VERA Exercise

In VERA, you will develop models of predator-prey interaction scenarios that involve wolves, sheep, and grass. The scenarios form a series of increasingly complex situations. You will start by trying to model the species with default propagation parameters and then investigate how changing propagation parameters, resource availability, and emergence of predator affect the species’ populations.

Phase 1: Sheep and Grass (unlimited resource)

Please create a new project and a blank model named “Phase 1.” You’ll start by adding population of sheep to the model.

Model Instructions

  • On the left side of the Model Editor, click on the “Biotic Population” shape in the middle. Notice that there is now a shape that says “Biotic” in the Modeling Space.
  • Type “Sheep” and click “Apply.” You can change the color of the shape by clicking on the “Fill” rectangle on the right side of the screen. Please note that the purpose of changing the color is to set the color of the line in the graph that shows this size of this population during the simulation. By setting different colors, you can easily distinguish each component in the graph.
  • Click the “Simulation” tab on the top right corner.
  • Click the “Start” to simulate the model.

Questions

  • Please write your observation. How do sheep population change over time? Why do you think this happens?

Model Instructions

  • Add “Grass” as a biotic population.
  • To create a relationship between “Sheep” and “Grass”, deselect both shapes.
    • Hover your mouse over the edge of the “Sheep” shape. When a green circle appears, you can click and drag your mouse over to the edge of the “Grass” shape.
    • Notice that the default relationship is “Consumes.”
  • Simulate the resulting model.

Questions

  • Please write your observation. How do sheep and grass populations change over time? Why do you think this happens?

Model Instructions

  • Make the “Grass” as unlimited resource so that it never runs out. You can make the unlimited resource by adding the “minimum population” parameter.
    • Click on the “Grass” component and then click on the “Parameters” tab in the right pane of the screen.
    • Add “minimum population.”
  • Simulate the resulting model.

Questions

  • Please write your observation. How do sheep population change over time? Why do you think this happens?

Before Phase 2

  • Save your model.
    • In the Model Editor, click on “File” below the project title, and “Save.”
  • Clone your model “Phase 1.”
    • Next to the model name “Phase 1,” click on an icon that looks like double documents
    • Select “Existing project” and select the project
    • Change “Cloned model name” as “Phase 2”

Phase 2: Sheep and Grass

In this phase, instead of making “Grass” as unlimited resource, make it a depleting resource like in a real ecology system.

Model Instructions

  • Add “Sheep” and “Grass” biotic populations as in the phase 1.
  • Add the “Consumes” relationship between “Sheep” and “Grass” as in the phase 1.
  • Click on the “Grass” component and then click on the “Parameters” tab in the right pane of the screen.
    • Remove “minimum population”

Now, let the “Grass” population be controlled by abiotic substance, “Sunlight.”

Model Instructions

  • Add “Sunlight” as an abiotic substance.
  • Add the “Affects” relationship between “Grass” and “Sunlight.”
  • Simulate the model.
  • Go back to your model, and adjust the “Growth rate” in the “Affects” relationship. Click on the “Affects” relationship and then click on the “Parameters” tab in the right pane of the screen.
  • Adjust “Growth rate” (min:0; max:1) so that grass population doesn’t run out but doesn’t explode.
  • Simulate the model again and compare the results.

Questions

  • Please write your observation. How do sheep and grass populations change over time? Compare the model with the model without sunshine in the previous step.

Phase 3: Sheep and Grass (controlled propagation parameters)

Please created a blank model named “Phase 3.” In the previous steps, you used default propagation parameters for the “Sheep” population. This time, you will add real propagation parameters such as lifespan, reproductive maturity, and offspring count as in real ecology systems using Smithsonian’s Encyclopedia of Life (EOL) (http://eol.org).

Model Instructions

  • Add “Sheep” biotic population.
    • On the left side of the Model Editor, click on the “Biotic Population” shape in the middle.
    • Type “Sheep” and click “Lookup species on EOL.”.
    • Wait until the list is retrieved from EOL.
    • Select one from the list (e.g. domestic sheep).
    • Click “Apply”
  • Add “Grass” biotic population.
    • On the left side of the Model Editor, click on the “Biotic Population” shape in the middle.
    • Type “Grass” and click “Apply.”
    • Make the “Grass” as the base population so that it never runs out. You can make the base population by adding the “minimum population” parameter.
  • Add the “Consumes” relationship between “Sheep” and “Grass” as in the phase 1 and 2.
  • Click on the “Sheep” component and then click on the “Parameters” tab in the right pane of the screen.
    • You will see the preset parameters as shown in Table 1.
  • Simulate the model.

Questions

  • Please write your observation. How do sheep and grass populations change over time? Compare the model with the model without parameterization in the previous step.

Phase 4: Sheep and Grass and Wolf

For this phase, please clone the model in the phase 3 and rename it as “Phase 4.” You will add a predator in this phase.

Model Instructions

  • Add “Wolf” as a biotic population.
    • On the left side of the Model Editor, click on the “Biotic Population” shape in the middle.
    • Type “Gray Wolf” and click “Lookup species on EOL.”
    • Wait until the list is retrieved from EOL.
    • Select one from the list (e.g. gray wolf).
    • Click “Apply”
  • Add the “Consumes” relationship between “Gray Wolf“ and “Sheep”.
  • Click on the “Gray Wolf” component and then click on the “Parameters” tab in the right pane of the screen.
    • You will see the preset parameters as shown in Table 2.
    • EOL may not have data for some species. In that case, you can manually add the parameters using other knowledge sources well as your knowledge.
  • Simulate the model.

Questions

  • Please write your observation. How do sheep and grass populations change over time? Does your simulation result look like the graph below? If not, how can you revise your model to make a stable predator-prey cycle? Please share your successes and failures.