Population size is important for
> assessing the vulnerability to extinction or extirpation of a species
> or the value of a wildlife stock
> understanding a species rate of evolution ( gene flow over time, founder effects, and historic moments such as plague /catastrophe)
> predator prey relationships
> ecological dynamics
> foraging behaviors
It is therefore important to be able to estimate the sizes of populations. It is often difficult or impossible to count all the individuals in a population in fact it might be harmful. One of the most practical ways to estimate a population size is to use the MARK-CAPTURE SAMPLING method.

This method ass/u/me/s :   that >
a) the population is widely dispersed
b) every individual in the pop. is equally likely to be caught
c) population size does not change dramatically during sampling (seasonal diebacks, migration, harm)

Example - lets say you are a U of T student living in the Annex in Toronto and you want to know precisely how many cockroaches are living in your dorm room you could capture as many roaches one night in a non lethal roach motel and and then MARK them on the back with white out.

 

 5 of the roaches have been marked and then released back into
the apartment! (our total sample size here does not matter)

> #marked [M] = 5 roaches (who then disperse among other apartment dwelling roaches)
Roaches [N] are now milling about in near dispersion and then a roach trap is set up
to capture a second sample [n] over night (below).
 

A count of the sample reveals that there are about 19-20 roaches and there are 2 marked [m] roaches in this second sample [n].

total# marked [M]      =           # recaptured [m]
total population [N]          size of second sample [n]

      N = Mn   =   5x20    =  50 roaches approximately
              m              2
- this system might be very accurate in determining  the number of field mice per hectare but when it comes to wide ranging populations it becomes unreliable.
- what harm, might come to the roach in the trap?