3. Sampling Techniques

What you need to know...
  • Sampling plants and animals using quantitative techniques including quadrats and pitfall traps.
  • Evaluation of limitations and sources of error in pitfall traps and quadrats.
  • Measuring abiotic factors including light intensity, temperature, pH and soil moisture.

Source: SQA

As you already know, this unit as all about the study of life on earth. But how do we study it? How do we know which species live where and which abiotic factors affect their distribution? Well, that's what this topic is all about. In this topic we learn about two techniques for sampling living things and how we can go about measuring abiotic factors.


Let's start with sampling. Imagine your were asked to count the number of each of the different plant species in the field below. How would you go about it?

A pretty daunting task? Impossible really. For most species it's simply impractical to count every single individual organism. Instead, we use sampling. Sampling is a way of getting a quantitative (numerical) measurement which is an estimate of the actual number. It involves finding the average number of organisms of a species in a particular area and then multiplying this by the total area being studied. In order for this to work many samples need to be replicated and then averaged to ensure the estimate is as reliable a figure as possible.

The method used to sample the organisms depends on the type of species being studied. For plants and animals which don't move much (i.e. limpets) quadrats are normally used. A quadrat consists of a wooden or plastic square of a known area (normally 1m2) which is subdivided into equal sized squares using string. You can see a quadrat being used in the image below.

There are different ways of using a quadrat, but the researcher will use a consistent method for each replicate. Quadrats are placed at random, normally using randomly generated coordinates on a calculator. The researcher then records the presence and abundance of each plant or animal species in the quadrat gird. You can see a quadrat in use in the video below.

As already mentioned, quadrats can only be used with organisms which either don't move at all, or so slowly that they would't move in the course of recording a replicate. Even with plant species, only relatively small species can be sampled with a quadrat. Using quadrats to determine a reliable measurement is difficult to do well and many measurements taken with quadrats are therefore relatively inaccurate. Sources of error when using quadrats include the difficulty in identifying the species present and deciding whether or not to include a plant which is partially present in the grid. Researchers therefore develop and agree consistent protocols to be used in the field to help remove the need for each individual researcher to use their own judgement.

Check out this useful quadrat animation [flash required].

If that's how we sample plants and limpets, how do we sample faster moving animals? There are many different methods depending on the size and speed of the animal. The one technique you need to know about it is the pitfall trap. This is a simple method which is used to sample small invertebrates such as insects. There are many different ways of constructing a pitfall trap, but they all essentially involve a container buried in the ground up to the level of the container opening. The container is filled with a fluid which kills and/or preserves the insects such as water or ethanol and a cover is placed over the trap to protect it from rainfall, debris and larger animals. One example of pitfall trap is shown in the image below.

Another example of a pitfall trap setup is shown in the detailed demonstration in the video below, or there's another here.

As with quadrats, pitfall traps are replicated numerous times in a particular area in order to obtain a reliable representative sample of the species being studied. Once again, pitfall traps are limited in use. They can only be used with relatively small, non-flying, insects. Sources of error when using pitfall traps include having an insufficient number of of traps, traps which are the wrong size or not placing the traps randomly.

This animal sampling animation includes pitfall traps [flash required].

Measuring abiotic factors

When studying the species in a habitat it is normally important to record the abiotic factors in the ecosystem also. Researchers use a variety of instruments in order to record the abiotic factors. The following video explains this in quite some detail.

You need to know what sort of instrument would be used to measure each of the following abiotic factors:

Abiotic Factor Instrument
Light Intensity Light Meter
Temperature Thermometer
pH pH Meter
Soil Moisture Moisture Meter

When measuring abiotic factors it is important that the equipment used is well maintained and clean. Researchers will record the abiotic conditions using the same method at each sample point in order to reduce sources of error. For example, if recording the temperature whilst using quadrats, the temperature will be measured at the same height from the soil each time. As with sampling, numerous measurements are made for each abiotic factor and average calculated in order to increase the reliability of the measurements taken.