Random sampling

It is often impractical or impossible to measure every single individual when investigating population traits. Random sampling is important to avoid bias and ensure the samples are representative of the whole population.

How random sampling works:

• Choose an area.
• Randomly generate coordinates across the area - This prevents sampling bias by removing human involvement in choosing samples.
• Collect samples from random coordinates - This gives us samples that are representative of the population.
• Repeat this several times - This gives us a large sample size and minimises the effects of chance.
• Analyse the data collected - This lets us identify any relationships.

Random sampling techniques

Once you have generated random coordinates, there are are a few different techniques you need to know that can be used to collect samples of plants and animals.

Sampling animals

There are a several techniques for sampling animals you need to know.

Techniques for sampling animals:

1. Pooter - This samples small insects by sucking air containing the insects into a plastic container via a tube.
2. Sweep net - This samples insects in long grass or air, where the net is swept in a 'figure of eight' motion.
3. Pitfall trap - This samples small, ground-crawling animals like insects and spiders by catching them in a hidden trap.
4. Tree beating - This samples the invertebrates living in a tree or bush by shaking or beating the tree to dislodge insects onto a white sheet below.
5. Kick sampling - This samples river organisms by kicking a river bank and catching organisms downstream in a net.

Sampling plants and non-motile animals using quadrats

A quadrat is a frame used to sample the organisms in an area. They are typically used to sample plants, or animals that don’t move around much (non-motile animals).

There are two types of quadrat you can use:

1. Frame quadrat - This is a square frame divided into a grid. The type and number of species within each section of the quadrat is recorded.
2. Point quadrat - This is a frame with a horizontal bar, through which pins are pushed at set intervals to touch the ground. Each species the pin touches is recorded.

Methods of estimating the number of individuals in a frame quadrat:

Species density per quadrat - The number of individuals of a species in a 1 m² quadrat.

Species frequency per quadrat - Calculated as $\frac{\text{number of squares in a quadrat a species is found in}}{\text{number of total squares in the quadrat}}\times 100$.

Percentage cover per quadrat - An estimate of the area within a quadrat that a species covers.