Required Practical: Ecosystems (AQA GCSE Combined Science: Trilogy) : 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

• You will:

  • 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

Investigating the effect of a factor on the distribution of this species

• 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