globals [grass]  ;; keep track of how much grass there is
;; Sheep and wolves are both breeds of turtle.
breed [sheep a-sheep]  ;; sheep is its own plural, so we use "a-sheep" as the singular.
breed [wolves wolf]
breed [goats goat]
turtles-own [energy]       ;; both wolves and sheep have energy
patches-own [countdown]

to setup
  clear-all
  ask patches [ set pcolor green ]
  ;; check GRASS? switch.
  ;; if it is true, then grass grows and the sheep eat it
  ;; if it false, then the sheep don't need to eat
  if grass? [
    ask patches [
      set pcolor one-of [green brown]
      if-else pcolor = green
        [ set countdown grass-regrowth-time ]
        [ set countdown random grass-regrowth-time ] ;; initialize grass grow clocks randomly for brown patches
    ]
  ]
  set-default-shape sheep "sheep"
  create-sheep initial-number-sheep  ;; create the sheep, then initialize their variables
  [
    set color white
    set size 1.5  ;; easier to see
    set label-color blue - 2
    set energy random (2 * sheep-gain-from-food)
    setxy random-xcor random-ycor
  ]
  set-default-shape goats "goat"
  create-goats initial-number-goats  ;; create the goats, then initialize their variables
  [
    set color white
    set size 1.5  ;; easier to see
    set label-color blue - 2
    set energy random (2 * goat-gain-from-food)
    setxy random-xcor random-ycor
  ]
  set-default-shape wolves "wolf"
  create-wolves initial-number-wolves  ;; create the wolves, then initialize their variables
  [
    set color black
    set size 2  ;; easier to see
    set energy random (2 * wolf-gain-from-food)
    setxy random-xcor random-ycor
  ]
  display-labels
  set grass count patches with [pcolor = green]
  reset-ticks
end

to go
  if not any? turtles [ stop ]
  ask turtles [
    if breed = sheep [
      move
    if grass? [
      set energy energy - 1  ;; deduct energy for sheep only if grass? switch is on
      sheep-eat-grass
    ]
    death
    reproduce-sheep
    ]
    if breed = goats [
      move
    if grass? [
      set energy energy - 1  ;; deduct energy for goats only if grass? switch is on
      goat-eat-grass
    ]
    death
    reproduce-goats
  ]
    if breed = wolves [
      move
      set energy energy - 1  ;; wolves lose energy as they move
      catch-prey
      death
      reproduce-wolves
  ]]
  if grass? [ ask patches [ grow-grass ] ]
  set grass count patches with [pcolor = green]
  tick
  display-labels
end

to move  ;; turtle procedure
  rt random 50
  lt random 50
  fd 1
end

to sheep-eat-grass  ;; sheep procedure
  ;; sheep eat grass, turn the patch brown
  if pcolor = green [
    set pcolor brown
    set energy energy + sheep-gain-from-food  ;; sheep gain energy by eating
  ]
end

to goat-eat-grass  ;; goats procedure
  ;; goats eat grass, turn the patch brown
  if pcolor = green [
    set pcolor brown
    set energy energy + goat-gain-from-food  ;; goats gain energy by eating
  ]
end

to reproduce-sheep  ;; sheep procedure
  if random-float 100 < sheep-reproduce [  ;; throw "dice" to see if you will reproduce
    set energy (energy / 2)                ;; divide energy between parent and offspring
    hatch 1 [ rt random-float 360 fd 1 ]   ;; hatch an offspring and move it forward 1 step
  ]
end

to reproduce-goats  ;; goat procedure
  if random-float 100 < goat-reproduce [  ;; throw "dice" to see if you will reproduce
    set energy (energy / 2)                ;; divide energy between parent and offspring
    hatch 1 [ rt random-float 360 fd 1 ]   ;; hatch an offspring and move it forward 1 step
  ]
end

to reproduce-wolves  ;; wolf procedure
  if random-float 100 < wolf-reproduce [  ;; throw "dice" to see if you will reproduce
    set energy (energy / 2)               ;; divide energy between parent and offspring
    hatch 1 [ rt random-float 360 fd 1 ]  ;; hatch an offspring and move it forward 1 step
  ]
end

to catch-prey  ;; wolf procedure
  let prey-sheep one-of sheep-here                     ;; Grab a random sheep on this patch.
  if-else prey-sheep != nobody                         ;; Did I get one?  If so,
    [ ask prey-sheep [ die ]                           ;; kill it and
      set energy energy + wolf-gain-from-food ]        ;; get energy from eating.
    [let prey-goat one-of goats-here                   ;; If not, grab a random goat on this patch.
     if (prey-goat != nobody) and (random 101 < wolf-goat-pref)   ;; Did I get one?  Shall I eat it?
       [ ask prey-goat [ die ]                         ;; If so, kill it and
         set energy energy + wolf-gain-from-food]      ;; gain energy from eating.
     ]
end

to death  ;; turtle procedure
  ;; when energy dips below zero, die
  if energy < 0 [ die ]
end

to grow-grass  ;; patch procedure
  ;; countdown on brown patches: if reach 0, grow some grass
  if pcolor = brown [
    ifelse countdown <= 0
      [ set pcolor green
        set countdown grass-regrowth-time ]
      [ set countdown countdown - 1 ]
  ]
end

to display-labels
  ask turtles [ set label "" ]
  if show-energy? [
    ask wolves [ set label round energy ]
    if grass? [ ask sheep [ set label round energy ] ]
    if grass? [ ask goats [ set label round energy ] ]

  ]
end


; Copyright 1997 Uri Wilensky.
; See Info tab for full copyright and license.
@#$#@#$#@
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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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NetLogo 6.0.2
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set grass? true
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