BIOL 3370
MARK-RECAPTURE LAB
An important step in an ecosystem study is determining population size of the species
present. Determining population size of organism that are mobile and can hide is obviously more
difficult. This laboratory includes an exercise in estimating the population size of animals using the
"mark-recapture" method.
The mark-recapture sampling method, also called capture-recapture, relies on a simple idea.
During a given period of time one captures as many members of the population as possible and marks
them in some fashion. Mice might be marked by small punch-holes in the ears, or beetles might be
marked by a small dot of paint. At some later time, the capturing procedure is repeated and the
number of both marked and unmarked captured animals is recorded. If certain assumptions are
correct, the proportion of marked individuals in the second sample will be the same as the
proportion of individuals marked previously compared to the "unknown" number of individuals in the
total population.
Suppose:
N = number of individuals in the total population
M = number of individuals captured and marked
C = total number of animals captured during the
second capture period
R = number of marked animals caught (recaptured)
during the second capture period
Then:
M/N = R/C
and if this is true, then:
N = MC/R
This gives us an estimate of the size of the population, N.
Theory is all well and good, but experience shows that there is some bias in this equation,
and that, in the long run, a more accurate estimate comes by using the following formula.
N = M (C+1)/(R + 1)
The capture-recapture method rests on several assumptions:
1. All individuals in the population have an equal chance of being captured.
2. There is no change in the ratio of marked to unmarked animals between capture times. This
means that there have been no significant additions of unmarked individuals to the population
and that decreases in the population have affected marked and unmarked individuals
proportionally.
3. Marked individuals distribute themselves in the population homogeneously after being released.
4. Capturing and marking an organism does not affect its chances of being recaptured.
5. Marks are not lost or overlooked.
Field Procedures
(Depending on weather conditions we may do an in-lab practice exercise of this method)
The mark-recapture method of estimating population size will be used to estimate the population
size of grasshoppers in a field.
In the field, the first mark-recapture lab of the week will capture & mark grasshoppers. The
number of marked individuals will be recorded.
Forty-eight hours later, the second lab-group will collect in the same sample area, recording the
total number captured, and noting how many of these were already marked (number recaptured).
Assignment:
1. Prepare a word document that includes a table where you show the numbers used for “M”, “C”,
and “R” in order to arrive at your estimate(s) of the population size of grasshoppers. Indicate
what your (final) estimate of grasshopper population size is.
2. In the same word document address each of the assumptions involved in using this method to
calculate population size (stated above) and note whether or not you think it was met, and why.
For one of the assumptions, tell how the estimate would change if it was not met (i.e. over- or
underestimated?).
Extra Credit (2pts) Complete the online assessment for this course:
* Go to http://www.salgsite.org/student
* Fill in your email address
* Enter the instrument number: 69436
* Enter the super-secret instrument password: ecology
I will not know the results of this assessment until after the grades have been reported, nor
will I know which assessment result was done by what student.