Organisms can be collected using a variety of methods. These methods include:
A pitfall trap is often used to get a sample of small invertebrates living on the ground, such as beetles, spiders and slugs.
It consists of a container, such as a yoghurt carton, buried in the ground. The top of the container is level with the soil surface, and it is covered by a piece of wood with a slight gap to allow insects to climb in. It is important to check the trap regularly to avoid the animals escaping or being eaten before they are counted.
It is often impractical to count all the organisms in a population. For example:
As a result, biologists have developed different ways to estimate the size of a population, including the use of a quadrat.
A quadrat is usually a square made of wire. It may contain further wires to mark off smaller areas inside, such as 5 × 5 squares or 10 × 10 squares. The organisms underneath, usually plants, can be identified and counted. Quadrats may also be used for slow-moving animals, eg slugs and snails.
There are four dandelion plants inside a 0.25 m2 quadrat. The whole field is 50 m2 in area. The estimated population size of dandelions in the field would be:
4 × (50 ÷ 0.25) = 4 × 200 = 800
When using a quadrat: