Estimating Population Abundance: Mark-Recapture

            Ecologists are often interested in understanding population dynamics.  That is, how populations change over time in terms of genetic make up and number of individuals.  The size of any population is dependent on many things.  Population size and genetic make-up (gene pool) may change due to immigration/emigration, drift, and factors influencing births and deaths.  How do ecologists estimate the size of any population of interest?  This can be a perplexing issue, especially if the population is quite large or cryptic.  It would be inefficient in terms of time and effort to count each and every individual, so ecologists estimate population size using several methods.  The methods used vary depending mainly on the organism of interest.  Plants are sessile, whereas most animals are mobile organisms.  A line-transect estimate may be useful for enumeration of sessile organisms, but not so for mobile organisms, or ones that are difficult to observe.  Mark-recapture is a useful method for determining the abundance of many animal populations.  There are several types of mark-recapture estimates available.  We will utilize the Petersen method of mark-recapture to estimate the population abundance of Drosophila over time.

            In general mark-recapture studies are done with or without replacement.  Individuals counted in a recapture sample may or may not be released back into the population.  In this lab we will introduce recaptured individuals back into the population.  Therefore, we will use a population estimator that assumes sampling with replacement.        

The Petersen Method

           

The Petersen method is a simple technique, in that it is very intuitive.  The size of a population is estimated based on a proportion of marked individuals in a recapture experiment.  The formula for estimating population abundance based on the Petersen method is:

                   N/M = C/R         therefore:     N= CM/R

Where  N = size of the population at time of sampling

            M = # marked in the initial sample

            C = total captured in second sample

            R = number marked in second sample

             = estimate of population size at time of marking

Unfortunately, this estimate tends to bias the results by overestimating population size.  It may also be extremely biased with a small sample size.  An unbiased estimator based on the Petersen method was developed by Bailey (1952), where:

        N   =  M (C+1) / (R+1)

This estimator assumes sampling with replacement, and is nearly unbiased when the number of recaptures (R) > 7.