Measuring Number & Distribution of Species

Applied Science: BTEC Level 3 National Extended Diploma, topic 3.5.2

Quadrats

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.
Some quadrants can have a point frame attached.
A point frame contains ten pins. When placed on a quadrat, you note down the plant species each pin touches.
Point frames can be used to measure plant cover & distribution.

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 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.

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).

A line transect records the species that makes contact with the tape measure at regular intervals along the 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.