Predator- Prey Relationships
All organisms have basic needs which consist of food, water, and space. More often than
not, these resources are limited. Organisms compete for these limited resources and the
organisms best adapted to their environment will survive and reproduce.
Populations are constantly changing due to both the interactions between the organisms
found in the population as well as external factors like human interference or abiotic factors.
Prey consists of any organism that is eaten by a predator. Predators are organisms that feed
directly on other living organisms (even if it does not kill the prey). For instance, all herbivores,
carnivores and omnivores are predators because they feed on live prey but scavengers,
detritivores and decomposes are not predators because they feed on dead prey. In a sense,
parasites can also be considered predators because they steal resources from their host organism.
Predators often feed on the sick, old, or weak prey because they are easy targets.
As the number of carnivores in a community increases, they eat more and more of the
herbivores, decreasing the herbivore population. It then becomes harder and harder for the
carnivores to find herbivores to eat, and the population of carnivores decreases. In this way, the
carnivores and herbivores stay in a relatively stable equilibrium, each limiting the other's
population. This equilibrium ensures that each species does not overpopulate and deplete the
ecosystem’s resources. A similar equilibrium exists between plants and plant-eaters.
Predator-prey relationships are often cyclic, meaning that the populations of both
predator and prey increase and decrease in a cycle or a pattern. These are often called boom and
bust cycles- boom refers to when the organisms increase exponentially due to an abundance of
food and the bust refers to when the populations crash due to a lack of food.
Answer the questions below based on the reading above:
a) What is the term for when organisms are fighting for limited resources?
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b) Define predator: ______________________________________________
c) Is a mushroom a predator? Why or why not?
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d) How do predator and prey maintain a steady equilibrium in the population?
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e) Define boom and bust cycle. __________________________________________
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The Lynx & the Hare
The Canada lynx and its primary prey, the snowshoe hare, are a
classic example of the fluctuation of predator prey populations. The lynx
has other food sources besides the hare but they are
not as abundant as the hare and will not sustain a large lynx population.
Hares eat grass and seeds to survive the cold Canada climate. Scientists
have collected data over many years on the two populations have created
the graph below.
Use the graph above to answer the following questions:
1. What patterns do you notice when comparing the hare and the lynx populations ?
(describe the shape of the graph.)
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2. Around 1865, why was the hare population so high? Explain.
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3. What made the hare population decrease right before 1865? Explain.
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4. Are there usually more lynx or more hares? Why is this? (Consider energy
pyramids).
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5. When the hare populations increases, what happens to the lynx population? Why?
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6. When the hare populations decrease, what happens to the lynx population? Why?
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7. What other factors in the ecosystem could affect the hare and lynx populations?
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8. What would happen to the hare population if all/most of the lynxes were hunted and
killed by trappers for their fur?
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9. How would this change in #9 affect the rest of the ecosystem over time?
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10. Scientists say that a good predator-prey relationship keeps the population in
balance/equilibrium. Explain how the lynx and hare model displays this concept.
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