Required Practical: Ecosystems (AQA GCSE Biology): Revision Note

RP9: Investigating ecosystems

Aim:

• Measure the population size of a common species in a habitat

• Use sampling techniques to investigate the effect of a factor on the distribution of this species

Sampling

• Population size can be measured by taking random samples within a habitat

  • Sampling reduces the time spent counting organisms

  • The sample represents the population as a whole, so must be large enough, and must be unbiased

Method: estimating population size

• Use a quadrat to estimate the population size of a plant species in a survey area 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 for your chosen species, e.g.

     • The number of individuals within the quadrat

     • The percentage cover within the quadrat

  4. Record abundance in a suitable results table

  5. Repeat steps 2-4 a number of times in order to obtain a representative sample

  6. Estimate the population of the survey area using the equation:

  $\mathrm{estimated}\mathrm{population}\mathrm{size}=\left(\frac{\mathrm{total}\mathrm{area}}{\mathrm{area}\mathrm{sampled}}\right)\times \mathrm{number}\mathrm{of}\mathrm{individuals}\mathrm{counted}$

Method: effect of a factor on species distribution

• The effect of a factor on species distribution can be studied using a transect as follows:

  1. Set the a tape measure across the gradient to be studied, e.g. ascending up the side of a hill

  2. Place a quadrat at equal intervals along the transect, e.g. every 5 m

  3. Record the number of the chosen plant species inside each quadrat

  4. Record the abiotic factor, e.g. altitude, for each quadrat

  5. Record the results in a table

  6. Plot the data in a graph and describe any relationship that can be observed

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