Activity 2: Interactions
Name: ________________________________
PSI Biology
Competition
Competition occurs when organisms try to obtain the same resources at the same place at the same
time. Competition takes place when there is resource limitation in an ecosystem. That is, when there is
not enough of a particular resource (food, shelter, water etc) to meet the needs of all organisms.
There are two different types of competition:


Interspecific competition – competition between individuals of different species
Intraspecific competition – competition between individuals of the same species
Read the following description about damselfish and answer the questions that follow.
Damselfish are tropical fish that live on coral reefs. Growing up to 36 cm long, they feed on crustaceans,
plankton and algae. Damselfish are extremely territorial. Fish that have a territory protect it aggressively
while fish who do not have a territory are constantly trying to obtain one. Suppose that a threespot
damselfish without a territory attacks another threespot damselfish with a territory. The threespot with
a territory is injured, thereby opening up a vacancy in territory. Within minutes, another threespot
damselfish as well as a cocoa damselfish converge on the territory. After an aggressive fight, one of the
threespot damselfish wins the territory.
1. What is the habitat of the damselfish?
2. What type of consumer is the damselfish?
3. What resource limitation occurs in the damselfish community?
4. What type of competition occurred between the two original threespot damselfish?
5. What type of competition occurred between the threespot and cocoa damselfish?
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Biology
Ecology
Multiple Interactions
Some populations are affected by multiple interactions. Tribolium include the flour beetles that are
known to infest grain products. Thomas Park is a scientist who did several experiments using two
different species of flour beetles: T. confusum and T. castaneum.
To determine who the better competitor was, Park grew the two species separately at 34⁰C and 70%
humidity as well as together at the same conditions. When grown separately, both species thrived.
When grown together, however, T. castaneum persisted while T. confusum died off.
Park also introduced a protozoan parasite, Adelina tribolii to the beetle populations. While Adelina
produced sickness and death in the flour beetle populations, he also noticed interesting changes in the
competition between the beetle species. In the presence of Adelina, T. confusum was able to
outcompete T. castaneum for resources.
6. Park initially studied the flour beetles together and separately. He maintained both
experimental treatments at 34⁰C and 70% humidity. Why was it important to maintain constant
conditions for all treatments?
7. What type of competition occurs between T. castaneum and T. confusum?
8. What is the result of competition between the flour beetles?
9. What type of interaction occurs between Adelina and the flour beetles?
10. How does the presence of Adelina affect competition between T. castaneum and T. confusum?
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Biology
Ecology
Answer Key – 13 points
1. 1 point
Damselfish live on coral reefs.
2. 1 point
Damselfish are omnivores.
3. 1 point
Damselfish populations are limited in territory.
4. 1 point
Intraspecific competition
5. 1 point
Interspecific competition
6. 2 points
It is important to maintain controlled variables in all experimental treatments in order to assure
that no outside factor is causing the observed results.
7. 1 point
Interspecific competition
8. 2 points
T. castaneum is a better competitor and is able to outcompete T. confusum.
9. 1 point
Parasitism
10. 2 points
The presence of Adelina reverses competition between the flour beetles.
www.njctl.org
Biology
Ecology