Investigating Population Size
Biology: AQA A Level, topic 7.4.4
Quadrats
The population size of immobile (e.g. plants) or slow-moving organisms (e.g. limpets) can be estimated using random samples of quadrats.
Quadrat
- Quadrats are normally 1m2 square frame divided into 100 squares.
- Each square represents 1% of the total area of the quadrat.
- If an individual covers more than half of a square, it is counted as representing 1% of the quadrat.
- Quadrats are used to count the population size within a large area.
Sampling
- If you wanted to investigate the size of a population, it would take hours to count every individual.
- Taking samples (a small section of the total area) allows the population size to be estimated.
- It is important that the samples accurately represent the total population size.
Random sampling
- Random sampling can be applied by dividing the area of interest into a grid and labelling the grid with coordinates.
- A random number generator provides a set of random coordinates.
- The quadrats can be placed using the coordinates.
Transects
- Quadrats could also be placed at regular intervals along a transect.
- A transect is a line across a habitat, usually placed using a tape measure.
- Transects are used when there is a change, or gradient, in abiotic conditions across the habitat.
- Transects are used when looking for changes in the abundance or distribution of a species across the environment (e.g. how abundance changes from open field to forest).
Line transect
- A line transect records the species that makes contact with the tape measure at regular intervals along the transect.
Belt transect
- A belt transect uses quadrats.
- Quadrats are placed at regular intervals along the transect.
- An abundance of different species in the quadrat can be measured by:
- Percentage cover – estimate how much of the quadrat is covered by a particular species
- Frequency – count how many individuals of a particular species are present in the quadrat.
Mark-Release-Recapture
Mark-release-recapture is a technique used to investigate population size of mobile organisms (e.g. a tree shrew). The steps involved are:
1) Capture
- The first step in mark-release-recapture involves the capturing of animals.
- Small mammals can be captured using a Sherman trap (a box with a trap door) and ground insects can be captured using a pitfall trap.
2) Mark
- Captured animals are marked in some way (e.g. using tags, bands, paint, or other body markings).
- Marking the animals allows you to identify which animals have already been counted and which animals have not.
3) Release
- The marked animals are released back into the environment and the traps are set up again.
- Releasing the animals back into the environment allows them to mix with the rest of the population.
4) Recapture
- A new sample is collected from the traps. The unmarked individuals that are captured are counted, marked and released.
5) Calculate population size
- Total population size can be calculated using the following equation:
- population size = \frac{number\ caught\ in\ 1st\ sample\ ×\ number\ caught\ in\ 2nd\ sample}{number\ marked\ in\ 2nd\ sample\}
Effect of Environmental Factors
Belt transects can be used to investigate the effect of abiotic factors on species distribution. The steps involved are:
1) Marking the belt transect
- A belt transect is used to investigate how population size varies across an environment.
- In this investigation, the belt transect should be marked along an area where one environmental factor changes.
- E.g. Light exposure will vary along a transect that is marked across an open field and into dense tree cover.
- E.g. Soil pH will vary along a transect in a coastal ecosystem.
2) Place the quadrat
- Place a 1m2 quadrat at regular intervals along the belt transect.
- Use the quadrat to identify which species are present and count the percentage cover of the different species.
- If a species covers over half of one square on the quadrat, it has 1% cover of the quadrat.
3) Measure environmental factor
- Record how the environmental factor you are investigating varies at each sample point along the belt transect.
- If you are investigating light intensity, use a light meter.
- If you are investigating soil pH, use a pH probe.
4) Record results
- Plot a graph that shows how the distribution of species correlates to the environmental factor.
Test yourself on Investigating Population Size
Which of the following statements about quadrats are true?
In what sort of habitat are transects used?
Mark-Release-Recapture
Which of the following statements about mark-release-recapture are true?
What is the benefit of marking animals?