; Script: HunterGatherers.nlogo ; Author: David Hyland-Wood (dwood@itee.uq.edu.au) ; Date: 31 January 2004 ; Modification History: ; 1 February 2004: Turtles expend energy and recover it by eating. ; Patches recover energy with time. ; Turtles die if they don't eat. ; Turtles hunt for food (non-randomly). ; 5 February 2004: Xenophobic turtles (simple avoidance). ; Turtles leave tracks (to allow xenophobic response). ; Hunger can override xenophobia if no other option exists. ; Forests can degrade so badly that they turn into Plains. ; 9 February 2004 Documentation. ; Pri ; 1. TODO: Implement capacity to learn to exploit environements (and forgetfulness) ; 2. TODO: Implement population changes when food surpluses/famine exist ; 3. TODO: Remove DBGs ; 4. TODO: Pull details into procedures ; Nice: Define breed for hunter-gatherers and use (instead of default turtles) ; Nice: Allow environment to be pre-defined ; NB: If amountTurtlesEat/Burn is the same, they can't make up for lost energy. ; Variable Definitions globals [ authorEmailAddress ;; Contact info for bug reports turnTime ;; Number of seconds for a unit time ;; numberOfTurtles ;; Total number of turtles ;; maxTurtleEnergy ;; Amount of energy each turtle has initially and can later attain ;; maxForestGrowthLimit ;; Maximum energy that a Forest patch can attain ;; maxPlainsGrowthLimit ;; Maximum energy that a Plains patch can attain initialForestFoliage ;; Initial energy of a Forest patch initialPlainsFoliage ;; Initial energy of a Plains patch forestDegradationLimit ;; Amount of energy a Forest has to reach to degrade into Plains ;; amountTurtlesEat ;; Amount of energy turtles extract from a patch per unit time (if they can) ;; amountTurtlesBurn ;; Amount of energy turtles use per unit time ;; amountForestsRecover ;; Amount of energy forests recover per unit time ;; amountPlainsRecover ;; Amount of energy plains recover per unit time foliageColors ;; Colors for foliage, a list from lowest to highest energy levels foliageSumForest ;; Total energy stored in Forest patches foliageSumPlains ;; Total energy stored in Plains patches tracksExpireAfter ;; How many turns do turtles tracks stay on patches? ] turtles-own [ myEnergyLevel] ;; myEnergyLevel = A turtle's energy level patches-own [ patchType ;; A patch's type (Forest or Plains) myFoliage ;; A patch's energy level myGrowthLimit ;; How much energy a patch can have tracks ;; Tracks left by a turtle (kept for tracksExpireAfter turns) turtleTracksID ;; The turtle which left the tracks ] to startup ;; Ensure setup is run when the model is opened. This prevents an error setup ;; if the "go" or "step" buttons are pressed before the "setup" button. end to setup ca ;; clear the screen, kill all turtles, reset global variables to 0! ; Global Variable Settings ; CHANGE THESE TO IMPACT THE MODEL set authorEmailAddress "dwood@itee.uq.edu.au" ;; Define the contact details for bug reports set turnTime 0.5 ;; Define the length of time in seconds for one turn ;; set numberOfTurtles 2 ;; Define the number of turtles to use ;; set maxTurtleEnergy 610 ;; Define the initial energy of a turtle ;; set maxForestGrowthLimit 1500 ;; Define the amount of energy a Forest patch may attain ;; set maxPlainsGrowthLimit 200 ;; Define the amount of energy a Plains patch may attain set initialForestFoliage maxForestGrowthLimit ;; Define the initial energy of a Forest patch set initialPlainsFoliage maxPlainsGrowthLimit ;; Define the initial energy of a Plains patch set forestDegradationLimit maxPlainsGrowthLimit / 2 ;; Define how much energy a Forest patch has when it gets permanently degraded ;; set amountTurtlesEat 210 ;; Define how much energy turtles gain from eating per unit time ;; set amountTurtlesBurn 200 ;; Define how much energy turtles lose per unit time ;; set amountForestsRecover 1 ;; Define how much energy forests gain per unit time ;; set amountPlainsRecover 1 ;; Define how much energy plains gain per unit time set foliageColors [ 9 69 68 67 66 65 64 63 ] ;; Define patch colors from lowest to highest energy state set foliageSumForest 0 ;; Initialise energy recorded in Forest patches set foliageSumPlains 0 ;; Initialise energy recorded in Plains patches set tracksExpireAfter 20 ;; Define how long to keep tracks on patches ;; Set up the turtles crt numberOfTurtles ;; Make the turtles ask turtles [ setxy ( random-normal 0 5 ) ( random-normal 0 5 ) set color 90 + random 6 ;; TODO: Change this is a unique list (not green, wraps when needed) set myEnergyLevel maxTurtleEnergy ;; Start with fully charged turtles ] ;; Set up a Forest and Plains ask patches ;; TODO - change amount of foliage to variable based on neighbors? [ set tracks -1 ;; Start with no turtle tracks set