# Rabbit-Grass-Carrots

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## WHAT IS IT?

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.

## HOW TO USE IT

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.

## THINGS TO NOTICE

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.

## THINGS TO TRY

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?

## NETLOGO FEATURES

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 ]
```

## RELATED MODELS

Wolf Sheep Predation is another interacting ecosystem with different rules.

## HOW TO CITE

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

For the model itself:

- Wilensky, U. (2001). NetLogo Rabbits Grass Weeds model. http://ccl.northwestern.edu/netlogo/models/RabbitsGrassWeeds. Center for Connected Learning and Computer-Based Modeling, Northwestern University, Evanston, IL.

Please cite the NetLogo software as:

- 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 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.

## Comments and Questions

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 clear-all ask patches [ set pcolor 53 ] ;setup display of grass and carrots grow-grass 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 ] reset-ticks end 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 eat-grass eat-carrot reproduce death ] ask carrots [ carrots-growth ;the carrots grow or dry out lifetime1 ] tick end 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 ] ] ] end to move ;rabbit procedure rt random 50 lt random 50 fd 1 set energy energy - 0.5 ;moving takes some energy to the rabbits end 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 end 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 end 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 ] ] end to death ; rabbit procedure ;; die if you run out of energy if energy < 0 [ die ] end to carrots-growth ;carrots procedure if lifetime > 10 ;carrots grows randomaly (depends on their lifetime) [hatch 1 [lt 45 fd 45]] end 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 end

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

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