; Modifed Wolf Sheep Predation model to include a new breed "humans" that eat wovles ; ; Original model from netlogo models library > biology > wolf sheep predation ; Copyright 1997 Uri Wilensky. ; See Info tab for full copyright and license. 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 [humans human] 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 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 ] set-default-shape humans "person" create-humans initial-number-humans [ set color yellow set size 1.5 set energy random (2 * human-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 sheep [ move if grass? [ set energy energy - 1 ;; deduct energy for sheep only if grass? switch is on eat-grass ] death reproduce-sheep ] ask wolves [ move set energy energy - 1 ;; wolves lose energy as they move catch-sheep death reproduce-wolves ] ask humans [ move set energy energy - 1 catch-wolves death reproduce-humans ] 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 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 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-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 reproduce-humans if random-float 100 < human-reproduce [ set energy (energy / 2) hatch 1 [rt random-float 360 fd 1] ] end to catch-sheep ;; wolf procedure let prey one-of sheep-here ;; grab a random sheep if prey != nobody ;; did we get one? if so, [ ask prey [ die ] ;; kill it set energy energy + wolf-gain-from-food ] ;; get energy from eating end to catch-wolves let prey one-of wolves-here if prey != nobody [ ask prey [ die ] set energy energy + human-gain-from-food ] 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 ] ask humans [ set label round energy ] if grass? [ ask sheep [ set label round energy ] ] ] end ; Copyright 1997 Uri Wilensky. ; See Info tab for full copyright and license. @#$#@#$#@ GRAPHICS-WINDOW 685 10 1154 500 25 25 9.0 1 14 1 1 1 0 1 1 1 -25 25 -25 25 1 1 1 ticks 30.0 SLIDER 3 150 177 183 initial-number-sheep initial-number-sheep 0 250 100 1 1 NIL HORIZONTAL SLIDER 3 187 177 220 sheep-gain-from-food sheep-gain-from-food 0.0 50.0 4 1.0 1 NIL HORIZONTAL SLIDER 3 222 177 255 sheep-reproduce sheep-reproduce 1.0 20.0 4 1.0 1 % HORIZONTAL SLIDER 181 150 346 183 initial-number-wolves initial-number-wolves 0 250 50 1 1 NIL HORIZONTAL SLIDER 181 186 346 219 wolf-gain-from-food wolf-gain-from-food 0.0 100.0 20 1.0 1 NIL HORIZONTAL SLIDER 181 222 346 255 wolf-reproduce wolf-reproduce 0.0 20.0 5 1.0 1 % HORIZONTAL SWITCH 5 87 99 120 grass? grass? 0 1 -1000 SLIDER 106 88 318 121 grass-regrowth-time grass-regrowth-time 0 100 30 1 1 NIL HORIZONTAL BUTTON 8 28 77 61 setup setup NIL 1 T OBSERVER NIL NIL NIL NIL 1 BUTTON 90 28 157 61 go go T 1 T OBSERVER NIL NIL NIL NIL 0 PLOT 16 311 332 508 populations time pop. 0.0 100.0 0.0 100.0 true true "" "" PENS "sheep" 1.0 0 -13345367 true "" "plot count sheep" "wolves" 1.0 0 -2674135 true "" "plot count wolves" "grass / 4" 1.0 0 -10899396 true "" "if grass? [ plot grass / 4 ]" "human" 1.0 0 -987046 true "" "plot count humans" MONITOR 54 264 125 309 sheep count sheep 3 1 11 MONITOR 129 264 211 309 wolves count wolves 3 1 11 MONITOR 215 264 291 309 NIL grass / 4 0 1 11 TEXTBOX 8 130 148 149 Sheep settings 11 0.0 0 TEXTBOX 186 130 299 148 Wolf settings 11 0.0 0 TEXTBOX 9 68 161 86 Grass settings 11 0.0 0 SWITCH 167 28 303 61 show-energy? show-energy? 1 1 -1000 SLIDER 348 149 512 182 initial-number-humans initial-number-humans 0 100 50 1 1 NIL HORIZONTAL SLIDER 348 186 511 219 human-gain-from-food human-gain-from-food 0 100 100 1 1 NIL HORIZONTAL SLIDER 349 222 511 255 human-reproduce human-reproduce 0 100 1 1 1 % HORIZONTAL MONITOR 300 263 363 308 humans count humans 17 1 11 @#$#@#$#@ ## WHAT IS IT? This model explores the stability of predator-prey ecosystems. 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. In the first variation, wolves and sheep wander randomly around the landscape, while the wolves look for sheep to prey on. Each step costs the wolves energy, and they must eat sheep in order to replenish their energy - when they run out of energy they die. To allow the population to continue, each wolf or sheep has a fixed probability of reproducing at each time step. This variation produces interesting population dynamics, but is ultimately unstable. The second variation includes grass (green) in addition to wolves and sheep. The behavior of the wolves is identical to the first variation, however this time the sheep must eat grass in order to maintain their energy - when they run out of energy they die. Once grass is eaten it will only regrow after a fixed amount of time. This variation is more complex than the first, but it is generally stable. The construction of this model is described in two papers by Wilensky & Reisman referenced below. ## 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 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 SHEEP-REPRODUCE: The probability of a sheep reproducing at each time step WOLF-REPRODUCE: The probability of a wolf 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 takes ## 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? ## 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? ## 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? ## NETLOGO FEATURES 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. ## RELATED MODELS 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. (2006). Thinking like a Wolf, a Sheep or a Firefly: Learning Biology through Constructing and Testing Computational Theories -- an Embodied Modeling Approach. Cognition & Instruction, 24(2), pp. 171-209. http://ccl.northwestern.edu/papers/wolfsheep.pdf ## HOW TO CITE If you mention this model in a publication, we ask that you include these citations for the model itself and for the NetLogo software: * 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. * 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 1997 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 http://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. This model was created as part of the project: CONNECTED MATHEMATICS: MAKING SENSE OF COMPLEX PHENOMENA THROUGH BUILDING OBJECT-BASED PARALLEL MODELS (OBPML). The project gratefully acknowledges the support of the National Science Foundation (Applications of Advanced Technologies Program) -- grant numbers RED #9552950 and REC #9632612. This model was converted to NetLogo as part of the projects: PARTICIPATORY SIMULATIONS: NETWORK-BASED DESIGN FOR SYSTEMS LEARNING IN CLASSROOMS and/or INTEGRATED SIMULATION AND MODELING ENVIRONMENT. The project gratefully acknowledges the support of the National Science Foundation (REPP & ROLE programs) -- grant numbers REC #9814682 and REC-0126227. 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