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

The grass and weeds can be adjusted to grow at different rates and give the rabbits differing amounts of energy. The model can be used to explore the competitive advantages of these variables.


Click the SETUP button to setup the rabbits (red), grass (green), and weeds (violet). Click the GO button to start the simulation.

The NUMBER slider controls the initial number of rabbits. The BIRTH-THRESHOLD slider sets the energy level at which the rabbits reproduce. The GRASS-GROWTH-RATE slider controls the rate at which the grass grows. The WEEDS-GROWTH-RATE slider controls the rate at which the weeds grow.

The model's default settings are such that at first the weeds are not present (weeds-grow-rate = 0, weeds-energy = 0). This is so that you can look at the interaction of just rabbits and grass. Once you have done this, you can start to add in the effect of weeds.


Watch the COUNT RABBITS monitor and the POPULATIONS plot to see how the rabbit population changes over time. At first, there is not enough grass for the rabbits, and many rabbits die. But that allows the grass to grow more freely, providing an abundance of food for the remaining rabbits. The rabbits gain energy and reproduce. The abundance of rabbits leads to a shortage of grass, 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. NetLogo provides a new way of studying predatory-prey systems and other ecosystems.


Leaving other parameters alone, change the grass-grow-rate and let the system stabilize again. Would you expect that there would now be more grass? More rabbits?

Change only the birth-threshold of the rabbits. How does this affect the steady-state levels of rabbits and grass?

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 birth-threshold. What would happen if each rabbit had a different birth-threshold? What if the birth-threshold of each new rabbit was slightly different from the birth-threshold of its parent? How would the values for birth-threshold evolve over time?

Now add weeds by making the sliders WEEDS-GROW-RATE the same as GRASS-GROW-RATE and WEEDS-ENERGY the same as GRASS-ENERGY. Notice that the amount of grass and weeds is about the same.

Now make grass and weeds grow at different rates. What happens?

What if the weeds grow at the same rate as grass, but they give less energy to the rabbits when eaten (WEEDS-ENERGY is less than GRASS-ENERGY)?

Think of other ways that two plant species might differ and try them out to see what happens to their relative populations. For example, what if a weed could grow where there was already grass, but grass couldn't grow where there was a weed? What if the rabbits preferred the plant that gave them the most energy?

Run the model for a bit, then suddenly change the birth threshold to zero. What happens?


Notice that every black patch has a random chance of growing grass or weeds each turn, using the rule:

if random-float 1000 < weeds-grow-rate
  [ set pcolor violet ]
if random-float 1000 < grass-grow-rate
  [ set pcolor green ]


Wolf Sheep Predation is another interacting ecosystem with different rules.


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

For the model itself:

Please cite the NetLogo software as:


Copyright 2001 Uri Wilensky.


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.

This model was created 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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Click to Run Model

globals [birth-threshold]

breed [rabbits rabbit]                        ;create rabbits breed, owns energy
rabbits-own [ energy ]

breed [carrots carrot]                        ;create carrots breed, owns lifetime
carrots-own [ lifetime ]

to setup
  ask patches [ set pcolor 53 ]               ;setup display of grass and carrots
  set birth-threshold 10
  set-default-shape rabbits "rabbit1"
  create-rabbits מספר-ארנבים-התחלתי           ;create rabbits by the user choise button
    set color white
    set size 3
    setxy random-xcor random-ycor
    set energy random 10                      ;start with a random amt. of energy for each rabbit
  set-default-shape carrots "carrot"
  create-carrots קצב-גידול-גזרים                ;create carrotss by the user choise button
    set color orange
    set size 3
    set lifetime random 7
    setxy random-xcor random-ycor

to go
  if not any? rabbits [ stop ]               ;if there are no rabbits, stop the model
  grow-grass                                 ;grass keep growing as long as it get eaten by rabbits
  ask rabbits
    move                                     ;the rabbits move, eat grass and carrots, reproduce and loose energy because of reproduction or movement
  ask carrots
    carrots-growth                           ;the carrots grow or dry out

to grow-grass
  ask patches [
    if pcolor = 53 or pcolor = brown[        ;grow grass on the setup defaule color or on brown patch (mud)
      if random-float 1000 < קצב-גידול-דשא    ;the grass grow back as a function of user choise
        [ set pcolor green ]
  ] ]

to move      ;rabbit procedure
  rt random 50
  lt random 50
  fd 1
  set energy energy - 0.5                   ;moving takes some energy to the rabbits

to eat-grass  ;rabbit procedure
  if pcolor = green
  [ set pcolor brown
    set energy energy + דשא-אנרגיה ]         ;gain energy by eating grass as a function of user choise

to eat-carrot  ;rabbit procedure
  let prey one-of carrots-here
  if prey != nobody  [
    ask prey [ die ]
    set energy energy + גזר-אנרגיה]           ;gain energy by eating carrots as a function of user choise

to reproduce    ;rabbit procedure
  if energy > birth-threshold
    [ set energy energy / 2                 ;give birth to a new rabbit, but it takes lots of energy
      hatch 1 [ fd 1 ] ]

to death       ; rabbit procedure
  ;; die if you run out of energy
  if energy < 0 [ die ]

to carrots-growth  ;carrots procedure
  if lifetime > 10                          ;carrots grows randomaly (depends on their lifetime)
  [hatch 1 [lt 45 fd 45]]

to lifetime1      ;carrots procedure
  set lifetime lifetime + 1
  if lifetime > 12 [ die ] ; if carrot pass 12 he gets dry and "die", otherwise he gets eaten by a rabbit

There is only one version of this model, created about 3 years ago by bosmat bar.

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