1.2 Investigating populations

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Factors to think about when using a quadrat

Three factors to consider when using quadrats:

  • The size of the quadrat - Dependent on the size of the organism being counted, i.e larger species need larger quadrats. Also if the species is in groups rather than even distribution, smaller quadrats should be used.
  • The number of quadrats being used - the larger the number, the more reliable the results. The more different species present, the more quadrats needed.
  • The position of each quadrat in the area - to produce stat significant results random sampling must be used.
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Random sampling- using quadrats.

  • It is important to avoid bias, by using random sampling, so the data collected is valid.
  • It is not effective enough to just throw a quadrat randomly, as you would unintentionally favour some areas more than others.

1. Lay out out two tape measures at the the two sides of the area.

2. Use a computer generator to randomly create coordinates.

3. Place the quadrat at the coordinates and record the species within it. 

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Systematic sampling

  • When there is transition in species present, systematic sampling is more useful.
  • For example, the change in communities of organisms as you farther from a lake, could be measured using a transect and systematic sampling.
  • A transect, i.e a tape stretched across the ground in a straight line, is placed, and any organism present on this line is recorded.
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Measuring abundance

Abundance - number of individuals of a species within a given space.

  • Percentage cover - when an organism is too abundant to count, or too difficult to count. It is an estimate of what percentage area the plant covers, calculated by the number of quadrats full of the species, divided by the total number of quadrats placed and then x100.


  • It doesn't show you how the organism is distributed, or how dense it is.
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Other abundance measuring techniques do not work on animals as they move around, and also are difficult to find.


A number of animals are caught, marked then released back into the wild. Later, you randomly collect another group of animals, and count the number which were marked.

 estimated population = no. in 1st sample x no. in 2nd sample/ no. of marked recaptured


  • the marked animals distribute themselves randomly among the rest of the population.
  • there is no immigration, emmigration, birth or death in the population
  • the mark isn't lost before recapture
  • the marking doesn't increase the probability of death (i.e. isn't toxic, or makes them more likely for predation)
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Analysing your data

  • First you have to make your data into a graph or table.
  • Any relationship between the variables could be affected by other factors.
  • The effect of chance is checked using a stat test: chi-test, standard deviation, spearman rank correlation. METHODS FOUND IN ISA
  • Postive correlation: both variables go UP
  • Negative correlation: when independent goes UP, dependent goes DOWN.
  • But correlation does not equal causation. The correlation could be something, ie they are both affected by an environmental factor.
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Hi, I have started using these notes to help me revise for my ISA and they are really helpful! Wondering if you had any tips? x

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