Question

What can you say about the general effect of marking effort on the variability associated with population size estimates in mark-recapture studies?

Answer 1

To understand how different physical and biological factors influence the distribution or abundance of species, population is measured and studied for changes that occur in them with respect to space and time. On one extreme ecologists can go all the way and try to count every single individual of that species and understand the changes and on the other extreme or the crude method, ecologists can count all the individuals in one small area and extrapolate their findings to generalize for the entire population. Both of these methods have their own advantages and disadvantages.

Many methods for estimating population size falls inbetween these two extreme methods. One of them is the Mark and recapture method which is often used in fisheries and in game reserves.

This methods involves marking a number of individuals in a natural population, returning them to that population and subsequently recapturing some of them as a basis for estimating the size of the population at the time of marking and release. It is often referred to as Lincoln Index or the Petersen Index to mark the name of the persons who first used it.

The principle behind this method is that if a proportion of the population was marked initially and returned to the original population and after a while a second sample was taken, the proportion of marked individuals in the second sample would be the same as was marked initially in the total population.

or

There are various assumptions that are made during data collection for this methos and they include

Assumption 1: The population under study is assumed to be "closed".This means that during the interval between the preliminary marking period andthe subsequent recapture period, nothing has happened to upset the proportions of marked to unmarked animals.

Assumption 2:The chance for each individual in the population to be caught are equal and constant for both the initial marking period and the recapture period. That means it should not be easier or become difficult to capture the individuals of the population.

Assumption 3 : Sufficient time has to be provided for the marked individuals of the population to evenly disperse amongst the rest of the population.

Assumption 4: Animals do not lose thier markings and so the markings should besuch that it cannot be lost due to shedding or molting or for any other environmental reason.

All these assumptionsare consideredto rule out the variability associated with population size estimates in mark-recpature studies.