NetLogo User Community Models
WHAT IS IT?
This model explores the stability of predator-prey ecosystems in the presence of a refuge area for preys. Such a system is called unstable if it tends to result in extinction for one or more species involved. In contrast, a system is stable if it tends to maintain itself over time, despite fluctuations in population sizes.
HOW IT WORKS
There are two main variations to this model.
HOW TO USE IT
1. Set the GRASS? switch to TRUE to include grass in the model, or to FALSE to only include wolves (red) and sheep (white).
THINGS TO NOTICE
When grass is not included, watch as the sheep and wolf populations fluctuate. Notice that increases and decreases in the sizes of each population are related. In what way are they related? What eventually happens?
Once grass is added, notice the green line added to the population plot representing fluctuations in the amount of grass. How do the sizes of the three populations appear to relate now? What is the explanation for this?
Why do you suppose that some variations of the model might be stable while others are not?
Varying the size of the refuge, it is visible that this size affects the relationship dynamics between predators and prey, altering there survival. What do you notice when refuge is increased or decreased?
THINGS TO TRY
Try adjusting the parameters under various settings. How sensitive is the stability of the model to the particular parameters?
Can you find any parameters that generate a stable ecosystem that includes only wolves and sheep?
Try setting GRASS? to TRUE, but setting INITIAL-NUMBER-WOLVES to 0. This gives a stable ecosystem with only sheep and grass. Why might this be stable while the variation with only sheep and wolves is not?
Notice that under stable settings, the populations tend to fluctuate at a predictable pace. Can you find any parameters that will speed this up or slow it down?
Try changing the reproduction rules -- for example, what would happen if reproduction depended on energy rather than being determined by a fixed probability?
Try studying the model variation several times for each refuge size, with and without grass. What do you see?
EXTENDING THE MODEL
There are a number ways to alter the model so that it will be stable with only wolves and sheep (no grass). Some will require new elements to be coded in or existing behaviors to be changed. Can you develop such a version?
Try creating a model in which the refuge can be used even without the location of origin being in the center of the world. You can also try creating a refuge with a different format (e.g. a square in the center) or a partial refuge, in which predators can enter at precise moments or situations. Is this possible?
Note the use of breeds to model two different kinds of "turtles": wolves and sheep. Note the use of patches to model grass.
Note use of the ONE-OF agentset reporter to select a random sheep to be eaten by a wolf.
Look at Rabbits Grass Weeds for another model of interacting populations with different rules.
CREDITS AND REFERENCES
Wilensky, U. & Reisman, K. (1999). Connected Science: Learning Biology through Constructing and Testing Computational Theories -- an Embodied Modeling Approach. International Journal of Complex Systems, M. 234, pp. 1 - 12. (This model is a slightly extended version of the model described in the paper.)
Wilensky, U. & Reisman, K. (in press). Thinking like a Wolf, a Sheep or a Firefly: Learning Biology through Constructing and Testing Computational Theories -- an Embodied Modeling Approach. Cognition & Instruction.
To refer to this model in academic publications, please use: Wilensky, U. (1997). NetLogo Wolf Sheep Predation model. http://ccl.northwestern.edu/netlogo/models/WolfSheepPredation. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.
Daniel Torrão, Hugo Mendes, Jonathan Marques, Maria Anjo, Melania Falcón & Ruben Cunha (2008). Dinâmica Populacional (Supervisor: Prof. Dr. José M. N. Azevedo). Departamento de Biologia. Universidade dos Açores.
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