Students will develop an understanding of how populations interact with each  other within a community, discussing ideas concerning carrying capacity, competition, and interdependence. From there students will use models to explain the connection between genetic drift, natural selection, and speciation.

Competition Between Populations

Purpose

How do populations affect each other in ecosystems?

Model Rules

In this model, a population of bugs are able to wander the world, eating grass as they go.  As the bugs eat grass, they gain energy, and as they move they lose energy.  If they run out of energy, the bugs will die, and if they gain enough energy, they will reproduce.

In this scenario, you will investigate how the bug population behaves with no outside influences.

Set the region % grassland to 100 in and the initial-birds to zero in both ecosystems before you begin.

Model Rules

In the next model birds will be introduced into the ecosystem.  If a bird catches a bug it gains energy.  As it moves it loses energy.   A bird can die if it loses all of its energy. A bird can have offspring if it collects enough energy.

Make a Prediction

In the ecosystem you looked at in the previous activity, there were no predators.

Now, you will run an investigation about how introducing birds to the ecosystem affects carrying capacity and fluctuation size for the bug population. The model allows you to setup two ecosystems.

• On the left, setup an ecosystem that has no birds. On the right create an ecosystem that has birds.
• Determine values of other variables that you will keep constant. Ensure that all variable values are the same on the right and left side, except for the number of birds.
• Setup and re-run the model a few times until you feel like you have generated enough evidence to reliably claim that the carrying capacity for the bugs and size of fluctuation for the bugs are the same, slightly different or noticeably different in the two ecosystems.

Now, you will run an investigation about how different levels of resources in the ecosystem affects carrying capacity for the bug population. The model allows you to setup two ecosystems.

• Make sure that both ecosystems (left and right) have no birds.
• On the left, set up an ecosystem with 50% grassland.  On the right, set up an ecosystem with 100% grassland.
• Determine values of other variables that you will keep constant. Ensure that all variable values are the same on the right and left side, except for the amount of grassland.
• Setup and re-run the model a few times until you feel like you have generated enough evidence to reliably claim that the carrying capacity for the bugs and for the bugs are the same, slightly different or noticeably different in the two ecosystems.

In this activity you will examine the effects of a competitor species on a population.  The competitor species in this case is called invaders.  The invaders eat grass, just like the bugs.  The invaders do not eat bugs, and the bugs do not eat invaders.

1. Set the initial number of birds to zero and the grassland region to 100%.

2. Press SETUP and then press GO/STOP. As the model is running, press the LAUNCH AN INVASION button on one side, sometime between a time of 100 and 500 .

3. Run the model until it pauses on its own.

4. Record your observations below.

You have separately examined the effects of predators and competing species on a bug population.  However, in real ecosystems it is typical that both predators and competing species will be present at the same time.  In the next scenario you will examine the behavior of the bug population when both of these are present.

Before moving on to that exploration, answer the questions below and think about your observations from the earlier activities.

1. Choose an initial number of bugs, invaders, and birds to include in each environment.  Make each at least 5.

2. Set the initial values for the amount of food bugs eat, the amount of food invaders eat, and the region amount of grassland to different combinations you decide.  If you want to test the effect of changing one variable between the ecosystems, remember to make the other variables the same in both ecosystems.

3. Press SETUP and then press GO/STOP. As the model is running, press the LAUNCH AN INVASION button, sometime between a time of 100 and 500 .

4. Run the model until it pauses on its own.

5. Record your observations below. Try getting the two outcomes you selected by trying different combinations of values. If you can't find at least one set of values for each of the two outcomes you selected, then test some new possible combinations of values if time permits.

Design Your Investigation

Pick two of the following outcomes to generate in the model:

Outcome A: All three populations (bugs, invaders, and birds) die off.

Outcome B: The predator population dies off, but bugs or invaders survive.

Outcome C: The average predator population is above 100.

Try to find solutions that work consistently.  If you run the model several times with your parameters, your chosen outcome should happen most of the time.

The original purpose of this activity was to answer: "How can we describe population size changes In ecosystem?"