Population Size
Estimation: Plants &
Animals
Introduction:
One of the most important kinds of data collected in
ecological studies is the number of organisms existing in the
environment. This information forms the basis for
understanding of why some species are endangered, yet other
species are pests. It helps us understand the habitat
requirements of species and their interactions with other
species.
Plant populations and stationary animals are relatively
easy to count. Quadrants or transects randomly sample a
portion of the habitat. From this information we can infer
the total number present. Animals that move are much more
difficult to count.
The Mark and Recapture Method assesses animal population
size by capturing critters, marking them, releasing them back
into the population and making a second capture a day to week
later. This method can be used on any animal as long as the
marks don’t hurt the animals and animals in the population
move about freely. Danish biologist C.G.J. Peterson developed
the standard mark and recapture method nearly a century ago
for studying fish populations. Extensions of the method
include ongoing mark and release (more than two captures) and
different marks for different capture periods. These methods
permit the estimation of recruitment (births and immigration)
as well as losses from the population by death and emigration.
Much work has been done on marking techniques: paint or
dye markers, radioactive tracers, leg banding, ear tagging,
fin or toe clipping. Clearly one assumption of this method is
that the marking technique not harm the animal or make it more
likely to be eaten by predators once released.
Simulation in the Classroom:
1. First,
found in a
the number
the end of
we will estimate the size of a population of beans
cup. Examine the contents of the cup and estimate
of beans. Record this data in the data table at
this lab handout.
2. Reach into the cup and remove a small handful of beans
(around 20 beans, the actual number is not critical). Mark
each bean removed with the permanent marker. Let the beans
dry and return them to the cup.
3. Put your hand over the cup and shake it well so the beans
disperse. Remove another handful of beans. Count the number
of beans in this sample and count the number of captured
(marked) beans.
4. You can estimate the number of beans by using the
mathematical equation below:
Marked and Recaptured on Day 2 (R)
Total Captured Day 2 (S)
Total Marked Day 1 (M)
=
Estimated Population Size (N)
where:
# of Marked Beans you collected with your second handful
= R
Total number of beans you marked = M
Total number of beans you collected on your second
handful = S
(marked & unmarked)
Estimated Total population size = N
Written with just the variables identified, we have:
R
S
SxM
=
Rearranges
to:
N= R
M
N
5. Show your work and record this calculated data in the data
table at the end of this lab handout.
6. Take another handful of beans from the cup (about 20).
Mark more of the beans using your permanent marker making sure
to mark only previously unmarked beans. Add the number of
marked beans in step 2 with those marked in this step. Return
the beans to the cup, shake it well, and remove another
handful of beans. Calculate the estimated bean population
size again using your new numbers.
7. Show your work and record this calculated data in the data
table at the end of this lab handout.
8. Dump all the beans on the table and count them all. Record
this data in the data table at the end of this lab handout.
9.
Answer questions 1 – 3.
Mark and Recapture with Grasshoppers
Bioretention Flora Counts
Things to take to the Field Site:
Nail Polish
Grasshopper Nets
Grasshopper Collection Bin
100 foot tape measures
Field Guide to Wildflowers
Bioretention Blank Map
Clipboards
Population Size
Estimation: Plants &
Animals
Data & Questions Sheet
Estimation of Number of Beans in the cup: _______________
Calculated Number of Beans in the cup: _______________
Display of Calculation:
New Calculated Number of Beans in the cup: _______________
Display of New Calculation:
Actual Number of Beans in the Cup: _________________
Questions:
1. Which calculation was closer to the actual number of beans? The first or the second?
2. Does this make sense with what you would have predicted? Why or why not?
3. Think about this experiment ‘in the wild’, how is this method of population estimation
flawed? In other words, how would an animal population be different from the bean
population and how would this affect your estimated population number?
4. Number of Marked Grasshoppers (Day 1): ____________
5. Total Number of Grasshoppers Caught (Day 2): ____________
6. Number of Marked Grasshoppers Caught (Day 2): ____________
7. Calculated Number of Grasshoppers: _______________
8. Display of Calculation:
9. Density of Grasshoppers: _______________
10. Display of Calculation:
11. Our population size estimate could be inaccurate for a number of reasons. List four
reasons that would cause our estimate to be inaccurate.
a)
b)
c)
d)
Flower counts:
Species
Cell 1
Cell 2
Cell 3
Pond