turtleTracksID -1 ifelse ( pxcor > -10 ) [ ;; Forest set patchType "Forest" set myGrowthLimit maxForestGrowthLimit set myFoliage initialForestFoliage set pcolor findColor myFoliage ] [ ;; Plains set patchType "Plains" set myGrowthLimit maxPlainsGrowthLimit set myFoliage initialPlainsFoliage set pcolor findColor myFoliage ] ] end to go locals [ myEnergeticNeighbors ;; Semi-cardinal neighbors with the most energy for a given turtle maxLocalFoliage ;; The highest amount of foliage in any neighbor of a turtle energeticPatch ;; A patch with the most energy neighboring a turtle foundSuitablePatch ;; Boolean test to determine whether a turtle has found somewhere to move currentTurtle ;; The current turtle ID, so it may be used in a patch context ] set foliageSumForest 0 ;; Record total energy stored in Forest each turn set foliageSumPlains 0 ;; Record total energy stored in Plains each turn ask turtles [ set currentTurtle who ;; Initialise the current turtle ID set foundSuitablePatch false ;; Initialise tracking of next turtle's new patch ;; Make turtles look for food. ;; Find the most energy in neighboring patches set maxLocalFoliage myFoliage-of (max-one-of neighbors [myFoliage]) while [ foundSuitablePatch = false ] [ ;; Try to randomly choose a high-energy neighbor that has not had recent visitors. set energeticPatch random-one-of neighbors with [ (myFoliage = maxLocalFoliage) and ( (turtleTracksID < 0) or (turtleTracksID = currentTurtle) ) and ( count turtles-here <= 0 ) ] ifelse ( is-patch? energeticPatch ) ;; Don't cause a runtime exception if there is [ ;; no energeticPatch. set foundSuitablePatch true ;; Found a suitable patch! ] ;; end if condition [ ;; If that didn't work, try to find just a neighbor that has not had recent visitors. set energeticPatch random-one-of neighbors with [ (myFoliage != maxLocalFoliage) and ( (turtleTracksID < 0) or (turtleTracksID = currentTurtle) ) and ( count turtles-here <= 0 ) ] ifelse ( is-patch? energeticPatch ) ;; Don't cause a runtime exception if there is [ ;; no energeticPatch. set foundSuitablePatch true ;; Found a suitable patch! ] ;; end if condition [ ;; If that didn't work, try to just find a place to go! ;; TODO: NB: Violates xenophobia when pushed. set energeticPatch random-one-of neighbors with [ ( count turtles-here <= 0 ) ] ifelse ( is-patch? energeticPatch ) ;; Don't cause a runtime exception if there is [ ;; no energeticPatch. set foundSuitablePatch true ;; Found a suitable patch! ] ;; end if condition [ ;; If can't even find a place to go (because we are surrounded by patches ;; containing turtles, a rare case!), just stay put. set energeticPatch patch-here ifelse ( is-patch? energeticPatch ) ;; Don't cause a runtime exception if there is [ ;; no energeticPatch. set foundSuitablePatch true ;; Found a suitable patch! ] ;; end if condition [ ;; This condition should never be reached. user-message "Error! Stopped. Turtle " + who + " could not find an energetic patch." + " This should never happen and is evidence of a serious bug. Please report it to " + authorEmailAddress stop ] ;; end ifelse is-patch? ] ;; end ifelse is-patch? ] ;; end ifelse is-patch? ] ;; end ifelse is-patch? if (foundSuitablePatch = true) [ setxy ( pxcor-of energeticPatch ) ( pycor-of energeticPatch ) ;; Move turtle to new patch set turtleTracksID who ;; Tell the patch who is on it set tracks tracksExpireAfter ;; Start track counter on this patch ] ] ;; end while ;; TODO: Vary amount turtles eat/burn in this area to implement learning. set myEnergyLevel (myEnergyLevel - amountTurtlesBurn) ;; All turtles burn energy every turn ;; Turtles only feed if the patch will support it if ( myFoliage > amountTurtlesEat ) [ set myFoliage (myFoliage - amountTurtlesEat) ;; Patches are eaten... ifelse ( patchType = "Forest" ) ;; Patches aren't allowed to grow this time [ set myFoliage (myFoliage - amountForestsRecover) ] [ set myFoliage (myFoliage - amountPlainsRecover) ] set myEnergyLevel (myEnergyLevel + amountTurtlesEat) ;; Turtles get energy by eating if (myEnergyLevel > maxTurtleEnergy) [ set myEnergyLevel maxTurtleEnergy ] ;; but not too much! ;; TODO: Add population when lots of excess food? ] ;;plot-energy-of-turtle-0 ;; DBG: Plot the energy of turtle 0 if ( myEnergyLevel < 0 ) [ die ] ;; Turtles die if they run out of energy ] ;; end ask turtles ask patches [ if ( ( patchType = "Forest" ) and ( myFoliage <= forestDegradationLimit ) ) [ set patchType "Plains" ] ;; Degrade badly damaged Forest into Plains ifelse ( patchType = "Forest" ) [ set myFoliage (myFoliage + amountForestsRecover) ;; Grow foliage set foliageSumForest foliageSumForest + myFoliage ;; Determine total energy level ] [ set myFoliage (myFoliage + amountPlainsRecover) set