Practical: Investigating Population Size (Edexcel IGCSE Biology: Double Science)

• Ecology is the branch of biology that studies:
  • The distribution of species
  • The abundance of species
  • Interactions between species
  • Interactions between a species and its abiotic environment
• Ecologists are biologists that study these interactions by investigating ecosystems
• One piece of equipment that might be used to investigate population size is a quadrat

Quadrats

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

Using a quadrat to investigate population size or distribution

• Quadrats can be used to measure abundance by recording:
  • The number of an individual species: the total number of individuals of a single species (eg. buttercups) is recorded
  • Species richness: the total number of different species (but not the number of individuals of each species) is recorded
  • Percentage cover: the approximate percentage of the quadrat area in which an individual species is found is recorded (this method is used when it is difficult to count individuals of the plant species being recorded eg. grass or moss

How to estimate percentage cover of one or more species using a quadrat

Investigating population size in 2 different areas using quadrats

Apparatus

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

Method

How to estimate the population size of a plant species in a survey area. You must repeat steps 1-5 in the second study area.

Results

• Once the results have been collected and the averages calculated, we can compare the abundance of the study 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