Sampling Methods

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Sampling Methods

Measuring biodiversity across large areas and in difficult locations can be impractical so scientists take samples. It's just like being at a food market!

Samples

Scientists study samples of large areas of land because it would be super hard to measure ALL the animals etc. (...biodiversity, if you will) across the whole area.
- Samples are designed to give a representation of biodiversity in an area.
- Samples involve measuring indices of diversity such as species richness over small areas and extrapolating the data to estimate diversity across large habitats.

Sampling methods

There are several different techniques to taking samples of populations and communities.
There are advantages and disadvantages for each method, and different techniques are more applicable in different scenarios.

Random sampling

Individuals or locations to measure are selected at random from a population or habitat.
Random sampling ensures minimal selection bias is introduced to the sampling process.
Random sampling is truly random and occasionally your data may not be entirely relevant to your investigation, especially if certain variables you are interested in occur infrequently.

Systematic sampling

Individuals or locations are selected at regular intervals within the target population or area.
Systematic sampling is very easy to implement.
Can introduce bias if the sampling interval coincides with underlying patterns within the sampling population.

Stratified sampling

The sampling site or population is partitioned into groups that each reflect a certain trait and then sampling takes place from each of the groups.
For example, studies in humans often stratify the population into males and females, this ensures equal representation from men and women in the final study.
Stratified sampling can produce more meaningful results and can be more accurate than a random study.
In order for accuracy to be achieved, knowledge of what constitutes a certain characteristic is required or the study risks misrepresenting the results.

Field data collection techniques

Collecting samples of organisms in the field requires safe, repeatable and ethical techniques.
Sweep netting is used to catch airborne insects and organisms present above ground in meadow-like environments. It can also be used for water-borne organisms.
Pitfalls passively catch organisms that fall into the holes that have been dug in the ground.
Pooters retrieve targeted organisms that have been identified for selection.

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.

Sampling Animals

Most animals are more mobile than plants, so they can't be accurately sampled using quadrats or transects.

Pooters, nets, and keys

Pooters and nets can be used to catch crawling insects and flying animals, respectively.
Keys can then be used to identify organisms.

Capture-recapture

Animals are caught, marked and then released.
Then another sample is taken, and the number of recaptured animals (those that have been marked) is recorded. This gives information on the population size.