Lesson 4.2 - Predator-Prey Interactions

Populations are changing in size constantly. In good years when food and water are plentiful, the population can increase rapidly. In bad years when food and water are scarce, the population can decrease rapidly. Other limiting factors can change quickly the population as well. What is the relationship between predator-prey populations?

Read the Find Out Activity on page 266 in your textbook. Then. answer the following questions.

Question 1. In the graph, look at year 1855. How many lynx and hare were harvested? What do these numbers imply?

Question 2. Look at the graph from 1855 to 1860. What is happening to the lynx and hare populations?

Question 3. Look at the graph from 1860 to 1865. What is happening to the lynx and hare populations?

Question 4. Look at the graph from 1865 to 1870. What is happening to the lynx and hare populations?

Question 5. Now look at the graph to see if you can find any relationship between the number of lynx and the number of hares. What relationship did you find?

Question 6. Are there any times when the graph does not show the typical predator-prey relationship?

Question 7. Do you think other species would show similar cycles in their populations?

Check your answers with those that follow.

 

 

 

 

 

 

 

 

 

 

 

 

Answers to Questions:

Question 1. In the graph, look at the graph at year 1855. How many lynx and hare were harvested? What do these numbers imply?

From the graph, just over 20 000 lynx were harvested and about 20 000 hares were harvested. These numbers imply that the number of hares were very low and the number of lynx was high in comparison to the number of hares. If the lynx depend on the hares for food, many lynx will starve and the survivors will produce few offspring.

Question 2. Look at the graph from 1855 to 1860. What is happening to the lynx and hare populations?

The lynx population is declining steadily as predicted in question 1. There is not enough food for the lynx, and the harvested number drops to about 7 000 in 1860. The hare population increases rapidly from 1855 to 1860. This is because the number of lynx that prey on them has been in rapid decline.

Question 3. Look at the graph from 1860 to 1865. What is happening to the lynx and hare populations?

The lynx population begins to increase rapidly until it peaks at a harvested population of almost 70 000 in 1865. There is abundant food for the lynx, so many will survive and produce many offspring. The hare population continues to increase rapidly from 1860 to 1865. This is because the number of lynx that prey is still trying to catch up to limit the population.

Question 4. Look at the graph from 1865 to 1870. What is happening to the lynx and hare populations?

The hare population peaks in 1865 at almost 160 000 hares harvested. The lynx population also peaks in 1865 at about 70 000 harvested. Then, both populations begin to collapse until in 1870 both harvested populations are at about 10 000.

Question 5. Now look at the graph and see if you can find any relationship between the number of lynx and the number of hares. What relationship did you find?

There seems to be a relationship between the lynx and hare population. Most of the time, the lynx population follows the hare population closely. When the hares increase, the lynx also increase. When the hares decrease, the lynx also decrease. This is typical of a predator-prey relationship.

Question 6. Are there any times when the graph does not show the typical predator-prey relationship? What do you think the reason for this might be?

The typical predator-prey relationship does not happen between 1840 and 1855. The hare population increases while the lynx population decreases; then, the hare population collapses and the lynx population increases only slightly. The reason for this could be disease, some other predator population changing, weather, or several other possible factors.

Question 7. Do you think other species would show similar cycles in their populations?

It is likely that you would see similar cycles in almost all animal populations for reasons similar to the lynx-hare predator-prey relationship.

 


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