Carrying Capacity

;; Carrying Capacity model.
;;
;; Coded in 2019 by Lin Xiang; Last revised in 2021 by Lin Xiang (lxiang75@gmail.com; lin.xiang@uky.edu)
;;
;; If you mention this model in a publication, we ask that you include the citations below.
;;
;; Xiang, L. (2019). Carrying Capacity. Department of STEM Education, University of Kentucky, Lexington, KY.
;;
;;-----------------------------------------
;;CREATIVE COMMONS LICENSE
;;This code is distributed by Lin Xiang under a Creative Commons License:
;;Attribution-ShareAlike 4.0 International (CC BY-SA 4.0)
;;https://creativecommons.org/licenses/by-sa/4.0/
;;
;;-----------------------------------------

globals [Available-grass total-grass h-size]

Breed [elks elk]
breed [wolves wolf]
breed [rabbits rabbit]
turtles-own [erg]


;------------------------------------;
;        SETUP PROCEDURES            ;
;------------------------------------;

to setup
  ca

  setup-habitat                       ;setup habitat

  ask patches with [pcolor = 53]      ;create elk
  [sprout-elks 1 [
    set shape "elk"
    set size 0.75
    set color 37.5
    setup-position
      set erg 30]]

  set infinite-grass? false
  if number-of-wolves != 0 [setup-wolves]
  if number-of-rabbits != 0 [setup-rabbits]
  RESET-TICKS
end 

;------------------------------------;
;        GO PROCEDURES               ;
;------------------------------------;

to go
  tick

competition
eat
death
reproduce
predation
update-habitat-size


regrow-grass

  if count elks >= 10000 [ user-message "There are more than 10,000 elks now. They have taken over this world!" stop]
  if count elks <= 0 [user-message "All elks die." stop]
end 

;------------------------------------;
;        SUB-PROCEDURES              ;
;------------------------------------;

to setup-habitat
  ask patches
  [set pcolor 94]
  ask patches with [abs pxcor <= habitat-size and abs pycor <= habitat-size] [set pcolor 53]

  set total-grass count patches with [pcolor = 53]
  set h-size habitat-size
end 

to setup-position
  setxy  (-1 * habitat-size) + random-float (habitat-size * 2)   (-1 * habitat-size) + random-float (habitat-size * 2)
end 

to move
    ifelse [pcolor] of patch-at dx dy = 94 [setup-position][fd 1 rt random 360]
end 

to eat
 ask elks [
    move
    set erg erg - 1
    ifelse infinite-grass? = false [
    if pcolor = 53
    [set erg erg + 3 set pcolor 0]][set erg erg + 3]
  ]
end 

to death
  ask elks [
    if erg <= 0 [die]
  ]

  ask turtles [
  if pcolor = 94 [die]
]
end 

to reproduce
  ask elks [ if erg >= 60
    [hatch 1 [set erg 30]
      set erg erg - 30
    setup-position]
  ]
end 

to regrow-grass
  ask patches [ if pcolor = 0
   [if random 100 < grass-growth-rate [set pcolor 53]
    ]
  ]
 set Available-grass count patches with [pcolor = 53] / total-grass
end 

to setup-wolves
         create-wolves number-of-wolves [
          set shape "wolf"
          set color 32
          set size 1
          set erg 20
          setup-position]
end 

to predation

  ifelse number-of-wolves = 0
  [ask wolves [die]]
  [let wolf-number count wolves
      if wolf-number != number-of-wolves
        [ask wolves [die]
        setup-wolves]
    ask wolves [
      move
      let prey one-of elks-here
      if prey != nobody [ask prey [die]]
  ]]
end 

to setup-rabbits
  create-rabbits number-of-rabbits
        [set shape "rabbit1"
          set color 9
          set size 0.5
          setup-position]
end 

to competition
   ifelse number-of-rabbits = 0
  [ask rabbits [die]]
  [let rabbit-number count rabbits
      if rabbit-number != number-of-rabbits
        [ask rabbits [die]
          setup-rabbits]
    ask rabbits [
      move
      if pcolor = 53 [set pcolor 0]
  ]]
end 

to update-habitat-size

  if h-size != habitat-size [
    clear-patches
    setup-habitat]
end 

to-report grass-growth-rate
 report -0.25 * (temperature ^ 2) + 12.5 * temperature - 56.25
end