Random and non-random sampling

Non-random sampling involves selection based on specific criteria or patterns and the sample is not chosen at random.

Differences between random and non-random sampling:

1. Random sampling removes human bias in sample selection, improving reliability.
2. Non-random sampling does not use random selection so is prone to bias.

Non-random sampling techniques

Systematic sampling is the main non-random sampling technique you need to know.

Key features of systematic sampling:

• It is a non-random sampling method where different areas in a habitat are sampled at regular intervals to avoid bias.
• It allows for the study of how species' distributions change across different areas within a habitat, for example, from a woodland to a lake.
• It involves marking a line (transect) and taking samples along this line at regular intervals.

Transects

There are two types of transect sampling you need to know:

1. Continuous belt transects - These have quadrats placed continuously along the transect line to study populations and detect any changes across an environmental gradient.
2. Interrupted belt transects - These have quadrats placed at regular intervals (e.g. every 5 m) along the line rather than continuously, which is faster but provides less precise data on population changes.

Both types of transect can reveal transitions and patterns in species distribution related to environmental factors.

Measuring abiotic factors

Abiotic factors directly affect the distribution of organisms in a habitat, so they are useful to measure when sampling.

We can measure abiotic factors like:

• Light, humidity, and temperature (using sensors).
• pH and wind speed (using probes).
• Dissolved oxygen (using specialised probes).

Advantages of measuring abiotic factors:

• They can detect rapid changes.
• They can reduce human error in taking readings.
• They can achieve a high degree of precision
• They allow data to be stored and tracked on a computer.

Investigating the effect of abiotic factors on distribution

For example, there may be seashore vegetation gradients based on shell deposit acidity, where the pH is lower closer to the sea due to rotting organic matter.

This could be investigated by:

1. Placing the transect line perpendicular to the shore.
2. Taking quadrat samples at regular intervals.
3. Recording the percentage cover of the target species.
4. Measuring the environmental variable (e.g. pH) at regular intervals.
5. Producing a graph of the data and analysing the relationship.