BIOL 3370
MARK-RECAPTURE LAB
An important step in an ecosystem study is determining population densities
of the species present. This laboratory includes an exercise in estimating the
population density 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 = 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)
Practice exercise
(Depending on weather conditions we will either do the practice exercise or the field
portion of this lab)
Mark-Recapture Simulation
In the first week, we will be simulating mark-recapture and calculating
population estimates based on the data we generate. Our population for this lab
will be green peas suspended in a matrix of flour. The peas represent our "insect"
(or sample organism), the flour represents the field in which we will be sampling.
The sampling procedure is given below.
1.
2.
3.
4.
5.
6.
7.
8.
Begin by adding a known number of peas to a bag of flower.
Shake your bag to evenly distribute the peas throughout the flour.
Simulate 'capture' by collecting a sample of peas (dip the cup into the
bag).
Separate the peas from the flour and return the flour to your bag.
DO NOT PUT THE PEAS BACK IN THE BAG!
Count the number of peas obtained and mark all of your captured peas
with a magic marker. DO NOT PUT THE PEAS BACK IN THE BAG!
Shake your bag again to redistribute the peas.
Repeat the above procedure several* times.
PUT ALL THE PEAS BACK IN THE BAG
[This portion simulates what the first lab section will be doing]
9.
Shake your bag to redistribute the peas.
10.
Collect a sample by dipping your cup into the bag.
11.
Repeat the same number of times as above.
12.
Count the total number of marked and unmarked peas
13.
Obtain your population estimate using the formulas provided.
Repeat the above procedure for an initial number of peas of 50, 100, and
200.
* The number of times you capture peas (dip the cup into the bag) represents
your sampling effort. For each initial number of peas you will estimate
population size several times using different degrees of ‘sampling effort’.
Part 1: Compare the resulting population estimates with the actual population
size (number of peas).
a. Which formula gave a better estimate of size?
b. Did your actual population size affect the accuracy of your estimate?
Which size worked best?
c. How did population estimates change with sampling effort?
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 via births or immigration and that decreases in the
population (death or emigration) have affected marked and unmarked individuals
proportionally.
3. Marked individuals distribute themselves in the population homogeneously after
being released.
4. Capturing an organism does not affect its chances of being recaptured.
5. Marks are not lost or overlooked.
Part 2: Each group should choose one of these assumptions and devise a
method of testing what would happen to the population estimate if that
assumption were not true. Would there be an over-estimation or an underestimation of the true population? Carry out the tests.
FIELD
(Depending on weather conditions we will either do the in lab practice exercise or the
field portion of this lab)
The mark-recapture method of estimating population size will be used to estimate
the population size of a particular species of insect in a field.
In the field, the first lab-group of the week will capture & mark either insects or
fish. 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.
ASSIGNMENT
If the indoor simulation was done:
1. Show all your calculations. Each set of calculations should have a heading
indicating the initial (or actual) population size and the sampling effort.
2. Address questions a through c of “Part 1”
3. From “Part 2” note which of the assumptions your group decided to test, and
explain the method used to test what would happen if this assumption were
violated.
a. Show your calculations.
b. How did your estimate of population size change with the violation of
the assumption?
If the field experiment is done:
1. Show your calculations for your estimate of the population size of
grasshoppers, and indicate the estimated population size. Calculate a
separate estimate for each lab period in which grasshoppers were
recaptured a second time.
2. Identify two assumptions involved in using this method to calculate
population size (stated above), and for each assumption note whether you
think the assumption was met and how the estimate would change if it was
not (over- or underestimated).