Predation and prey modeling lab
Introduction:
In 1970, the deer population of an island forest preserve was about 2000 animals. Although the island
had excellent vegetation for feeding, the food supply had limits. Therefore, the forest management staff
worried that overgrazing might lead to mass starvation of the deer. Since the area was too isolated for
hunters, the wildlife service decided to bring in natural predators to control the deer population. This is
an example of a predator/prey relationship. It was hoped that natural predation would keep the deer
population from becoming too large and increase the deer health, as the deer population approached
the carrying capacity of the island. Carrying capacity is the maximum number of organisms that can
survive in a habitat without impacting other organisms in that habitat. Limits on the carrying capacity
include food, water, cover, prey and predator species.
In 1971, ten wolves were flown into the island.
The results of this program are shown in the following table. The population change is the
number of deer born minus the number of deer that died during that year. The herd population started
at 2000 when this study began.




Calculate the number of deaths (predation + starvation).
To determine the deer population change, subtract the number of deaths from births (births deaths), this can be a positive number, indicating growth, or a negative number which indicates
a population decline.
Calculate the deer population by adding/subtracting the population change from the population
the year before
The year 1971 has been completed for you as an example.
Year Wolf
Deer Predation Starvation Number of
population births
deer deaths
1970
1971 10
800
1972
1973
1974
1975
1976
1977
1978
1979
1980
920
1000
944
996
836
788
766
780
790
12
16
22
28
24
21
18
19
19
Deer
Deer population
population
change
Starting population
2000
400
100
400+100=500 8002000+300=2300
500=300
480
240
640
500
880
180
1120
26
960
2
840
0
720
0
760
0
760
0
Graph the wolf vs. deer populations. You can do this in Excel or draw the graphs. You should
use a bar graph to model the population. (Hint: you can copy/paste the table above into Excel).
You can include both deer and wolves on one graph OR make separate graphs for each.
Here is an example:
Example bar graph
2500
2000
1500
1000
500
0
Wolf population
Deer population
1970
Year
Wolf
population
Deer
births
1971
Predation
Starvation
Number of
deer deaths
Deer
population
change
Starting Population
Deer
population
1970
0
2000
1971
10
800
400
1972
1973
1974
1975
12
16
22
28
920
1000
944
996
480
640
880
1120
240
500
180
26
580
740
980
1220
340
260
-36
-224
2340
2260
1964
1776
1976
1977
1978
1979
1980
24
21
18
19
19
836
788
766
780
790
960
840
720
760
760
2
0
0
0
0
1060
940
820
860
860
-224
-152
-54
-80
-70
1776
1848
1946
1920
1930
100 400+100=500
800500=300
2300
Changes in Population
2500
2300
2340
2000
2260
2000
1964
Population
1776
1776
1848
1946
1920
1930
1500
1000
500
0
10
12
16
22
1970
1971
1972
1973
1974
28
24
21
18
19
19
1975
1976
1977
1978
1979
1980
0
Years
Deer population
Wolf population
Analysis: Answer the following questions using complete sentences.
1. Describe what happened to the deer population between 1971 and 1980.
As years passed the deer population decreased, due to the introduction of wolves in the
ecosystem.
2. When was the wolf population the highest? What happens to the deer population when
the wolf population increases?
The wolf population was the highest in 1975, and as a matter of fact the deer population
was the lowest that year. Thus, it shows that if the wolf population increases the deer population
decreases.
3. What do you think would have happened to the deer on the island had wolves NOT been
introduced?
Their population would have kept growing constantly, but they would have kept dying of
starvation constantly as well. Also, if the population kept increasing, there would be always less
and less food, thus making the deer population decrease drastically at some point.
4. What do you think would have happened to the ecosystem on the island if wolves had
NOT been introduced?
Probably the deer population would have kept increasing, leading to more food being
consumed, and the whole ecosystem would have been damaged. Also, due to the deer eating
most of the food, other species in the ecosystem would have probably died of starvation. This
might have led to a complete destruction of the ecosystem and all the species.
5. While the deer are native to the island, the wolves are not native. What do you think the
possible impacts of the wolves might be on the island ecosystem?
The introduction of wolves might affect the island's current food chain by feeding not just
on deer but also on other indigenous species, thereby causing population fluctuations and
changing the ecosystem's overall balance. Also, if there are other native predators on the island
that depend on the same prey species as the wolves, they may experience greater food
competition, which might have a detrimental effect on their numbers. Finally, bringing in nonnative wolves might transmit new parasites or illnesses that would impact both the island's
natural animals and the wolves themselves.
6. Hypothesize what might happen if a non-native prey species such as a rabbit was
introduced to the island. What would happen to the deer? What about the wolves? (Hint:
Larger rabbits can be prey for wolves).
Rabbits and deer may fight over the same food supplies, which may cause the deer
population to drop as a result of the competition. Additionally, if the wolves switch their
attention from the deer to the rabbits because they are a simpler or more plentiful food supply, it
may lessen the burden of predation on the deer population. As a result, there may be a rise in the
deer population, which might result in more starvation. Finally, the introduction of rabbits could
disrupt the island's food chain, affecting not only the deer and wolves but also other species
within the ecosystem.
Adaption from: Deer: Predation or Starvation, www.biologycorner.com