Measuring the distribution of a species

Collecting organisms

Organisms can be collected using a variety of methods. These methods include:

  • sweep nets
  • pooters
  • pitfall traps

Pitfall traps

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.

Cross-section of a bug trap showing an insect at the top of a hole. The hole is covered by a board which is raised from the ground at an angle by stones either side of the hole.Pitfall trapping involves the placement of open containers in the ground

Counting organisms

It is often impractical to count all the organisms in a population. For example:

  • animals may move into - or out of - an area during counting
  • some may die during the time it takes to complete the counting
  • it may be difficult to find all the animals during counting

As a result, biologists have developed different ways to estimate the size of a population, including the use of a quadrat.

Quadrats

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.

Man working within a quadrat, a rectangular plot
A quadrat is a tool used to isolate a physical area for study

Worked example

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:

  • it should be placed randomly so that a representative sample is taken
  • the validity and reliability of the results increases as the results from more quadrats are analysed