NetLogo Models Library:
This model explores the stability of consumer producer ecosystems and both temporary disturbances and more sustained environmental changes affect the stability of the population and the ecosystem.
Bugs wander randomly around one of two regions (left or right). Each region is a self contained ecosystem in the world. Bugs in one region never go into the other region. As bugs reach the edge of their region, they wrap around to other side of their own region.
Each time step, each bug loses one unit of energy and they must consume a food source (grass) to replenish their energy. When they run out of energy, they die. To allow the population to continue, each bug must have enough energy to have an offspring. When that threshold is reached, the offspring and parent split the energy amongst themselves.
Different amounts of grassland can be assigned to each region. Within each region, those grassland spots will regrow grass at a fixed rate, when it is eaten. Grass is eaten when a bug moves over that patch. When this happens, a fixed amount of grass energy is deducted from the patch (square) where the grass was eaten.
Different disturbances can be tested in this system, including temporary removal of grass (simulating a fire) and infection of some percentage of the bugs (simulating transmittable disease).
Adjust the slider parameters (see below), or use the default settings.
Press the SETUP button.
Press the GO button to begin the model run.
View the LEFT REGION POPULATION SIZE VS. TIME and RIGHT REGION POPULATION SIZE VS. TIME plot to watch the bug and grass populations fluctuate over time in each of the regions on the screen.
View the BUGS IN LEFT ECOSYSTEM and BUGS IN RIGHT ECOSYSTEM monitors to keep track of the total number of new bugs that that are in each region.
View the number of ticks the simulation has run for in the TIME monitor.
CONSTANT-SIMULATION-LENGTH: When turned "on" the model run will automatically stop at 1000 ticks. When turned "off" the model run will continue running without automatically stopping.
LEFT-REGION-%-OF-GRASSLAND and RIGHT-REGION-%-OF-GRASSLAND: The percentage of patches in the world & view that produce grass in that region. The grasses that don't produce grass will show "gray" rocks/gravel in that patch.
FOOD-LEFT-BUGS-EAT and FOOD-RIGHT-BUGS-EAT: Sets the amount of energy that all bugs in that region gain from eating grass at a patch as well as the amount of energy deducted from that grass.
LEFT-BUGS-TO-ADD and RIGHT-BUGS-TO-ADD: Determine the initial size of bug population in that region and also determines how many bugs will be added to the model run when the L-ADD-MORE-BUGS or R-ADD-MORE-BUGS button is pressed.
LEFT-BUGS-TO-INFECT and RIGHT-BUGS-TO-INFECT: Determine the % the bug population in that region that will be instantly infected with a disease. The disease is transmitted by contact with other bugs in that region. Bugs die after a fixed time period of carrying the disease.
L-START WILD FIRE and R-START WILD FIRE: start a wild fire in a patch that has grass on the left side of the region. Fire moves from burning patches to adjacent patches that have grass in that region.
Watch as the grass and bug populations fluctuate. How are increases and decreases in the sizes of each population related?
Adding bugs, or infecting bugs affects the size of the populations in the short term, but not in the long term. What causes this behavior?
Different percentages of grassland values affect the carrying capacity (average values) for both the bugs and grass. Why?
Try adjusting the parameters under various settings. How sensitive is the stability of the model to the particular parameters. Does the parameter setting affect the amount of fluctuations, the average values of bugs and grass, or does it lead to the collapse of the ecosystem (death of all the bugs)?
In this model, all the bugs are identical to each other and follow the same rules. Try modeling variation in the bug population that would make it easier for some bugs to get food.
The visualization of fire embers uses the transparency value for the color to gradually fade out the color of the fire and let the background show through, before the embers disappear completely.
Refer to Bug Hunt Predators and Invasive Species - Two Regions for extensions of this model that include predators (birds that eat bugs) and an invasive species (another population of consumers).
This model is part of the Ecology & Population Biology unit of the ModelSim curriculum, sponsored by NSF grant DRL-1020101.
For more information about the project and the curriculum, see the ModelSim project website: http://ccl.northwestern.edu/modelsim/.
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Copyright 2015 Uri Wilensky.
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