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NetLogo Models Library:
Sample Models/Biology

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Daisyworld

[screen shot]

If you download the NetLogo application, this model is included. (You can also run this model in your browser, but we don't recommend it; details here.)

## WHAT IS IT?

This model explores the "Gaia hypothesis", which considers the Earth as a single, self-regulating system including both living and non-living parts. In particular, this model explores how living organisms both alter and are altered by climate, which is non-living. The example organisms are daisies and the climatic factor considered is temperature.

Daisyworld is a world filled with two different types of daisies: black daisies and white daisies. They differ in albedo, which is how much energy they absorb as heat from sunlight. White daisies have a high surface albedo and thus reflect light and heat, thus cooling the area around them. Black daisies have a low surface albedo and thus absorb light and heat, thus heating the area around them. However, there is only a certain temperature range in which daisies can reproduce; if the temperature around a daisy is outside of this range, the daisy will produce no offspring and eventually die of old age.

When the climate is too cold it is necessary for the black daisies to propagate in order to raise the temperature, and vice versa -- when the climate is too warm, it is necessary for more white daisies to be produced in order to cool the temperature. For a wide range of parameter settings, the temperature and the population of daisies will eventually stabilize. However, it is possible for Daisyworld to get either too hot or too cold, in which case the daisies are not able to bring the temperature back under control and all of the daisies will eventually die.

## HOW IT WORKS

White daisies, black daisies, and open ground (empty patches) each have an albedo or percentage of energy they absorb as heat from sunlight. Sunlight energy can be changed with the SOLAR-LUMINOSITY slider (a value of 1.0 simulates the average solar luminosity of our sun).

Each time step, every patch will calculate the temperature at that spot based on (1) the energy absorbed by the daisy at that patch and (2) the diffusion of 50% of the temperature value at that patch between its neighbors. Open ground patches that are adjacent to a daisy have a probability of sprouting a daisy that is the same color as the neighboring daisy, based on a parabolic probability function that depends on the local temperature (where an optimum temperature of 22.5 yields a maximum probability of 100% of sprouting a new daisy). Daisies age each step of the simulation until they reach a maximum age, at which point they die and the patch they were in becomes open.

## HOW TO USE IT

START-%-WHITES and START-%-BLACKS sets the starting percentage of the patches that will be occupied by daisies (of either color) after pressing SETUP.

Selecting PAINT-DAISIES-AS and pressing PAINT-DAISIES allows the user to draw or erase daisies in the VIEW, by left clicking on patches.

ALBEDO-OF-WHITES and ALBEDO-OF-BLACKS sets the amount of heat absorbed by each of these daisy colors. ALBEDO-OF-SURFACE sets the amount of heat absorbed by an empty patch.

The SOLAR-LUMINOSITY sets the amount of incident energy on each patch from sunlight. But this value only will stay fixed at the user set value if the SCENARIO chooser is set to "maintain current luminosity". Other values of this chooser will change the albedo values. For example "ramp-up-ramp-down" will start the solar luminosity at a low value, then start increasing it to a high value, and then bring it back down again over the course of a model run.

SHOW-TEMP-MAP? shows a color map of temperature at each patch. Light red represents hotter temperatures, and darker red represents colder temperatures.

## THINGS TO NOTICE

Run the simulation. What happens to the daisies? Do the populations ever remain stable? Are there ever population booms and busts? If so, what causes them? (Hint: how do the daisies affects the climate? How does the climate then affect the daisies?)

What happens if boom and bust cycles just keep getting bigger and bigger? The swings can't keep getting bigger forever.

Does the planet ever become completely filled with life, or completely devoid of life?

Try running the simulation without the daisies. What happens to the planet's temperature? How is it different from what happens with the daisies?

Can the Daisyworld system be said to exhibit "hysteresis"? Hysteresis is a property of systems that do not instantly follow the forces applied to them, but react slowly, or do not return completely to their original state. The state of such systems depend on their immediate history.

## THINGS TO TRY

Try running the model with SHOW-DAISIES? off and SHOW-TEMP-MAP? on. You might be able to see interesting spatial patterns that emerge in temperature concentrations and periodic redistricting of temperature regions more easily in this mode.

Try adjusting the fixed temperature diffusion setting in the procedures (change it from 0.5). What happens to the behavior of Daisyworld if temperature is never diffused (set to 0.0)?

## EXTENDING THE MODEL

Black and white daisies represent two extreme types of daisies that could exist in this world. Implement a third species of daisy. You will need to choose what your daisy does and how it is different from black and white daisies. How does your new daisy affect the results of this model?

Sunlight is only one aspect that controls the growth of daisies and other forms of life. Change the model so different parts of the world have different levels of soil quality. How will this affect the outcome?

Many people feel that the Gaia hypothesis can be disturbed by human causes. Implement pollution in the model. Does this cause the daisies to die off quicker or more often?

Can you think of any other ways in which living organisms both alter and are altered by their environment?

## NETLOGO FEATURES

Uses the `diffuse` primitive to distribute heat between patches.

## RELATED MODELS

An alternate Daisyworld model is listed on the [User Community Models](http://ccl.northwestern.edu/netlogo/models/community/) page. It uses patches only, no turtles.

## CREDITS AND REFERENCES

The Daisyworld model was first proposed and implemented by Lovelock and Andrew Watson. The original Gaia hypothesis is due to Lovelock.

Watson, A.J., and J.E. Lovelock, 1983, "Biological homeostasis of the global environment: the parable of Daisyworld", Tellus 35B, 286-289. (The original paper by Watson and Lovelock introducing the Daisyworld model.)

http://www.carleton.edu/departments/geol/DaveSTELLA/Daisyworld/daisyworld_model.htm

## 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:

* Novak, M. and Wilensky, U. (2006). NetLogo Daisyworld model. http://ccl.northwestern.edu/netlogo/models/Daisyworld. 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 2006 Uri Wilensky.

![CC BY-NC-SA 3.0](http://i.creativecommons.org/l/by-nc-sa/3.0/88x31.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.

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