7.4.4

# Investigating Population Size

Test yourself

The population size of immobile (e.g. plants) or slow-moving organisms (e.g. limpets) can be estimated using random samples of quadrats.

• 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.

• 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.