Key terms

  • Biodiversity – a measure of the number of different species living in an area.
  • Population – the number of organisms of the same species living in an area.
  • Habitat – where a population lives.
  • Environment – an organism’s surroundings. It contains factors that influence the organism. These factors can be divided into abiotic and biotic factors.
  • Abiotic – non-living factors e.g. temperature.
  • Biotic – living factors e.g. a predator.
  • Community – several populations of different species living in habitats close together.
  • Ecosystem – an area where a community of organisms live and are affected by a range of environmental factors.


The distribution of organisms in a habitat may be affected by physical factors, such as temperature and light.

When investigating the number of organisms it is difficult to cover a large area because of time constraints.

A large grassy ecosystem.

To overcome this, a number of small sections within a larger area are investigated or sampled.

It is important that sampling methods are:

  • Reliable - a large sample needs to be obtained. For example 20 small sections should be sampled if the area is large.
  • Representative - it needs to be representative of the entire area being investigated. In other words sampling should take place randomly across the larger area, not just concentrated in one small part.

Collecting data

The method of data collection depends on the organism being sampled. A quadrat (a square frame) can be used if the organism moves slowly or not at all.

A student using a quadrat

Organisms found inside the quadrat can be counted or their percentage cover can be estimated.

Percentage cover is the percentage of the quadrat covered by each species, which is usually rounded up to the nearest 10%. However, if percentage cover is between 1-5%, it is rounded down to 1%.

When data for all the quadrats has been collected an average for each species can be calculated.

This gives an idea of the percentage cover of that organism across the entire area being investigated.