Practical: Investigating Population Size (Edexcel IGCSE Biology)

• Ecology is the branch of biology that studies species distribution, abundance, interactions between species, and their interactions with the abiotic environment
• Ecologists investigate ecosystems, often using tools like quadrats to study population size

Quadrats

• Quadrats are square frames made of wood or wire
• They can be a variety of sizes, e.g. 0.25 m² or 1 m²
• They are placed on the ground and the organisms within them are recorded
• Plants species are commonly studied using quadrats to estimate their abundance

Quadrats are used to investigate population size or species distribution

• Quadrats can be used to measure abundance by recording:
  • the number of individuals of a single species, e.g. buttercups
  • species richness: the number of different species present
  • percentage cover: the approximate percentage of the quadrat area in which an individual species is found
    • this method is used when it is difficult to count individuals of the plant species being recorded, e.g. grass or moss

Percentage cover can be assessed using a quadrat

Investigating population size in two different areas using quadrats

Apparatus

• 2 tape measures
• Quadrat
• Random number generator
• Species key

Method

The population size of a plant species in a survey area can be estimated using quadrats

Results

• Once the results have been collected and the averages calculated, we can compare the abundance of the species in each survey area
• Species abundance is likely to be influenced by biotic factors such as:
  • competition
  • predator-prey relationships
  • interactions with other organisms within the food chain or food web
• The abundance will also be influenced by abiotic factors such as:
  • light intensity
  • mineral availability
  • water availability
  • pH
  • temperature
  • salinity

Limitations

• It can be easy to miss individual organisms when counting in a quadrat, especially if they are covered by a different species
  • Solution: Use a pencil or stick to carefully move leaves out of the way to check if there is anything else underneath
• Identifying species may be tricky
  • Solution: Use a species key to identify the species

Applying CORMS to practical work

• When working with practical investigations, remember to consider your CORMS evaluation

CORMS Evaluation

• In this investigation, your evaluation should look something like this:
  • C - We are changing the study area where we are collecting the data
  • O - We will count the same species of organism in each quadrat
  • R - We will repeat the investigation several times to ensure reliability
  • M1 - We will count the number of the designated study species found across all quadrats
  • M2 -  this isn't really relevant in this scenario
  • S - We will control the size of the quadrat, the random way that quadrats are placed on the ground, the day that the results were collected