Investigating Populations

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1.2 - Investigating populations

We study a habitat by counting number of individuals of a species in an area (abundance) but it is virtually impossible to count and indentify every organism [too time-consuming and damaging to the habit] so we use smaller samples, which is representative of the habitat to ensure validity of the conclusion made.

Sampling techniques

  • Random sampling - frame/point quadrats
  • Systematic sampling - along transects


  • Size - depends on size of animals/plant being counted, and distribution (larger species needs a larger quadrat). A large sample of small quadrats is more representative when species are in groups and not evenly distributed.

  • Quadrat number - quadrat number relies on a balance between representative results and time to collect data

  • Placement - statistically significant results are the result of random sampling 

Random sampling 

Sampling must be random to avoid bias when collecting data.

1. Lay out two tape measures at right angles around sample area
2. Random numbers found (computer) to find co-ordinates in square
3. Take sample of species inside quadrats on the co-ordinates

Systematic sampling

 Sometimes more informative to sample this way and when there is a transition between communities of plants and animals. A line transect comprises of a string in a straight line over the ground and a belt transect is the…


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