Ecosystem - Rabbits and Grass

Submitted by librarian on Tue, 2006-08-08 12:56. :: Category

WHAT IS IT?

This project explores a simple ecosystem made up of rabbits and grass. The rabbits wander around randomly, and the grass grows randomly. When a rabbit bumps into some grass, it eats the grass and gains energy. If the rabbit gains enough energy, it reproduces. If it doesn't gain enough energy, it dies.

HOW TO USE IT

Click the SETUP button to set up the rabbits (red) and grass (green). Click the GO button to start the simulation.

The NUMBER slider controls the initial number of rabbits. The HATCH-THRESHOLD slider sets the energy level at which the rabbits reproduce. The GRASS-GROWTH-RATE slider controls the rate at which the grass grows. (Note: If you change the GRASS-GROWTH-RATE in the middle of a simulation, the change will not take effect until the next setup.)

THINGS TO NOTICE

Watch the TOTAL-RABBITS monitor to see how the rabbit population changes over time. At first, there is plenty of grass for the rabbits. The rabbits gain energy and reproduce. The abundance of rabbits leads to a shortage of grass, and soon the rabbit population begins to decline. This allows the grass to grow more freely, providing an abundance of food for the remaining rabbits, and the cycle begins again.

The rabbit population goes through a damped oscillation, eventually stabilizing in a narrow range. The total amount of grass also oscillates, out of phase with the rabbit population.

These dual oscillations are characteristic of predator-prey systems. Such systems are usually described by a set of differential equations known as the Lotka-Volterra equations. StarLogo provides a new way of studying predatory-prey systems and other ecosystems.

EXPLORATIONS

With the current settings, the rabbit population goes through a damped oscillation. By changing the parameters, can you create an undamped oscillation? Or an unstable oscillation?

In the current version, each rabbit has the same hatch-threshold. What would happen if each rabbit had a different hatch-threshold? What if the hatch-threshold of each new rabbit was slightly different from the hatch-threshold of its parent? How would the values for hatch-thresshold evolve over time?

STARLOGO FEATURES

This project uses turtles to "grow" the grass. It would be more "natural" for the patches to grow the grass, with a command like: if (random 1000) = 0 [setpc green] But that command would be much slower, since all of the patches would need to execute the command.

Turtle procedures



	
turtles-own [energy species [rabbit grass] ]
;turtles have an energy variable and can be of species rabbit or species grass (note:
;this could also be accomplished by using Starlogo's breed function.

;The turtle setup procedure. The turltes are moved to random positions, a certain number
;defined by the grass-rate slider are made into "grass" species, the rest are made rabbits
;and are given a random energy between 0-9.
to setup
  setxy random screen-width random screen-height
  ifelse who < grass-rate
   [setc black ht setspecies grass]
   [setcolor blue
    setspecies rabbit
    setenergy random 10]
  repeat 20 [grow]
end

;All grass turtles wiggle forward, and turn the patch ahead of them into grass.
to grow
  if species = grass [  ;only grass turtles should do this
    rt random 10 lt random 10
    fd 1
    if pc-ahead = green [stamp green]  ;grass only grows near other grass
  ]
end

;This combiniation procedure lays out the steps rabbits should follow each turn.
to move
  if species = rabbit [       ;only rabbit turtles should do this
    takestep
    eat-grass
    reproduce
    death
  ]
end

;If a turtle is on a green grass patch, it eats the grass by setting the patch color
;back to brown and increasing its own energy
to eat-grass
  if pc = green [stamp brown setenergy energy + 1]
  if energy > 30 [setenergy  30]
end

;The rabbits wiggle randomly, move forward, and decrease their energy with each step.
to takestep
  rt random 50
  lt random 50
  setenergy energy - .25
  fd 1
end

;When a rabbit's energy level is above the value given by the "hatch-threshold" slider,
;they reproduce and half their energy.
to reproduce
  if energy > (hatch-threshold)
       [setenergy energy * .5
        hatch [seth random 360 jump 1]
       ]
end

;When a rabbit's energy reaches zero, they die.
to death
  if energy < 0 [die]
end

Observer procedures




globals [mincolorgene maxcolorgene rangecolorgene avgcolorgene sdcolorgene temp totalcolorgene]

;The setup procedure prepares the model by clearing the screen and graph,
;painting the patches brown, creating the number of turtles set by the "number" slider,
;creating random patches of grass, and asking the turtles to run their own setup procedure.
to setup
  ca
  clearplot
  setbg brown
  crt number + grass-rate
  ask-patches [if (random 100) < 25 [setpc green]]
  ask-turtles [setup]
  setup-graph
end

;Initializes the graph.
to setup-graph
  pp1 ppreset setppc green
  pp2 ppreset setppc blue
  pp3 ppreset setppc black
  setplot-title "Rabbits and Grass"
  setplot-yrange 0 300
  setplot-xrange 0 50
end

;Creates plots for the number of grass patches (green line) and the number of
;rabbits (blue line)
to graph-it  ;graph the amount of grass (scaled) and turtles
  pp1 ppd plot (count-pc green) / 5
  pp2 ppd plot count-turtles-with [species = rabbit]
end

to total-rabbits
  output count-turtles-with [species = rabbit]
end



to go
 ;the movebutton, grassbutton, and graphbutton are pressed
  startmovebutton
  startgrassbutton
  startgraphbutton
  startextinction
end

to stop-it
  ;the movebutton, grassbutton, and graphbutton are stopped
  stopmovebutton
  stopgrassbutton
  stopgraphbutton
  stopextinction
end

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