Mark-Recapture

The Mark-Recapture technique is used to estimate the size of a population where it is impractical to count every individual. The basic idea is that you capture a small number of individuals, put a harmless mark on them, and release them back into the population. At a later date, you catch another small group, and record how many have a mark. In a small population, you are more likely to recapture marked individuals, whereas in a large population, you are less likely. This can be expressed mathematically using the equation below.

N = (M*C) / R

N = estimated Number of individuals in the population

M = number of individuals captured and Marked

C = total number Captured the second time (with and without a mark)

R= number of individuals Recaptured (those with a mark)

 

Example: Let's say that your neighborhood has a population of snails, which come out onto the sidewalk when the sprinklers come on, and the rest of the time they hide in the vegetation. If you catch 20, and mark them and, then, a week later, after they have had a chance to disperse into the population, you catch 15, and 6 have marks on them, what is the estimated size of the population?

N = (M*C) / R

N = (20 * 15) / 6

N = 50

snails

Assumptions: If any of the following assumptions or conditions are violated, it may affect the accuracy of the population estimate.

  1. If no marked individuals are recaptured, R = 0 and your result is undefined. Mark more individuals and try again.
  2. The marking technique must not harm the individual or affect its survival by predation.
  3. The mark must not wash off or wear away.
  4. There must be no immigration into or emigration out of the population.
  5. There must be no mortality between the mark and recapture times.
  6. The marking experience must not make an individual more or less likely to be recaptured.

Questions:

  1. Describe how a mark might affect the survival of an individual. Would that cause an over- or under-estimate of the population size?
  2. What if the mark washed off the animal? Would that cause an over- or under-estimate of the population size?
  3. What if many unmarked individuals are immigrating into the population? Would that cause an over- or under-estimate of the population size?
  4. What if many of the marked individuals are emigrating from the population? Would that cause an over- or under-estimate of the population size?
  5. What if most of the marked individuals die before the recapture is conducted? Would that cause an over- or under-estimate of the population size?
  6. Can you suggest some reasons why an individual might be more likely than average to be recaptured? Would that cause an over- or under-estimate of the population size?
  7. Can you think of a reason why an individual might be less likely to be recaptured? Would that cause an over- or under-estimate of the population size?

References:

https://www.youtube.com/watch?v=240806aPHVg

https://en.wikipedia.org/wiki/Mark_and_recapture