foliageSumPlains foliageSumPlains + myFoliage ] if ( myFoliage > maxForestGrowthLimit ) ;; Ensure patches can't overgrow [ set myFoliage maxForestGrowthLimit ] if ( myFoliage > maxPlainsGrowthLimit ) and ( patchType = "Plains" ) [ set myFoliage maxPlainsGrowthLimit ] set pcolor findColor myFoliage ;; Update patch coloring if ( tracks > 0 ) ;; If this patch has tracks... [ set tracks tracks - 1 ;; Expire tracks slowly if ( tracks <= 0 ) [ set turtleTracksID -1 ] ;; Expire turtle ID for tracks when tracks expire ] ] plot-patch-energy-levels-by-type ;; Plot total patch energy levels by type plot-number-of-turtles ;; Plot the total number of turtles wait turnTime ;; Slow progress end to-report findColor [ energyLevel ] ;; Returns a color to match a patch's energy level locals [ index ] ;; Index in the array foliageColors set index ( energyLevel * ( length foliageColors - 1 ) ) / maxForestGrowthLimit ;; NB: Sic (compares to maxForestGrowthLimit as global maximum) report item index foliageColors end to plot-patch-energy-levels-by-type ;; Plot patches' energy levels by type set-current-plot "Patch Energy Levels by Type" set-current-plot-pen "Forest" plot foliageSumForest set-current-plot-pen "Plains" plot foliageSumPlains end to plot-number-of-turtles ;; Plot the total number of turtles set-current-plot "Number of Turtles" set-current-plot-pen "Turtles" plot count turtles end to plot-energy-of-turtle-0 ;; Plot the energy of turtle 0 set-current-plot "Turtle 0" set-current-plot-pen "default" plot myEnergyLevel-of turtle 0 end @#$#@#$#@ GRAPHICS-WINDOW 365 11 726 393 17 17 10.03 1 10 1 1 1 CC-WINDOW 18 310 356 392 Command Center BUTTON 272 11 338 44 Setup setup NIL 1 T OBSERVER T BUTTON 273 50 338 85 Go go T 1 T OBSERVER T PLOT 258 401 491 555 Patch Energy Levels by Type Time Energy 0.0 100.0 0.0 70.0 true false PENS "Forest" 1.0 0 -11352576 true "Plains" 1.0 0 -16777216 true SLIDER 14 10 186 43 numberOfTurtles numberOfTurtles 0 100 4 1 1 NIL BUTTON 274 92 337 125 Step go NIL 1 T OBSERVER T PLOT 17 401 246 554 Number of Turtles Time Turtles 0.0 100.0 0.0 20.0 true false PENS "Turtles" 1.0 0 -16777216 true SLIDER 16 158 218 191 maxForestGrowthLimit maxForestGrowthLimit 0 3000 1500 1 1 NIL SLIDER 14 46 186 79 maxTurtleEnergy maxTurtleEnergy 0 1000 610 1 1 NIL SLIDER 17 234 215 267 maxPlainsGrowthLimit maxPlainsGrowthLimit 0 1500 200 1 1 NIL SLIDER 14 82 186 115 amountTurtlesEat amountTurtlesEat 0 500 210 1 1 NIL SLIDER 15 120 188 153 amountTurtlesBurn amountTurtlesBurn 0 500 200 1 1 NIL SLIDER 17 196 205 229 amountForestsRecover amountForestsRecover 0 20 1 1 1 NIL SLIDER 18 272 196 305 amountPlainsRecover amountPlainsRecover 0 50 1 1 1 NIL @#$#@#$#@ WHAT IS IT? ----------- This model represents a number of hunter-gatherer bands in an environment that represents forest and plains areas. HOW IT WORKS ------------ Turtles (hunter-gatherer bands) burn energy every turn and they try to eat every turn to gain energy. When they eat, they degrade the environment which then recovers at a slower rate. Turtles have a maximum energy limit, above which they stop growing. The amount of foliage in the environment is color-coded. The more green a patch is, the more foliage (energy) is has. Forest areas start very green and Plains start much less green, by default. Each type of environment has a maximum energy limit, above which it stops growing. Similarly, each type of environment has a recovery rate. If a forest is degraded below a certain threshold level, it will change to a plains environment type and therefore not recover as much. Each turtle tries to find a place to go on the next turn by choosing a semi-cardinal patch from their current patch. Turtles will randomly choose one of the patches with the most energy (food). Turtles leave tracks, which expire after a while. Turtles are xenophobic in that they will avoid other turtles' tracks, which can override the search for high-energy patches. If no food can be found elsewhere, hunger overrides xenophobia. HOW TO USE IT ------------- Click the "Setup" button to randomly place some turtles in the environment. Click the "Go" button to run the model continuously and the "Step" button to run it one turn at a time. The number of turtles in the model is set with the numberOfTurtles slider (4 by default). Four turtles is a stable population for the default environment size and variable settings. The remainder of the variables are closely interrelated! Please play with them, but do not be surprised if the model varies wildly once they are changed. It is the relative proportions of these variables more than their individual amplitudes that matters. The maximum amount of energy that a turtle may have is set with the maxTurtleEnergy