NetLogo User Community Models
by MBI Summer Program 2009 NetLogo Group (Submitted: 08/13/2009)
WHAT IS IT?
This project explores a simple ecosystem made up of rabbits, grass, and weeds. The rabbits wander around randomly, and the grass and weeds grow randomly. When a rabbit bumps into some grass or weeds, it eats the grass and gains energy. If the rabbit gains enough energy, it reproduces. If it doesn't gain enough energy, it dies.
The grass and weeds can be adjusted to grow at different rates and give the rabbits differing amounts of energy. The model can be used to explore the competitive advantages of these variables.
HOW TO USE IT
Click the SETUP button to setup the rabbits (red), grass (green), and weeds (violet). Click the GO button to start the simulation.
The NUMBER slider controls the initial number of rabbits. The BIRTH-THRESHOLD slider sets the energy level at which the rabbits reproduce. The GRASS-GROWTH-RATE slider controls the rate at which the grass grows. The WEEDS-GROWTH-RATE slider controls the rate at which the weeds grow.
The model's default settings are such that at first the weeds are not present (weeds-grow-rate = 0, weeds-energy = 0). This is so that you can look at the interaction of just rabbits and grass. Once you have done this, you can start to add in the effect of weeds.
THINGS TO NOTICE
Watch the COUNT-RABBITS monitor and the POPULATIONS plot window to see how the rabbit population changes over time. At first, there is not enough grass for the rabbits, and many rabbits die. But that allows the grass to grow more freely, providing an abundance of food for the remaining rabbits. The rabbits gain energy and reproduce. The abundance of rabbits leads to a shortage of grass, and the cycle begins again.
The rabbit population goes through a damped oscillation, eventually stabilizing in a narrow range. The total amount of grass also oscillates, out of phase with the rabbit population.
These dual oscillations are characteristic of predator-prey systems. Such systems are usually described by a set of differential equations known as the Lotka-Volterra equations. NetLogo provides a new way of studying predatory-prey systems and other ecosystems.
THINGS TO TRY
Leaving other parameters alone, change the grass-grow-rate and let the system stabilize again. Would you expect that there would now be more grass? More rabbits?
Change only the birth-threshold of the rabbits. How does this affect the steady-state levels of rabbits and grass?
With the current settings, the rabbit population goes through a damped oscillation. By changing the parameters, can you create an undamped oscillation? Or an unstable oscillation?
In the current version, each rabbit has the same birth-threshold. What would happen if each rabbit had a different birth-threshold? What if the birth-threshold of each new rabbit was slightly different from the birth-threshold of its parent? How would the values for birth-threshold evolve over time?
Now add weeds by making the sliders WEEDS-GROW-RATE the same as GRASS-GROW-RATE and WEEDS-ENERGY the same as GRASS-ENERGY. Notice that the amount of grass and weeds is about the same.
Now make grass and weeds grow at different rates. What happens?
What if the weeds grow at the same rate as grass, but they give less energy to the rabbits when eaten (WEEDS-ENERGY is less than GRASS-ENERGY)?
Think of other ways that two plant species might differ and try them out to see what happens to their relative populations. For example, what if a weed could grow where there was already grass, but grass couldn't grow where there was a weed? What if the rabbits preferred the plant that gave them the most energy?
Run the model for a bit, then suddenly change the birth-threshold to zero. What happens?
Notice that every black patch has a random chance of growing grass or
if (random-float 1000) < weeds-grow-rate and pcolor = black
Look at Wolf Sheep Predation for another interacting ecosystem with different rules.
CREDITS AND REFERENCES
To refer to this model in academic publications, please use: Wilensky, U. (2001). NetLogo Rabbits Grass Weeds model. http://ccl.northwestern.edu/netlogo/models/RabbitsGrassWeeds. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.
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