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This model explores the stability of consumer producer ecosystems when different initial populations of predator, prey, and producers are initialized into the model and when different numbers of invasive species are added to the model run.
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 step (tick), 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 for an individual bug, it has an offspring. The offspring and parent split the energy amongst themselves. Different amounts of grassland can be assigned to each region using the sliders. 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. Within each region, grass regrows at a fixed rate.
Birds wander randomly around one of two regions (left or right), following similar movement, energy loss, and reproduction rules as the bugs. Birds eat bugs, and not grass.
An invasive species can be introduced into either ecosystem in the model. Individuals of the invasive species wander randomly around the regions in which they are introduced. Their movement, energy loss, and reproduction rules are the same as the bugs. They also eat grass.
Monitors for BUGS, BIRDS, and INVADERS will report for both the left and right region, depending on where the monitors are located (the left side of the interface or the right).
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-INITIAL-BIRDS and RIGHT-INITIAL-BIRDS: Determines the initial size of the bird population in that region.
FOOD-LEFT-BUGS-EAT and FOOD-RIGHT-BUGS-EAT: Sets the amount of energy that bugs in that region gain from eating grass at a patch as well as the amount of energy deducted from that grass patch.
LEFT-INITIAL-BUGS and RIGHT-INITIAL-BUGS: Determines the initial size of the bug population in that region.
LEFT-REGION-%-OF-GRASSLAND and RIGHT-REGION-%-OF-GRASSLAND: The percentage of patches in the world & view that produce grass in that region. The grass patches that don't produce grass will show "gray" rocks/gravel.
FOOD-LEFT-INVADERS-EAT and FOOD-RIGHT-INVADERS-EAT: Sets the amount of energy that all invaders in that region gain from eating grass at a patch as well as the amount of energy deducted from that grass.
LEFT-INVADERS-TO-ADD and RIGHT-INVADERS-TO-ADD: Determines the size of invader population to add to that region, when the corresponding LAUNCH INVASION button is pressed.
Watch as the grass, bug, bird, and invader populations fluctuate. How are increases and decreases in the sizes of each population related?
Different % of grassland values affect the carrying capacity (average values) for both the bugs, grass, birds, and invasive. Why?
Different food consumption values (FOOD-LEFT-BUGS-EAT and FOOD-RIGHT-BUGS-EAT and FOOD-LEFT-INVADERS-EAT and FOOD-RIGHT-INVADERS-EAT) may lead to different levels of stability as well as whether one population (bugs vs. invaders) outcompetes the other.
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 carrying capacity of bugs, grass, invaders, or birds, or does it lead to the collapse of one population in the ecosystem?
The two regions in this model are represented by shapes that add another "wall" that surround the region.
Refer to Bug Hunt Disruptions and Bug Hunt Environmental Changes for extensions of this model that include temporary disturbances such as fire and disease.
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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