Mark_Recapture Lab 2

Mark_Recapture Lab 2

For many animals, estimates of population size by direct counts are impractical (ie. Think of our first lab when we estimated how many blades of grass).  For mobile or secretive forms, it is difficult to obtain direct estimates of population density, even in areas of very small size.  Often, however, estimates of population size can be obtained by marking a segment of the population on one occasion and sampling the numbers of marked and unmarked animals on one or more later occasions.

In every mark-recapture exercise, there are certain assumptions that we have to make about the data and/or population itself.  Some of these include:

–          Closed population: there is no immigration or emigration of individuals

–          All individuals have an equal chance at begin marked

–          The mark doesn’t come off

–          The mark doesn’t affect survival

–          No reproduction or mortality occurs

–          No metamorphosis (if dealing with insects)

–          A sufficient amount of time exists between recaps, allowing individuals to naturally disperse back into their environment

If we were doing this exercise in the lab each group would receive a bucket filled with wheat bran and an unspecified number of mealworms.  Mealworms are a commonly used in laboratory studies.  Their life cycle includes four stages: egg, larvae, pupae, adult (see below).  You would start off by marking 30 larval stage mealworms by painting a thick, black stripe down their back with a permanent marker, and then put them back in the bucket.  After 24 hrs you would return and use a small glass jar to extract three scoops of wheat bran, and place them in separate counting trays. You would count the total number of mealworms captured and take note of how many were marked.



You will see in the handout below that I have filled in the mark recapture data.  Follow the procedure on the next page to complete the assignment



Begin by calculating the population size and confidence limits for each recapture. 

–          Use the equations on pages 113-114 of the lab manual (I have inserted pictures of these pages below)

–          The first equation you encounter on pg 113 is for population size:

–          Use the text above this equation to figure out what n1 n2 and m2 mean

–          Next you need to figure out the confidence limits by first calculating the variance s2 (see the next equation on page 113)

o   Note: confidence limits are a range of values that fall below and above the mean population size.  In this exercise we are calculating 95% confidence limits.  This means that we are 95% certain that the true population size falls somewhere between the lower and upper value

–          After you find variance you need to convert the value to standard deviation (s).  You can do this by taking the square root (√) of variance (s2)

–          At the top of page 114 use the first equation to figure out the 95% confidence limit

–          Multiply the standard deviation (s) by 1.96, and add that number to your mean population size to find the upper value of you confidence limit.  Next subtract that number to find the lower limit

–          Once you have estimated the population size and confidence limits, complete the “Averaging method”.  First, find the average of the three population size values you calculated (this will be the “Mean estimate of population size”)

–          Next calculate the approximate standard error

–          Use the approximate standard error to find the confidence limits of the “Mean population size”

–          The final step is to answer the two questions

o   Note: for question 1, discuss which of the assumptions (see previous page) could have been violated during this procedure and why (for example, is it possible that some individuals could have escaped (emigrated)?

o   Note: For question 2, look at the confidence limits for each recap.  Did the range of values change significantly from the different recaps?  If they did then it was good that more than one recaptures was a good idea.

Type all your answers directly into the form below and email it back to me

This project is to be done independently (not in groups)





Mark and Recapture Lab


Initial number marked:  30

First recapture

              Date:                                 Total recapture:                123                                  Marked:   15

              Estimated size of population and confidence limits:


Second recapture

              Date:                                 Total recapture:                89                                    Marked:  9

              Estimated size of population and confidence limits:


Third recapture

              Date:                                 Total recapture:                136                                  Marked:  18

              Estimated size of population and confidence limits:


Averaging method:

              Approximate standard error:

              Mean estimate of population size:


1.       Is there evidence that the assumption of the mark and recapture have been violated?  Justify your answer




2.       Do your results suggest that doing the second and third recaptures was worthwhile?  Justify your answer.