The second model, Daisyworld, was invented by James Lovelock (1992), and here the key variable is temperature.
Lovelock JE. 1992. A numerical model for biodiversity. Philosophical Transactions of the Royal Society B 338:383-91.
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Daisyworld is an imaginary planet orbiting a star like the Sun and at the same orbital distance as the Earth. The surface of Daisyworld is fertile earth, sown uniformly with daisy seeds. As is true in our world, the daisies vary in color, and daisies of similar color grow together in patches. As sunshine falls on Daisyworld, the model tracks changes in the growth rate of each variety of daisy, and changes in the amount of the planet's surface covered by different colored daisies.
The simplest version of this model contains only two varieties of daisies, white and black. Black daisies absorb more heat than bare earth, while white daisies reflect sunlight. Consequently, clumps of same-colored daisies create a local microclimate for themselves: slightly warmer (if they are black) and slightly cooler (if white) than the mean temperature of the planet. Both black and white daisies grow fastest, and at the same rate, when their local effective temperature (the temperature within their microclimate) is 22.5°C. They respond identically, with a decline in growth rate, as the temperature deviates from this ideal. As a result, at a given average planetary temperature, black and white daisies experience different microclimates and therefore have different growth rates.
However, as the daisies cover a sufficiently large area of the surface of Daisyworld, their color affects not only their own microclimate, but also the albedo or reflectance of the planet as a whole. Like our own sun, the luminosity of Daisyworld's star has gradually increased. A simulation of life on Daisyworld begins in the past with a cooler sun. This enables the black daisies to spread until they warm the planet. Later on, as the sun grows hotter, the white daisies grow faster than black ones, cooling the planet. So over the history of Daisyworld, the warming sun gradually changes the proportion of white and black daisies, creating the global phenomenon of temperature regulation: the planet's temperature is held near an optimum for – and by – the daisies.
Imagine that a team of astronauts and planners is sent to investigate Daisyworld. They would have plenty of time to study the only living things on the planet, and they would almost certainly conclude that the daisies had evolved to grow best at the normal temperature of the planet, 22.5°C. But this conclusion would invert the actual state of affairs. The daisies did not adapt to the temperature of the planet; instead they adapted the planet to suit themselves. A Daisyworld without daisies would track the increase in the sun's luminance, rather than stabilizing near the ideal temperature for daisies. But the role of the daisies in keeping the planet at a cozy temperature would not be obvious to the newcomers. Only when the sun's luminosity gets too hot for the daisies to control would the daisy's former role in temperature stabilization become apparent.
The Daisyworld model soon became the architypal example of a self-organizing, self-regulating environmental system. Explore it for yourself below.
Questions:
The simplest version of this model contains only two varieties of daisies, white and black. Black daisies absorb more heat than bare earth, while white daisies reflect sunlight. Consequently, clumps of same-colored daisies create a local microclimate for themselves: slightly warmer (if they are black) and slightly cooler (if white) than the mean temperature of the planet. Both black and white daisies grow fastest, and at the same rate, when their local effective temperature (the temperature within their microclimate) is 22.5°C. They respond identically, with a decline in growth rate, as the temperature deviates from this ideal. As a result, at a given average planetary temperature, black and white daisies experience different microclimates and therefore have different growth rates.
However, as the daisies cover a sufficiently large area of the surface of Daisyworld, their color affects not only their own microclimate, but also the albedo or reflectance of the planet as a whole. Like our own sun, the luminosity of Daisyworld's star has gradually increased. A simulation of life on Daisyworld begins in the past with a cooler sun. This enables the black daisies to spread until they warm the planet. Later on, as the sun grows hotter, the white daisies grow faster than black ones, cooling the planet. So over the history of Daisyworld, the warming sun gradually changes the proportion of white and black daisies, creating the global phenomenon of temperature regulation: the planet's temperature is held near an optimum for – and by – the daisies.
Imagine that a team of astronauts and planners is sent to investigate Daisyworld. They would have plenty of time to study the only living things on the planet, and they would almost certainly conclude that the daisies had evolved to grow best at the normal temperature of the planet, 22.5°C. But this conclusion would invert the actual state of affairs. The daisies did not adapt to the temperature of the planet; instead they adapted the planet to suit themselves. A Daisyworld without daisies would track the increase in the sun's luminance, rather than stabilizing near the ideal temperature for daisies. But the role of the daisies in keeping the planet at a cozy temperature would not be obvious to the newcomers. Only when the sun's luminosity gets too hot for the daisies to control would the daisy's former role in temperature stabilization become apparent.
The Daisyworld model soon became the architypal example of a self-organizing, self-regulating environmental system. Explore it for yourself below.
Questions:
- What are the key parameters that change the distribution of black and white daisies?
- Over what solar luminosity range is the system stable?
- What happens when the solar luminosity gets too high?
References:
Lovelock JE. 1992. A numerical model for biodiversity. Philosophical Transactions of the Royal Society B 338:383-91.
Lovelock JE. 1992. A numerical model for biodiversity. Philosophical Transactions of the Royal Society B 338:383-91.