What's the point of the mark-recapture method?

Question

What's the point of it if one of the assumptions is "the population size doesn't increase or decrease during the sampling study"?

Isn't the point of the mark-recapture method to determine what the population is?

What would happen if this assumption wasn't taken into consideration?

Answer 1

Two things about the assumption - 1) Increase and decrease could be the difference between an "open" and "closed" population amd 2) Length of time between sampling periods.

For the first item I listed, you need to be relatively certain that the organisms you're sampling don't get up and more away, or that new members don't enter from outside (which would change the number of the population from when you did the first sample). Otherwise, what you're estimating as the "population" is only a part of the total population. So before you begin the study you (as the researcher) need to know things like the boundary and distance an organism is likely to range (size of its territory). The way the calculations work is that they assume that that the percentage of organisms that are marked are a percentage of a fixed number, so when you sample the second time, the percentage of "recaptures" allows you to estimate the total number of what was present DURING THE FIRST SAMPLING PERIOD!

For the second, if your samples are a year or more apart, reproduction will likely have taken place, adding to the population (or in the case of a natural disaster, such as a flood, drought, tornado, etc.) some may have died.

The point of doing the population study is to have an estimate for a given area within a given period of time. If organisms can enter, leave, be born, or die, because they are free to roam or too much time passes or unexpected events happen, your results won't be accurate.