Sampling for Abundance and Distribution (WJEC GCSE Biology)

The Principles of Sampling

• Sampling can be defined as:

The process of collecting a series of small data sets that can be used to represent an entire population

• Sampling in a habitat can be used to assess:

  • Abundance

    • The number of different species, or the number of individuals of a particular species

  • Distribution

    • Where a species is found within a habitat

• Sampling is a useful technique in large and complex habitats where it is not possible to find, identify, and count every organism that is present

• Sampling data can be scaled up to make an estimate for the total species numbers in a habitat

The importance of representative sampling

• Sampling involves taking a measure of abundance from a series of small areas within the habitat; these areas are used to represent the habitat as a whole

• In order to be representative it is essential that the following are considered:

  • Samples sites must be distributed randomly within a habitat to avoid bias

  • Enough samples must be taken to represent the entire habitat; generally the higher the number of samples the more representative the results will be

Using Quadrats to Investigate Abundance

• A quadrat is a square frame that is placed within the area to be studied to provide a sample

  • Quadrats can be used to study sessile organisms

  • Quadrats can be different sizes depending on the species being studied

    • A 1 m² quadrat can be used to study small organisms such as herbaceous plants in a grassland or limpets on a rocky shore

    • A 400 m² quadrat can be used to study large organisms such as trees

      • Quadrats like this will usually be marked out with string rather than a frame!

• Quadrats can be placed in a habitat randomly or along a transect

Quadrat diagram

Quadrats can be used to take samples when measuring biodiversity

Random quadrat sampling

• When studying the abundance of organisms throughout an entire habitat quadrats should be placed randomly

  • This avoids bias which might occur, e.g. if a student chose to take samples in a particular spot because the organisms look easier to count

• A series of random quadrat samples could be taken as follows:

  1. Lay out two tape measures at the edges of a habitat like a pair of graph axes; this creates a grid

  2. Use a random number generator to choose two numbers to use as grid co-ordinates

     • The quadrat is placed at this location on the grid

  3. Take a measure of abundance, e.g.

     • The number of species within the quadrat

     • The number of individuals of a particular species within the quadrat

     • The percentage cover of a particular species within the habitat

       • This may be easier for species where individual organisms cannot be easily distinguished, e.g. grass

  4. Points 2 and 3 should be repeated a number of times in order to obtain a representative sample

Random quadrat sampling diagram

Quadrats should be placed at random when measuring abundance throughout an entire habitat

Transect sampling

• Transects are lines laid out across a site; they are used to measure the effect of environmental factors on the distribution of organisms along a gradient, e.g.

  • At increasing distances from a feature such as a field margin or the edge of a water body

  • Along a soil pH gradient

  • Along a light intensity gradient

• Quadrats can be used during transect sampling as follows:

  1. Lay out a tape measure across the habitat along the chosen gradient; this forms the transect

     • E.g. if investigating the effect of altitude on abundance then an altitude gradient would be used

  2. Place the quadrat at set points along the tape measure, e.g. at 5 metre or 1 metre intervals

  3. Take a measure of abundance in each quadrat

  4. Points 2 and 3 should be repeated a number of times in order to obtain a representative sample

Transect diagram

Quadrats can be used together with transects to assess abundance along a gradient within a habitat, e.g. buttercup abundance at different altitudes