Lincoln-Peterson Population Lab
Mark and Recapture Technique
Introduction
Unlike plants, animals are often secretive and usually mobile. Consequently, simple
counts or plot sampling techniques are useless for determining population size. However,
for several reasons, including understanding population dynamics and setting harvest
limits, it is important to estimate animal abundance. Therefore, several techniques have
been developed to accomplish this task. Of the numerous possibilities that exist, markrecapture methods are frequently employed. The general procedure involves capturing
and marking animals (this phase is referred to as the marking period). Different animals
often require unique capture and marking techniques. Once captured, the marked animals
are then released. After a certain period of time (long enough for the animals to
redistribute themselves within the habitat) individuals in the mobile population are again
captured and counted. The second cohort of individuals is called the census population.
Within the census population a certain number of individuals will carry the mark applied
earlier and others will be unmarked. Those individuals carrying the mark belong to the
marked cohort. It is important to keep track of the number of marked and unmarked
individuals in the census population. This general procedure may be applied either short
term or long term and applies to both open and closed populations. Accordingly, the mark
recapture model that is used will depend on the population type and the duration of the
study (table one).
Table One
Mark-Recapture Model
Model type
Description
Lincoln-Peterson Method
Closed population
One marking period
Schnabel Method
Closed population
Multiple marking periods
Jolly-Seber Model
Open population
Multiple marking periods
Objectives
In today’s lab you will
1. Take part in estimating the population size of a wild bean population
2. Learn the basic assumptions underlying the Lincoln Peterson Model
3. Learn the potential errors and strengths in sampling populations
Assumptions of Lincoln-Peterson Method
1. All individuals have the same chance of getting caught in the first sample.
2. Marking individuals does not affect their behavior making them more likely to be
caught or predated.
3. The population is “closed,” so N is constant.
4. Emigration and mortality must be absent or equal for marked and unmarked
individuals.
5. Individuals must retain their markings indicating capture throughout the study.
Where:
M= number of animals captured and marked in 1st sample
C= number of animals captured in 2nd sample (marked +unmarked; census population)
R= number of marked animals captured in 2nd sample (marked cohort)
N= estimate of population size
The size of M represents the marking effort and the size of C represents the sampling
effort. The unknown population size (N) may be estimated by rearranging the above
formula:
Population = # Initially Captured & Marked x Total Captured Second Sample
# of Marked Individuals Recaptured
In the example below 50 turtles were caught and marked, 100 were then captured
later of which, 14 had been caught and marked in the original sample of 50.
Population = 50 X 100 = 35 estimated population
14
While this formula provides an estimate of the population size, it tends to over estimate N
because of a failure to meet one or more of the assumptions (listed above). However, we
can correct for the bias by using the following formula:
You have data from each year of the study, beginning with 2005. To do estimates of
population you can look at our data in a few ways. You could look at all the years
collectively and get something like this.
(Total # turtles found 2005-2013 +1) (Total number captured in 2014 +1)
(# of marked recaptures 2014 – 1)
Or you could use a couple of years and then compare them to other year “sets”.
(Total number of turtles found in 2010-2012 +1) (Total number captured in 2013 + 1)
(# of marked recaptures 2013 – 1)
Materials
 500 Kidney Beans
 1 Flat Tray or brown bag
 1 Marker
Procedure
We will be estimating the population size of beans rather than live animals so that you
can examine the effect of marking and sampling effort on the final population estimate.
You will mark a small number of beans to simulate low marking effort and a large
number of beans to simulate high marking effort. Sample effort depends on the size of
the census population.
Step 1: Capture and Mark 25 beans (5% Marked) on both sides and mix them in with
the rest. The without looking sample the beans by randomly removing 25 beans (5%
population) repeat 3 times and record data {low marking and low sampling effort}
Step 2: Now without looking, remove 100 beans (20% population) at random and
repeat three times - record data {low marking and elevated sampling effort}
Step 3: Now remove and mark an additional 75 beans (20% Marked) on both sides
and mix them in with the rest. The without looking sample the beans by taking out 25
beans (5% population) and record data {high marking and low sampling effort}
Step 4: Now without looking, remove 100 beans (20% population) at random and
repeat three times record data {high marking and elevated sampling effort}
Sample
Number
5% Marked
5% Captured
M
C
5% Marked
20% Captured
R
N
M
C
R
20% Marked
5% Captured
N
M
C
20% Marked
20% Captured
R
N
M
C
One
Two
Three
Mean of N
Mean of N
Mean of N
Mean of N
%Error
using
mean
%Error
using
mean
%Error
using
mean
%Error
using
mean
SD
SD
SD
SD
Calculations
1. Calculate the variance and then standard deviation
http://www.mathsisfun.com/data/standard-deviation.html
2. Calculate percent error
% Error = (Actual – Calculated) x 100%
Actual
R
N
DISCUSSION QUESTIONS
Assume all questions and thus answers are about the box turtle study here on the
Lovett Campus!
1. Recall Assumption 1: Each animal is equally likely to be captured. This
assumption is almost never true. Explain how the likelihood of capture is not the
same for all individuals at the first marking effort. How would you minimize this
problem?
2. Recall Assumption 2: Marking does not influence an individual animal’s
probability of recaptured. Describe how being initially captured and marked could
(a) increase and (b) decrease an individual’s probability of recapture.
3. Recall Assumption 3: Births and immigration do not occur between the marking
and recapture efforts. Can we assume a closed or open population at Lovett for box
turtles?
4. Recall Assumption 4: If mortality & emigration occur between capture and
recapture, marked animals have the same combined mortality-emigration rate
as unmarked animals. If the assumption is violated, is it more likely because marked
animals have a higher rate of mortality + emigration than unmarked animals, or a
lower rate? (Explain why) How can a researcher attempt to insure that the assumption
is met?
5. Recall assumption 5: Marks should not be lost between capture and recapture.
How can a researcher attempt to insure that the assumption is met? What would
happen to the estimate if marks were lost?
6. How many turtles should we capture from our sample area to have a dependable
estimate of population? Defend your answer. Use data available on the wiki from the
BT study.