quadratsPlants, sessile animals in uniform areaDensity, frequency, percentage cover
TransectDistribution along a gradientPresence/abundance by distance
Mark-release-recaptureMobile animalsEstimated total population

Link to 7.4.7 (required practical) and to statistical tests of correlation between an abiotic factor and species abundance.

Section 6 of 9Worked example and accuracy

A transect across a meadow has quadrats at 5 m intervals.

Worked example and accuracy
Counts of buttercups in a 0.25 m2 quadrat were 2, 4, 3, 5 and 1, so the mean is 3 per quadrat and the density is 3 / 0.25 = 12 per m2.
In a field of 3000 m2 the estimate is 36 000 buttercups.
The estimate is reliable only if the quadrats are numerous and representative.

The accuracy of the estimate improves with more samples, which can be judged by plotting the running mean against the number of quadrats and seeing where it levels off.

Worked example and accuracy
Variation can be shown with the standard deviation; a large standard deviation suggests that the plants are unevenly distributed (clumped).
Percentage cover is quicker than counting but more subjective, so use the same person to estimate, or a grid quadrat divided into 100 squares and count the squares at least half covered.
Remember always to describe the use of random numbers and not just 'throw the quadrat', which is biased by the person throwing.

Section 7 of 9Exam-style questions

  1. A student released 40 marked snails into a garden. A later sample contained 50 snails, of which 8 were marked. Calculate the estimated population size. [2]2 marks

  2. Describe how you would use quadrats to estimate the population size of daisies in a field of area 4000 m2. [5]5 marks

  3. Give three assumptions made when using the mark-release-recapture method. [3]3 marks

Section 8 of 9Key terms

TermMeaning
QuadratA frame of known area used to sample sessile organisms.
TransectA line along which samples are taken to study changes across a gradient.
Percentage coverEstimated percentage of the quadrat area covered by a species.
Random samplingSelecting sample sites so that each location has an equal chance of being chosen.
Mark-release-recaptureMethod of estimating the size of a mobile animal population by marking and recapturing.
Species frequencyThe proportion of sample areas in which a species is present.
DensityNumber of individuals per unit area.

Section 9 of 9Exam technique

1
Placing quadrats where the plants look most interesting rather than using random coordinates.
2
Using too few samples, so the mean is unreliable.
3
Forgetting to scale up to the whole area using the quadrat area.
4
Marking animals in a way that makes them more visible to predators.
5
Not giving the Lincoln index as (n1 x n2) / m.

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