breed [ sheep a-sheep ]

turtles-own [ energy ]  ;; agents own energy

patches-own [ grass-amount ]  ;; patches have grass

;; this procedures sets up the model
to setup
  clear-all
  ask patches [
    ;; give grass to the patches, color it shades of green
    set grass-amount random-float 10.0
    recolor-grass ;; change the world green
  ]
  create-sheep number-of-sheep [
    setxy random-xcor random-ycor
    set color white
    set shape "sheep"
    set energy 100
  ]
  reset-ticks
end

;; make the model run
to go
  if not any? sheep [
    stop
  ]
  ask sheep [
    wiggle        ;; first turn a little bit
    move          ;; then step forward
    check-if-dead ;; check to see if agent should die
    eat           ;; sheep eat grass
    reproduce     ;; the sheep reproduce
  ]
  regrow-grass    ;; the grass grows back
  tick
  my-update-plots ;; plot the population counts
end

;; check to see if this sheep has enough energy to reproduce
to reproduce
  if energy > 200 [
    set energy energy - 100  ;; reproduction transfers energy
    hatch 1 [ set energy 100 ] ;; to the new agent
  ]
end

;; recolor the grass to indicate how much has been eaten
to recolor-grass
  set pcolor scale-color green grass-amount 0 20
end

;; regrow the grass
to regrow-grass
  ask patches [
    set grass-amount grass-amount + grass-regrowth-rate
    if grass-amount > 10.0 [
      set grass-amount 10.0
    ]
    recolor-grass
  ]
end

;; sheep procedure, sheep eat grass
to eat
  ;; check to make sure there is grass here
  if ( grass-amount >= energy-gain-from-grass ) [
    ;; increment the sheep's energy
    set energy energy + energy-gain-from-grass
    ;; decrement the grass
    set grass-amount grass-amount - energy-gain-from-grass
    recolor-grass
  ]
end

;; asks those sheep with no energy to die
to check-if-dead
  if energy < 0  [
    die
  ]
end

;; update the plots in the interface tab
to my-update-plots
  plot count sheep
end

;; sheep procedure, the sheep changes its heading
to wiggle
  ;; turn right then left, so the average is straight ahead
  rt random 90
  lt random 90
end

;; sheep procedure, the sheep moves which costs it energy
to move
  forward 1
  set energy energy - movement-cost ;; reduce the energy by the cost of movement
end


; Copyright 2007 Uri Wilensky.
; See Info tab for full copyright and license.
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@#$#@#$#@
## ACKNOWLEDGMENT

This model is from Chapter Four of the book "Introduction to Agent-Based Modeling: Modeling Natural, Social and Engineered Complex Systems with NetLogo", by Uri Wilensky & William Rand.

* Wilensky, U. & Rand, W. (2015). Introduction to Agent-Based Modeling: Modeling Natural, Social and Engineered Complex Systems with NetLogo. Cambridge, MA. MIT Press.

This model is in the IABM Textbook folder of the NetLogo Models Library. The model, as well as any updates to the model, can also be found on the textbook website: http://www.intro-to-abm.com/.

## WHAT IS IT?

This is the fourth model in a set of five models that build towards a predator prey model of population dynamics.  This fourth model gives the sheep the ability to reproduce.

## HOW IT WORKS

The model creates a population of sheep that wander around the landscape. Moving around costs some energy, and when their energy gets too low, sheep die. However, the sheep can eat grass in the environment to regain energy and the grass regrows over time.  If the energy of a sheep gets above a certain level, it can reproduce.

## HOW TO USE IT

Set the NUMBER-OF-SHEEP slider and press SETUP to create the initial population. You can also change the MOVEMENT-COST slider to affect the energy cost of movement for the sheep. The GRASS-REGROWTH-RATE slider affects how fast the grass grows back, while the ENERGY-GAIN-FROM-GRASS slider affects how much energy the sheep can gain from eating the grass.

After this, press the GO button to make the sheep move around the landscape, and eat the grass.

## THINGS TO NOTICE

Is there any relationship between where the sheep die off and where they reproduce?

Do all the sheep reproduce at the same time? Why is that?

## THINGS TO TRY

How does reproduction affect the model results?  Is it easier to get the sheep to continue to live?  For instance, do the values of GRASS-REGROWTH-RATE, MOVEMENT-COST and ENERGY-GAIN-FROM-GRASS necessary to balance the sheep population change when the sheep reproduce?

## RELATED MODELS

The Wolf Sheep Predation Model in the Biology section of the NetLogo models library.

## CREDITS AND REFERENCES

This model is a simplified version of:

* 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.

## HOW TO CITE

This model is part of the textbook, “Introduction to Agent-Based Modeling: Modeling Natural, Social and Engineered Complex Systems with NetLogo.”

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

For the model itself:

* Wilensky, U. (2007).  NetLogo Wolf Sheep Simple 4 model.  http://ccl.northwestern.edu/netlogo/models/WolfSheepSimple4.  Center for Connected Learning and Computer-Based Modeling, Northwestern Institute on Complex Systems, Northwestern University, Evanston, IL.

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.

Please cite the textbook as:

* Wilensky, U. & Rand, W. (2015). Introduction to Agent-Based Modeling: Modeling Natural, Social and Engineered Complex Systems with NetLogo. Cambridge, MA. MIT Press.

## COPYRIGHT AND LICENSE

Copyright 2007 Uri Wilensky.

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

This work is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 License.  To view a copy of this license, visit https://creativecommons.org/licenses/by-nc-sa/3.0/ or send a letter to Creative Commons, 559 Nathan Abbott Way, Stanford, California 94305, USA.

Commercial licenses are also available. To inquire about commercial licenses, please contact Uri Wilensky at uri@northwestern.edu.

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