slider. THINGS TO NOTICE ---------------- Turtles will degrade the environment and the environment will (more slowly) recover. The total energy of the environment will drop until it finds a new equilibrium energy level. Turtles that start on the plains will (by default) not be able to extract enough energy to live. Therefore, their search for food will either end in death by starvation or in a move to the forest. Note that turtles in the forest will tend to stay in the forest. They will appear to "bounce off" of the forest-plain boundary due to the search for patches with available food. If the amountTurtlesEat/Burn are the same, they can't make up for lost energy. Therefore, turtle populations are unstable and will always eventually die. THINGS TO TRY ------------- Increase the number of turtles and run the simulation. If too many turtles try to share the environment, some (or all) of them will die. If the simulation starts with more turtles than the equilibrium level, the environment will degrade rapidly, causing the death of many turtles. Once the excess turtle population has been removed, the environment will recover to a new equilibrium state, defined by a stable energy level over time. That energy level will be proportional to the number of remaining turtles. If the number of turtles is increased drastically (try 100), the environment will degrade so rapidly as to kill all turtles. The environment will eventually recover, although some parts of the forest may have been damaged and only recover to the energy level of plains. Try starting the simulation with turtles on the plains vs in the forest. What happens to the turtles on the plains? By default, turtles cannot live on the plains for long so their search for food either ends in starvation or in a move to the forest. EXTENDING THE MODEL ------------------- Turtles all represent hunter-gatherer bands of the same size population (and have the same maximum energy level). The model could be extended by allowing turtles (bands) to have different population levels (and hence energy requirements). Turtles do not currently have individual members that are "born" or "die". It has yet to be shown whether modeling "generations" will be necessary or useful. Turtles do not currently learn in any way. The model could be extended by allowing turtles (bands) to learn more about their environment over time, which could result in a greater capacity for exploitation. Better exploitation capability could allow, for example, a band to extract more energy from an environment without degrading it as fast. Bands may only be able to exploit a patch if other turtles are not also using it (e.g. "farming" by one turtle may allow other turtles to "steal" from the patch, but not to also "farm"). Turtles may be able to learn about adjacent environments (e.g. forest turtles living near plains) through exposure. This has precidence in the detailed environmental knowledge held by extant hunter-gatherer bands. Turtles may "forget" what they have learned about an environment if they are not exposed to it for a long time. This may be modeled by direct forgetfullness or by losing "institutional memory", as generations pass. Can it be shown that learning through environmental exposure is sufficient to model the dynamics between agricultural and hunter-gatherer societies. NETLOGO FEATURES ---------------- This model uses the construct "random-one-of neighbors with ..." to find semi-cardinal patches near the current turtle with various properties. This construct is used in a cascade of loosening restrictions on turtle movement to implement a turtle's preferences in the hunt for food. Each patch has a variable which determines its environment type. RELATED MODELS -------------- This model was partially inspired by the Schelling's famous Segegration model and should evolve to become a superset of the Cooperation and Wealth Distribution models in order to account for those behaviors. CREDITS AND REFERENCES ---------------------- To refer to this model in academic publications, please use: Hyland-Wood, D. (2004). NetLogo HunterGatherer model. http://www.itee.uq.edu.au/~dwood/NetLogoModels/HunterGatherers. School of Information Technology and Electrical Engineering, University of Queensland, Brisbane, Australia. In other publications, please use: Copyright 2004 by David P. Hyland-Wood. All rights reserved. See http://www.itee.uq.edu.au/~dwood/NetLogoModels/HunterGatherers for terms of use. This model is built in the NetLogo environment: Wilensky, U. 1999. NetLogo. http://ccl.northwestern.edu/netlogo/. Center for Connected Learning and Computer-Based Modeling, Northwestern University. 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