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Wolf-Sheep Predator-Mediated Coexistence (FINAL)

by Teresa S. Alexandre, Vanessa Costa & Rúben Rego (Submitted: 09/03/2017)

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## WHAT IS IT?

This model explores the predator-mediated coexistence, which occurs when selective predation favours the coexistence of prey species that might otherwise exclude one another, contributing to an increase of the number of species in a community as long as the preferred prey species is the competitively dominant.

## HOW IT WORKS

There are two types of prey: sheep and goats. The competition between them can be changed in the interface.
Wolves look around searching for sheep and goats to eat. Wolves eat both sheep and goats, but they will prefer to eat a sheep if they notice the presence of one in a patch near them (this preference can also be changed in the interface).

## 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).
2. Adjust the slider parameters (see below), or use the default settings.
3. Press the SETUP button.
4. Press the GO button to begin the simulation.
5. Look at the monitors to see the current population sizes
6. Look at the POPULATIONS plot to watch the populations fluctuate over time

Parameters:
INITIAL-NUMBER-SHEEP: The initial size of sheep population
INITIAL-NUMBER-WOLVES: The initial size of wolf population
INITIAL-NUMBER-GOATS: The initial size of goats population.
SHEEP-GAIN-FROM-FOOD: The amount of energy sheep get for every grass patch eaten
WOLF-GAIN-FROM-FOOD: The amount of energy wolves get for every sheep eaten
GOAT-GAIN-FROM-FOOD: The amount of energy goats get for every grass patch eaten.
WOLF-GOAT-PREFERENCE: Preference that a wolf shows for a goat.
SHEEP-REPRODUCE: The probability of a sheep reproducing at each time step
WOLF-REPRODUCE: The probability of a wolf reproducing at each time step
GOAT-REPRODUCE: The probability of a goat reproducing at each time step.
GRASS?: Whether or not to include grass in the model
GRASS-REGROWTH-TIME: How long it takes for grass to regrow once it is eaten
SHOW-ENERGY?: Whether or not to show the energy of each animal as a number

Notes:
- one unit of energy is deducted for every step a wolf takes
- when grass is included, one unit of energy is deducted for every step a sheep or a goat takes

## THINGS TO NOTICE

Notice that when there's no wolves in the ecosystem, the dynamics between populations of goats and sheep will change. How so? Why does that happen?

## THINGS TO TRY

Try adjusting the parameters under various settings. How sensitive is the stability of the model to the particular parameters?

Try to change the wolf-goat-preference in the interface. How does that afect the populations?

Try to change the competitive dominace between goats and sheep by playing with the 'sheep-gain-from-food' or'sheep-reproduce' and the 'goat-gain-from-food' or 'goat-reproduce'. How does that affect the populations?

Take wolfs out of the equations changing the 'initial-number-wolves' to 0. What will happen?

## 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?

Can you modify the model so the sheep will flock?

Can you modify the model so that wolf actively chase sheep?

## RELATED MODELS

* 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.

## CREDITS AND REFERENCES

Townsend, C. R., Begon, M. & Harper, J. L. (2008). Essentials of Ecology. Third Edition. Blackwell Publishing, 522 pp.

## HOW TO CITE

If you mention this model or the NetLogo software in a publication, we ask that you include the citations below.

For the model itself:

* Alexandre, T. S., Costa, V. & Rego, R. (2017) NetLogo Wolf-Sheep Predator-Mediated Coexistence model. Model built under the orientation of Doctor José M. N. Azevedo. Biology Department, University of Azores, Ponta Delgada.

Please cite the NetLogo software as:

* 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 2017. Teresa Sofia Alexandre, Vanessa Costa e Rúben Rego.

![CC BY-NC-SA 3.0](http://ccl.northwestern.edu/images/creativecommons/byncsa.png)

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