Field investigations

It is important for ecologists to estimate the number of organisms in a population to:

It is almost always impossible to count all of the organisms in a population. So a small section of a population can be looked at to estimate distribution or numbers in the whole population.

This process requires sampling and the area or part of population looked at is called a sample.

Sampling

When sampling a population:

  • the numbers of organisms are counted within a sample site, and then the results multiplied to estimate the total number in the entire habitat
  • large animals and plants can often simply be counted, sometimes from an aerial photograph
  • smaller animals such as insects and smaller mammals need to be trapped first so they can be counted and then released afterwards - pitfall traps are small traps dug into the ground, which often have food inside to attract small mammals and the sides of these traps are smooth to stop the mammals escaping
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.
  • insects and other invertebrates can be collected and counted using large nets to sweep through grasses or leaves of trees in a process called sweep netting
  • aquatic organisms can be collected and counted using nets held downstream of an area of river bed which is then gently disturbed by the person doing the sampling - the small animals float into the net, and this is called kick-sampling
  • pooters are small devices used when sampling to suck up small insects safely without them going into your mouth
A pooter device being used to collect small insects

Using quadrats

Quadrats, are square frames of wire, often with an area of 0.25 m2. These are placed on the sampling area - for instance, the ground, a rockpool or tree trunk - to estimate the distribution of plants or slow-moving animals within them.

When investigating plants in quadrats, the following sampling techniques can be used:

  • Number of an individual species: the total number of individuals of one species (eg daisies) is recorded.
  • Species richness: the number of different plant or animal species is recorded but not the number of individuals within a species.
  • Percentage cover: the percentage of the quadrat area that is covered by one species (eg grass). This is easier to estimate if a quadrat has wires making smaller sections. Percentage cover rather than number of individuals is used when estimating plant frequencies if it is difficult to identify individual plants, such as grasses, moss or seaweeds on rocks.
Man working within a quadrat, a rectangular plot

Random or systematic sampling?

Random sampling using a quadrat involves the placing of quadrats at random coordinates.

Regardless of whether you are investigating the number of individual species, the species diversity or the percentage cover in different areas you would normally use random sampling.

Systematic sampling uses a line called a transect, sampling at regular intervals along it, often using a quadrat. Transects are used where the habitat is not uniform and may show different zones.

curriculum-key-fact
Most sampling is random.

Systematic sampling can be used if there is a trend or pattern across the habitat, such as distance up a beach, or altitude on a hillside.

If you are using the wrong kind of sampling method for your experiment, this can lead to biased results.