NetLogo Models Library:
## WHAT IS IT?
This model explores the stability of predator-prey ecosystems and how that stability is affected when new species are introduced into the ecosystem.
## HOW IT WORKS
Birds and bugs wander randomly around the landscape. Each step costs both animals energy and they must consume a food source (bugs must eat grass and birds must eat bugs & the invaders) to replenish their energy - when they run out of energy they die. To allow the population to continue, each bird or bug must have enough energy to have a litter of offspring and the offspring and parent split the energy amongst themselves. Grass grows at a fixed rate, and when it is eaten, a fixed amount of grass energy is deducted from the patch (square) where the grass was eaten.
Invasive species (mice) can be introduced to this ecosystem as well. The mice are indirect competitors of the bugs. They also eat grass. They reproduce following the same rules as bugs. The eating behavior of the mice and the bugs can be adjusted.
Other disruptions such as removing a portion of the bug population (simulating a disease) or burning down the grass (removing the food source for bugs and invasive species temporarily) can also be tested to see how these disruptions affect the stability of the ecosystem.
## HOW TO USE IT
1. Adjust the slider parameters (see below), or use the default settings.
2. Press the SETUP button.
4. Press the GO button to begin the simulation.
5. View the POPULATIONS plot to watch the populations fluctuate over time.
6. View the birds/bugs and invaders monitors to view current population sizes.
7. Adjust the NUMBER-INVADERS-TO-ADD and AMOUNT-OF-FOOD-INVADERS-EAT and press LAUNCH INVASION.
8. View the POPULATIONS plot to watch the populations fluctuate over time.
CONSTANT-SIMULATION-LENGTH: When turned "on" the model run will automatically stop at 2500 ticks. When turned "off" the model run will continue without automatically stopping.
AMOUNT-OF-GRASSLAND: The percentage of patches in the world & view that produce grass.
INITIAL-NUMBER-BUGS: The initial size of the bug population.
INITIAL-NUMBER-BIRDS: The initial size of the bird population.
AMOUNT-OF-FOOD-BUGS-EAT: The amount of grass consumed at each simulation step by each one of the bugs.
MIN-REPRODUCE-ENERGY-BIRDS: The amount of energy required for a bird to accumulate before it automatically produces an offspring.
GRASS-TO-BURN-DOWN: Sets the % of designated grassland patches that will have all their grass immediately removed when the BURN THE DOWN GRASS button is pressed.
%-OF-BUGS-TO-REMOVE: Sets the % of existing bug population that will be immediately removed when the REMOVE BUGS button is pressed.
NUMBER-INVADERS-TO-ADD: The number of the invasive species introduced when LAUNCH INVASION is pressed.
AMOUNT-OF-FOOD-INVADERS-EAT: The amount of grass consumed at each simulation step by each one of the invasive species.
## THINGS TO NOTICE
Watch as the grass, predators, and bug populations fluctuate. How are increases and decreases in the sizes of each population related?
How does changing the attributes of the bugs (AMOUNT-OF-FOOD-BUGS-EAT) or birds (MIN-REPRODUCE-ENERGY-BUGS) affect the stability and carrying capacity of the ecosystem?
How does introducing an invasive species that has the same diet as the bugs (i.e. grass) affect the stability of the ecosystem? How does setting the AMOUNT-OF-FOOD-INVADERS-EAT to a different value than AMOUNT-OF-FOOD-BUGS-EAT affect the stability of each population in the ecosystem?
## THINGS TO TRY
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 each population, or does it lead to the collapse of one of the populations in the ecosystem (death of all the bugs, birds, or invasive species)?
## EXTENDING THE MODEL
Additional invasive plant or invasive predator could be added.
The amount of food a predator or a bug eats could be adjusted so that faster food consumption also corresponds to higher metabolism (cost of energy per move per turn), so that there is a competitive disadvantage for rapid food consumption in some environments.
Traits such as amount of food a predator eats, or min-reproduction energy could be inherited in offspring and subject to slight random changes from mutations.
## NETLOGO FEATURES
scale-color is used to visualize the amount of food (energy) available for bugs or invaders at each patch. A whitish green (color of 59.9) is used to represent little grass foliage (not much food available at this patch) and green (color of 55) is used to represents lots of foliage (max. food available at this patch). All other values for food plant-energy are linearly scaled between this range of colors.
## RELATED MODELS
Look at the Wolf-Sheep model for a basic predator prey model example.
Look at Bug Hunt Consumers for a simpler version of this model that includes no predators or invaders.
## CREDITS AND REFERENCES
This model is a part of the BEAGLE curriculum (http://ccl.northwestern.edu/simevolution/beagle.shtml)
## HOW TO CITE
If you mention this model in a publication, we ask that you include these citations for the model itself and for the NetLogo software:
* Novak, M. and Wilensky, U. (2011). NetLogo Bug Hunt Predators and Invasive Species model. http://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.
## COPYRIGHT AND LICENSE
Copyright 2011 Uri Wilensky.
![CC BY-NC-SA 3.0](http://ccl.northwestern.edu/images/creativecommons/byncsa.png)
This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License. To view a copy of this license, visit http://creativecommons.org/licenses/by-nc-sa/3.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.
Commercial licenses are also available. To inquire about commercial licenses, please contact Uri Wilensky at email@example.com.