Daisyworld

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Daisyworld, a computer simulation, is a hypothetical world orbiting a sun whose temperature is slowly increasing in the simulation. Daisyworld was introduced by James Lovelock and Andrew Watson to illustrate the plausibility of the Gaia hypothesis in a paper published in 1983. The simulated planet is seeded with two different species of daisy as its only life form: black daisies and white daisies. White daisies have white flowers which reflect light, and the other species has black flowers that absorb light. Both species have the same growth curve (that is, their reproduction rate is the same function of temperature) but the black daisies are themselves warmer than the white daisies and bare earth. A planet with preponderance of white daisies is cooler than one with more black ones.

At the beginning of the simulation, Daisyworld is so cold that only a few black daisies, and almost no white daises, can survive. Whenever the planet's temperature decreases, the black flowers tend to predominate, they absorb a little heat from the sun, which causes the planet's temperature to rise, allowing a greater proliferation of black daisies, more absorption of heat, and so on. As the planet becomes hotter white daisies begin to breed as well, and eventually the planet reaches a point of temperature equilibrium. Any increase in temperature is combated by a greater proportion of white daisies; any decrease leads to more black daisies. Such a system is remarkably stable against varying solar input; the entire planet maintains homeostasis. Eventually the external temperature becomes too hot for the daisies to oppose, and heat overwhelms the planet.

When the simulation is run without the daisies, the planet's temperature proceeds in sync with that of the sun. With the daisies, at the beginning of the simulation there is enhanced warming, and at the end of the simulation enhanced cooling, resulting in a close to equilibrium temperature for most of the simulation. In this way the daisies are modifying the climate to make conditions more hospitable for themselves. However, the Daisyworld system exhibits hysteresis: for some solar constants, the planet has two distinct stable states, typically no life and one almost 100% occupied with life.

Later extensions of the Daisyworld simulation included rabbits, foxes and other species. One of the more surprising findings of these simulations is that the larger the number of species, the greater the improving effects on the entire planet (i.e., the temperature regulation was improved). These findings lent support to the idea that biodiversity is valuable, and sparked the modern biodiversity debate[citation needed].

Daisyworld has attracted a substantial amount of criticism. It bears scant resemblance to Earth; the system requires an ad-hoc death rate (γ) to sustain homeostasis; and the paper blurs the distinction between species-level phenomena and individual level phenomena. However, Daisyworld arguably demonstrates that biologically mediated homeostasis does not require a teleological explanation.

A version of the Daisyworld simulation was included in the Maxis video game SimEarth.

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