Grass, rabbits and foxes
This page provides an example of how to use StarLogo to create a model for the following situation: there is growing grass, rabbits that eat the grass and foxes that eat the rabbits. There are adjustable parameters that control various aspects of the coexistence of the three species and the balance between them. Small changes easily lead to the extinction of one or two species ... which unfortunately corresponds to what happens in the real world (up to now, with different species...).
This project explores a simple ecosystem made up of rabbits and grass. The rabbits wander around randomly, and the grass grows randomly. When a rabbit bumps into some grass, it eats the grass and gains energy. If the rabbit gains enough energy, it reproduces. If it doesn't gain enough energy, it dies.
For more information about StarLogo on the Atractor’s website, click here.
- If the foxes eat all the rabbits, they will start to die of starvation soon afterwards.
- If instead all the foxes disappear first, the amount on grass and rabbit populations oscillate in a cycle, although quite differently than before.
- In certain stable conditions over long time periods, if a dry year occurs there is less grass, causing an imbalance leading to a decrease in the number of rabbits and, as a consequence, the extinction of foxes (not rabbits, as one might expect). This is illustrated in the figures below, photos of successive stages in the evolution of the system. The last picture was taken shortly after halving the water supply for the grass...
This example (adapted, adjusted and modified from one of StarLogo’s accompanying documentation) requires some knowledge of the program and some programming skills that are obviously beyond what can be expected of an average student, even in high school. But the program’s operation itself is simple and has been tested with great success with a 9-year-old student. And exploring the program's reaction to the change of parameters is quite interesting, even for adults, and very instructive.
Warning: This is not a final version; if you notice any bizarre behavior, please notify Atractor.
Clicking ini, you get something like:
Then clicking Start, you will be watching a cycle, illustrated by the graph below; the phase in which the graph ends (a grass peak) is illustrated by the picture of the field with grass, rabbits and foxes existing at the time ...
A further stage of system development (minimum grass):
A dry season (obtained in the model by sliding watering + sun to about half of the initial value) caused a reduction in rate of grass growth. As you may see, the effect was felt in the number of rabbits, but it was just devastating for the foxes ...
Enjoy other experiences changing the parameters and check if the observed effects coincide with the expected ones! If you want to use this program without permanent connection to the internet (for that you need to have StarLogo installed on your computer), please import the file coelhos-_en.slogo and open it with the program. To install StarLogo, please go to StarLogo site.
What follows is a manipulable applet. Click ini; after the button resumes its initial color, click Start. The buttons below ini can be manipulated during the evolution of the system. The ones above ini do not take effect until you press ini again.