Sampling Animals

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Mark, release, recapture Method.

  • Some animals are trapped, e.g. using pitfall traps.
  • They are marked in some harmless way then released.
  • Traps are used a few days later.
  • The number of marked and unmarked animals are recorded.
  • The population size is estimated usinf this formula:

 Population size:   Number in 1st sample X Number in 2nd sample

                           Number in 2nd sample previously marked

  • E.g. 10 animals were trapped, marked and released. 2 days later, 20 animals were trapped, where 5 were found to be marked:

 Population size: 10 X 20  = 200 = 40                                                                                                                           5           5


  • Marked animals will mix with origninal population.
  • The marking doesn't affect their survival.
  • No deaths and no births - population size stays the same ( stable )
  • No movement of animals in and out of the area. 
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Animals that move are tricky to sample

  • Move around - could be counted twice.
  • A risk of not counting all the animals if they are moving about.
  • Some animals may be active at different times of the day.
  • Animals ma run away or hide from you as you approach them to sample them.
  • You cannot be certain if an animal actually belongs to the habitat you re sampling.


  • Large animals can be observed.
  • Smaller animals and microbes trapped using appropriate equipment.
  • Mark, release ,recapture method can be used to estimate the population. 
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