Vocabulary: carrying capacity, density-dependent limiting factor, density-independent limiting factor, limiting factor, population, population density
Prior Knowledge Questions (Do these BEFORE using the Gizmo.)
1. Suppose you had a pet rabbit. What would the rabbit need to stay alive and healthy?
Pet rabbits need food, fresh water, a clean living space, and shelter from the elements in order to stay alive and healthy.
2. A female rabbit can give birth to over 40 baby rabbits a year. Suppose all of her offspring survived and reproduced, all of their offspring survived and reproduced, and so on. If that happened, in only eight years the mass of rabbits would exceed the mass of Earth!
So, why aren’t we overrun with rabbits? What keeps the rabbit population in check?
Several environmental factors, for example limited food resources, keep a rabbit population from growing too large.
A population is a group of individuals of the same species that live in the same area. The size of a population is determined by many factors. In the Rabbit Population by Season Gizmo™, you will see how different factors influence how a rabbit population grows and changes.
1. Select the BAR CHART tab. What is the approximate size of the initial rabbit population? 40 rabbits
2. Select the TABLE tab. Click Play (), and allow the simulation to run for one year.
A. In which season did the rabbit population increase the most? Spring
B. In which season did the rabbit population increase the least? Winter
Get the Gizmo ready:
Click Reset ().
Question: What determines how large a population can grow?
1. Think about it: A limiting factor is any factor that controls the growth of a population. What do you think are some of the limiting factors for the rabbit population? Examples of limiting factors include competition, disease, living space, natural disasters, predation, and unusual weather.
2. Run Gizmo: Select the DESCRIPTION tab. Set the Simulation speed to Fast. Select the GRAPH tab. Click Play, and allow the simulation to run for at least 10 years. (Note: You can use the zoom controls on the right to see the whole graph.)
A. Describe how the rabbit population changed over the course of 10 years. For the first four years, the rabbit population increased in size. For the next six years, the rabbit population stayed approximately the same size.
B. What pattern did you see repeated every year?
The rabbit population increased in size at the beginning of the year (spring), and then decreased in size towards the end of the year (winter).
C. How could you explain this pattern?
The rabbits reproduce during the parts of the year when resources, such as food, are plentiful, but when resources are harder to find, such as during the winter, many of the rabbits die.
3. Analyze: The carrying capacity is the maximum number of individuals of a particular species that an environment can support. All environments have carrying capacities.
A. What is this environment’s approximate carrying capacity for rabbits? (Note: Average the summer and winter carrying capacities.) between 350 and 450 rabbits B. When did the rabbit population reach carrying capacity? Explain how you know. The rabbits reached carrying capacity during the fourth year of the simulation. After this year, the average size of the rabbit population does not change much
Density-dependent limiting factors
Get the Gizmo ready:
On the SIMULATION pane, make sure Ample is selected for the amount of LAND available.
Introduction: Population density is the number of individuals in a population per unit of area. Some limiting factors only affect a population when its density reaches a certain level. These limiting factors are known as density-dependent limiting factors.
Question: How does a density-dependent limiting factor affect carrying capacity?
1. Think about it: What do you think some density-dependent limiting factors might be? Density-dependent limiting factors include predation, competition, and contagious diseases
2. Predict: How do you think the amount of land available will affect the
3. Experiment: Use the Gizmo to find the carrying capacity with Ample, Moderate, and Little land. List the carrying capacities below.
Ample: 400 Moderate: 300 Little: 100
4. Analyze: How did the amount of space available to the rabbits affect how many individuals the environment could support? As the amount of space decreased, so did the number of rabbits the environment could support.
5. Infer: Why do you think limiting a population’s space decreases the carrying capacity? As the amount of available land decreases, vital resources such as food, water, and living space will decrease as well.
6. Challenge yourself: Other than space, what might be another density-dependent limiting factor? Explain. A contagious disease would be a density-dependent limiting factor.
Density-independent limiting factors
Get the Gizmo ready:
On the SIMULATION pane, select Ample for the amount of LAND available.
Introduction: Not all limiting factors are related to a population’s density. Density-independent limiting factors affect a population regardless of its size and density.
Question: How do density-independent limiting factors affect how a population grows?
1. Think about it: What do you think some density-independent limiting factors might be? Density-independent limiting factors include unusual
weather and natural disasters.
2. Gather data: Click Play. Allow the population to reach carrying capacity. Click Pause (). Select the GRAPH tab and click the camera () to take a snapshot of the graph. Paste the snapshot into a blank document. Label the graph “Normal Weather.”
3. Predict: How do you think a period of harsh winters will affect the rabbit population? The harsh weather will kill the rabbit population.
4. Investigate: Click Reset. Select Harsh winter from the CONDITIONS listed on the SIMULATION pane. Click Play, and observe the how the population changes over five years. Paste a snapshot of the graph in your document. Label the graph “Harsh Winter.”
A. How does the Harsh Winter graph differ from the Normal Weather graph? The carrying capacity is lower and there is a greater population decrease during the winter.
B. What do you think most likely caused the differences seen in the two graphs? The harsh winter weather is difficult to survive, so more members of the population die during the winter.
5. Predict: Rabbits reproduce in the spring. How do you think a period of cold springs will affect the rabbit population? The rabbits will not be able to reproduce.
(Activity C continued on next page)
Activity C (continued from previous page)
6. Investigate: Deselect Harsh winter. Select Cold spring. Click Play, and observe the how the population changes over a period of five years. Paste a snapshot of the graph in your document and label the graph “Cold Spring.”
A. How does the Cold Spring graph differ from the Normal Weather graph? The carrying capacity is lower and there is a smaller population increase during the spring
B. What do you think most likely caused the differences seen in the two graphs? The cold spring weather makes it difficult for the rabbits to reproduce as many new individuals in the spring, which is the main breeding season for the rabbits.
7. Predict: How do you think a period of hot summers will affect the rabbit population?
8. Investigate: Deselect Cold spring. Select Hot summer. Click Play, and observe the how the population changes over a period of five years. Paste a snapshot of the graph in your document. Label the graph “Hot Summer.”
A. How does the Hot Summer graph differ from the Normal Weather graph? The carrying capacity is lower, and the rabbit population shrinks instead of staying the same or growing during the summer.
B. What do you think most likely caused the differences seen in the two graphs? During hot summers, rabbits are stressed and are unable to reproduce. In addition, the weaker individuals may not be able to survive higher than normal temperatures.
9. Think and discuss: Other than unusual weather, what might be another density-independent limiting factor that could affect the rabbit population? If possible, discuss your answer with your classmates and teacher.
A natural disaster, such as a fire, would be another density-independent limiting factor. The fire would endanger all of the rabbits living in the environment, no matter how many rabbits were present at the time of the fire.