Scenario 1: Bug and Grass Population You will first examine the grass and bug population, where the bugs will feed on the grass. Change the number of initial-number-birds to 0 and leave the other settings as i

Demonstration

 For this exercise, you will use a simulation to observe the impact of a variety of conditions on the population size of grass, bugs, and birds. This simulation is run on NetLogo:

• Novak, M. and Wilensky, U. (2011). NetLogo Bug Hunt Predators and Invasive Species model. https://ccl.northwestern.edu/netlogo/models/BugHuntPredatorsandInvasiveSpecies. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.
• Wilensky, U. (1999). NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.

Setting Up NetLogo

Go to http://ccl.northwestern.edu/netlogo/ and click “Go to NetLogo Web.” Search the Models Library for “Curricular Models/BEAGLE Evolution/Bug Hunt Predators and Invasive Species” and select it.

Scenario 1: Bug and Grass Population

You will first examine the grass and bug population, where the bugs will feed on the grass. Change the number of initial-number-birds to 0 and leave the other settings as is for now. Press the “setup” button and then press “go/pause.” Let the simulation run for 1000 ticks.

1. Describe how the bug and grass population size is changing over time. What type of growth patterns are you seeing in each population? What factor(s) could be impacting the change in size?
2. Estimate the carrying capacity of each population and record it in the table below. You can do this by hovering over each line on the graph.

 Run the simulation four more times, but now start adjusting the values for amount-of-grassland, initial- number-bugs, and amount-of-food-bugs-eat (how much grass is consumed at each tick by each one of the bugs). The bugs need enough energy from the grass in order to reproduce. Talk with your group to decide on which adjustments you wish to make before each run and write them in the table below. Remember to press the "setup” button after making the adjustments so they are saved.

Before each run, make a prediction with your group about how each population will be affected. After each run, take notes on the change in the population size of both organisms and determine the carrying capacity. Think about what factors could be causing the changes in population size.   

3. How was changing the values, especially about how much food a bug eats, affecting the carrying capacity in these populations? Did you see scenarios where the bug or grass population collapsed (died?

Scenario 2: Bug, Bird, and Grass Population

You will now add birds back into this ecosystem. Birds are predators of bugs. Refresh the page to get the baseline values for all components back in this simulation. Press the “setup” button and then press “go/pause.” Let the simulation run for 1000 ticks.

1. Describe how the bird population is changing in size over time in relation to the grass and bug population.
2. What is the carrying capacity of the bird population in this run? Record this number in the table below.

As you did in the previous section, run the simulation four more times. There are now two additional values you can adjust: the initial-number-birds and the min-reproduce-energy-birds (amount of energy required for a bird to obtain before it produces offspring). You can adjust these two as well as the values you changed in the bug and grass simulation.

Before each run, make a prediction with your group about how each population will be affected. After each run, take notes on the change in the population size of all three organisms and determine the carrying capacity of each population. Think about what factors could be causing the changes in population size.  

3. How was changing the values, especially about how much energy birds require to reproduce, affecting the carrying capacity in these populations? Did you see scenarios where either of these populations collapsed (died)?

Ecosystems Disruptions

Lastly, let’s disrupt the population growth by bringing in some more population regulation factors. Refresh the page to get back the baseline values.

Scenario 1: You will simulate a fire that is burning down a portion of the grassland. For this scenario, you can adjust the value of %-of-grass-to-burn and you will press the “burn the grass down” button while a simulation is running. You can press this button one time or even several times during a simulation run. 

1. Is this scenario an example of a density-dependent or density-independent factor?
2. What is happening to the bug and bird populations as a result of the grass population being reduced in size? Can you adjust the other values to find a situation where the bug or bird population collapses?
   

Scenario 2: You will simulate a disease that is targeting the bug population. For this scenario, you can adjust the value of %-of-bugs-to-remove and you will press the “remove bugs” button while a simulation is running, Again, you can press this button one time or several times during a simulation run.

3. Is this scenario an example of a density-dependent or density-independent factor?
4. What is happening to the grass and bird population as a result of the bug population being reduced in size?

Scenario 3: You will simulate the introduction of invasive species into the ecosystem. This organism here is a mouse that eats grass and reproduces like bugs in the simulation. For this scenario, you can adjust the value of number-invaders-to-add and amount-of-food-invaders-eat. When you begin a simulation run, press the “launch an invasion” button only once.

5. Is this scenario an example of a density-dependent or density-independent factor?
6. How is the introduction of invasive species affecting the size of the bug population? What happens to the bug population if you decrease or increase the amount of food these invasive species eat?