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PREDATOR-PREY POPULATIONS
PRE-LAB
Is there a relationship between the population size of prey and predators in an ecosystem?
What type of factors can affect the population size of prey and predators in an ecosystem?
BACKGROUND
In this activity a new meadow develops in a forest as a result of a fire. Mice migrate into the
meadow and begin to reproduce. As the mouse population increases, a weasel is also attracted
into the meadow. In the following exercise the meadow (habitat) is represented by a Petri plate,
the mouse population (prey) by beads, and the weasel population (predator) by a spoon.
SAFETY
Follow all ASIM safety procedures.
MATERIALS
Plastic spoon
Petri dish
1 bag of 100 beads
paper cup or weigh boat
PROCEDURE
Assumptions:
1. The surviving mice of a generation always double their numbers.
2. In each generation at least 10 mice are initially present in the meadow (by immigration, if
necessary).
3. The maximum number of mice that the meadow can support, the carrying capacity, is
100.
4. In each generation at least one weasel is initially present in the meadow (by immigration,
if necessary).
5. In order for a weasel to survive in the meadow, it must capture at least 5 mice. (If a
weasel does not capture at least 5 mice, it will either starve in the meadow or leave in
search of food elsewhere.)
6. For each of the 5 mice that a weasel captures, it will produce 1 offspring. (If one weasel
captures 7 mice and a second captures 2, the first weasel will survive in the meadow and
produce 1 offspring, but the second will neither survive in the meadow nor produce
offspring.)
Use the following directions to complete the first 4 generations of Tables I and II on the Data
Sheet. Follow these directions exactly. One team member should fill in the Data Sheet while the
other(s) works with the dish, beads, and spoon.
Generation 1
1. Begin with 10 mice and one weasel in the meadow (place 10 beads in the dish).
2. The capture of mice by the weasel is simulated by scooping the spoon once through the
dish for each weasel present.
3. However, we will assume that in the first generation the weasel does not capture any
mice.
4. In Table I enter a 0 for prey captured by predator 1 of the first generation. Also enter a 0
for offspring produced by the predator.
5. Then fill in generation 1 of Table II.
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6. The 10 surviving mice double their numbers; therefore, add 10 more beads to the dish for
a total of 20 beads.
Generation 2
1. As there were no predator survivors or offspring in the first generation, we will assume
that another weasel immigrates into the meadow.
2. Scoop once through the dish, picking up 4 beads. Place the beads into the stomach,
represented by the paper cup, of the predator.
3. In Table I enter 4 for prey captured by predator 1 of the second generation and 0 for
predator offspring.
4. Then fill in generation 2 of Table II. Sixteen mice survive; therefore, add 16 more beads
to the dish since the surviving mice of a generation always double their numbers.
Generation 3
1. Again, assume that another weasel immigrates into the meadow.
2. Scoop once through the dish, picking up 7 beads.
3. In Table I enter 7 for prey captured by predator 1 of the third generation.
4. Also enter 1 for the offspring produced by this predator since the weasel captured 5 mice.
5. Fill in generation 3 of Table II.
6. Twenty-five mice survive; therefore, add 25 more beads to the dish.
Generation 4
1. There are 2 predators in generation 4 (add predator survivors and predator offspring of
generation 3).
2. Scoop once through the dish for the first predator, picking up 13 beads.
3. In Table I enter 13 for prey captured by predator 1 of the fourth generation and 2 for the
offspring produced by this predator.
4. Scoop again through the dish for the second predator, picking up 7 beads.
5. In Table I enter 7 for prey captured by predator 2 of the fourth generation, and 1 for the
offspring produced by this predator.
6. Then fill in generation 4 of Table II.
7. Thirty mice survive; therefore, add 30 more beads to the dish.
Completing the Exercise
1. Continue as outlined above to complete Table II.
2. Important: In simulating the capture of mice by the predator, scoop the spoon through the
diameter of the dish, do not look at the dish while doing this, and do not deliberately
attempt to scoop up all the beads.
3. In Table II remember that Initial Prey never falls below 10 and Initial Predators never fall
below 1.
4. When Table II is complete, graph the results as directed on the Data Sheet.
5. Complete the questions on the Data sheet.
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STUDENT DATA SHEET
J11Predator Prey Populations
NAME _______________________
Date _________________________
DATA TABLE I
Predator
1 C
O
2 C
O
3 C
O
P
R
4 C
O
E
D
5 C
O
A
T
6 C
O
O
R
7 C
O
G
8 C
O
E
N
9 C
O
E
R
10 C
O
A
T
11 C
O
I
O
12 C
O
N
S
13 C
O
14 C
O
1
0
0
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
GENERATIONS OF PREDATORS
6
7
8
9
10 11
12
19
20
C = number of prey that has been captured
O = number of new offspring produced by the predator
TABLE II
Initial Prey
Total Prey
Captured
Prey
Survivors
Initial
Predators
Surviving
Predators
Predator
Offspring
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1
10
0
2
3
4
5
10
1
0
0
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GRAPHING RESULTS
When Table II is complete, graph the recorded results, using two separate lines, in the Initial
Prey and Initial Predator rows. Use an “m” to locate each point on the graph for prey and a “w”
for each point on the graph for predator (or use different colors). Connect all prey points (m)
with a line and each predator point (w) with a separate line to complete the graph.
100
80
A
B
U
N
D
A
N
C
Y
60
40
20
10
8
6
4
2
1
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
GENERATION NUMBER
ANALYSIS QUESTIONS
1. Which population (predator, prey) shows the first increase in numbers? _______________
2. Does a peak in weasel population come at the same time as or after a peak in mouse
population? What is the explanation for this?
3. What factor seems to determine the size of the weasel population in the meadow in any given
generation?
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4. What factor seems to cause the decline of the mouse population in the meadow?
5. What density independent factor affected the populations in the meadow? Density dependent
factors?
6. The governing board of a national park would like to increase the moose population in the
park in order to attract more tourists. Since the main predator of the moose is the wolf, the
planning committee has formulated a plan that involves decreasing the wolf population. Do
you agree with this action? Why or why not? What is your prediction of the outcome of this
action?
7. Based on your prediction to question 6, recommend a possible solution to solve any
problem(s) that might evolve due to changes in the park's ecosystem.
8. Design a possible small scale lab that the governing board could develop to test the reaction
of removing the predators in a stable ecosystem before actually implementing their
suggestion.
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