Required Practical: Ecosystems (AQA GCSE Combined Science: Trilogy: Biology): Revision Note

RP9: Investigating ecosystems

• Aim:

  • to measure the population size of a common species in a habitat and use sampling techniques to investigate the effect of a factor on the distribution of this species

• Proceedure:

  • use a quadrat to estimate the population size of a plant species in a survey area

  • use a transect line and a quadrat to investigate the effect of a factor on the number of plants in a survey area

Measuring population size

• 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

• Samples are taken using quadrats, which are placed randomly within the habitat to avoid bias, e.g. so that students don't just sample the closest part of the habitat

  • Randomness is achieved using a grid and random number generator

Use these images

Estimating population size method:

1. Use two tape measures to lay out a survey area (e.g. 10 m × 10 m) in the chosen habitat, such as the school field

2. Use a random number generator to create a set of coordinates to place the first quadrat. E.g. a 4 and a 5 would mean to place the quadrat 4 m along the x-axis and 5 m along the y-axis

3. Count the number of the chosen plant species (e.g. dandelions) that are found within this quadrat

4. Record this number in a results table and repeat steps 1–3 to record the number of the chosen plant species in all 10 quadrats

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

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

Investigating the effect of a factor on species distribution

• Species are not usually evenly distributed throughout a habitat, but can be affected by factors in the environment, e.g. the presence of a river, or a large tree

• Sampling using a transect allows the effect of a particular factor on the distribution of a species to be assessed

Method:

1. Set the transect up through the area of investigation (e.g. a 30 m tape measure is placed up a hillside)

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) at each quadrat

5. Record the results in a table

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

Use this image