Appropriate Sampling Methods for an Ecological Study

When investigations into specific habitats need to occur for whatever reason, be it analysis of species present or the viability of the habitat under certain conditions, sampling becomes essential. It would be a tedious process to count every animal in the habitat in addition to gathering other information such as different sizes and colours of the inhabitants. To solve this problem, we take specimens to be studied from different areas of the habitat with the assumption that what we have gathered is an adequate representation of the habitat. The idea is to expand as little effort as possible in collection without compromising the data.

Different Sampling Techniques

There are many different sampling methods. The ones to be discussed here are: quadrat; pooters; bottles; jars; nets; line and belt transects; mark, release and recapture methods.

Quadrat

These are generally used for sampling plant communities and stationary animals. Two things to be noted about quadrant use are:

  • The purpose for using a quadrant is to allow for comparable samples to be obtained from areas of consistent size and shape. So it is essential that the same size and shape quadrant is used in the habitat being studied;
  • For proper use choose an area large enough to be representative of the specimen being studied. The area must not be so big that it cannot be sampled adequately or so small that the habitat can be damaged by trampling feet.

The quadrant can give important information relating to density, frequency and percentage cover. These are called methods of abundance and are discussed below.

Density

Count the number of individuals in several quadrants and take the mean to give the number per unit area.

Frequency

This is the number or percentage of sampling units in which a particular specie occurs. This avoids having to count the number of individuals. Consistency in determining the presence or absence in a sampling unit is essential. For example, if in one sampling unit you counted all the plants rooted and ignored the plants uprooted, then in another sampling unit you counted plants rooted and uprooted, you are not being consistent. You must decide whether or not you will count all uprooted plants OR all rooted plants OR all uprooted and rooted plants.

Percentage Cover

This is the percentage of ground covered by a species within the sampling unit. It can be found by counting the number of squares within the quadrant that the plant completely covers, then count those that are only partly covered and estimate the total number of full squares that would be covered by that species.

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Pooter

This is a device used to catch small insects. The user breathes in through the mouth piece which has a piece of net covering the end. The insects are sucked into the holding chamber via the inlet tube.

 bio_pooter

Nets

Nets can be used in a variety of settings to sample various insects, for example butterflies; or aquatic creatures, for example zooplankton in a pond. One example of a net would be a sweep net.

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Sweep Net

These are used in areas of long grass to catch organisms. They can also be used in ponds.

Bottles/Jars

Bottles are often used to sample water, for example samples may be collected in plastic bottles to test for pH and alkalinity or in a glass bottle for nitrogen content. The sample bottles are often thoroughly cleaned to ensure than they are free from contaminants as possible. The one thing that a jar may be used for that a bottle may not be used for is soil sampling.

Line and Belt Transects

Line transects are used when you wish to illustrate a particular gradient or linear pattern along which communities of plants or animals change. They provide a good way of being able to visualise changes taking place along the line. Where time is limited a line transect can be carried out faster than a belt transect.

Line Transect

Line Transect

Belt transects give information on abundance as well as presence or absence of species. It may be considered as a widening of the line transect to form a continuous belt or series of quadrats.

Belt Transect

Belt Transect

Mark and Release Capture Methods

Used for estimating population size of mobile animals where it is likely to be impossible to count all individuals from a given habitat. A number of animals are captured within a habitat and marked in a way that makes them easily recognised when they are encountered again. The marked animals are released into the same habitat and assumed that they will mix back into the rest of the local population. The same habitat is then re-sampled and the number of marked and unmarked animals caught in the second sampling is noted. The proportion of animals marked in the first sampling, that is recaptured in the second sampling can be used to estimate the overall population size.

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