Daisyworld
Encyclopedia
Daisyworld, a computer simulation
, is a hypothetical world orbit
ing a star
whose radiant energy is slowly increasing. It is meant to mimic important elements of the Earth-Sun system, and was introduced by James Lovelock
and Andrew Watson
in a paper published in 1983 to illustrate the plausibility of the Gaia hypothesis
. In the original 1983 version, Daisyworld is seeded with two varieties of daisy
as its only life forms: black daisies and white daisies. White petaled daisies reflect light
, while black petaled daisies absorb light. The simulation tracks the two daisy populations and the surface temperature of Daisyworld as the sun's rays grow more powerful. The surface temperature of Daisyworld remains almost constant over a broad range of solar output.
mechanisms. Daisyworld examines the energy budget
of a planet populated by two different types of plants, black daisies and white daisies. The colour of the daisies influences the albedo
of the planet such that black daisies absorb light and warm the planet, while white daisies reflect light and cool the planet. Competition between the daisies (based on temperature-effects on growth rates) leads to a balance of populations that tends to favour a planetary temperature close to the optimum for daisy growth. Lovelock and Watson demonstrated the stability of Daisyworld by forcing the sun
that it orbits to evolve along the main sequence
, taking it from low to high solar constant. This perturbation of Daisyworld's receipt of solar radiation caused the balance of daisies to gradually shift from black to white but the planetary temperature was always regulated back to this optimum (except at the extreme ends of solar evolution). This situation is very different from the corresponding abiotic world, where temperature is unregulated and rises linearly with solar output. Later versions of Daisyworld introduced a range of grey daisies and populations of grazers
and predators
, and found that these further increased the stability of the homeostasis
. More recently other research, modelling the real biochemical cycles of Earth, and using various "guilds" of life (e.g. photosynthesis
ers, decomposers
, herbivores and primary and secondary carnivores) has also been shown to produce Daisyworld-like regulation and stability, which helps to explain planetary biological diversity
.
This enables nutrient recycling
within a regulatory framework derived by natural selection
amongst species
, where one being's harmful waste becomes low energy food for members of another guild. This research on the Redfield ratio
of nitrogen to phosphorus shows that local biotic processes can regulate global systems (See Keith Downing & Peter Zvirinsky, The Simulated Evolution of Biochemical Guilds: Reconciling Gaia Theory with Natural Selection).
of the sun's rays increases, germination of black daisies becomes possible. Because black daisies absorb more of the sun's radiant energy, they are able to increase their individual temperatures to healthy levels on the still cool surface of Daisyworld. As a result they thrive and the population soon grows large enough to increase the average surface temperature of Daisyworld.
As the surface heats up, it becomes more habitable for white daisies, whose competing population grows to rival the black daisy population. As the two populations reach equilibrium, so too does the surface temperature of Daisyworld, which settles on a value most comfortable for both populations.
In this first phase of the simulation we see that black daisies have warmed Daisyworld so that it is habitable over a wider range of solar luminosity than would have been possible on a barren, gray planet. This allowed growth of the white daisy population, and the two populations of daisies are now working together to regulate the surface temperature.
The second phase of the simulation documents what happens as the sun's luminosity continues to increase, heating the surface of Daisyworld beyond a comfortable range for the daisies. This temperature increase causes white daisies, who are better able to stay cool because of their high albedo
or ability to reflect sunlight, to gain a selective advantage over the black daisies. White daisies begin replacing black daisies, which has a cooling effect on Daisyworld. The result is that Daisyworld's surface temperature remains habitable - in fact almost constant - even as the luminosity of the sun continues to increase.
In the third phase of the simulation, the sun's rays have grown so powerful that soon even the white daisies can no longer survive. At a certain luminosity their population crashes, and the barren, gray surface of Daisyworld, no longer able to reflect the sun's rays, rapidly heats up.
At this point in the simulation solar luminosity is programmed to decline, retracing its original path to its initial value. Even as it declines to levels that previously supported vast populations of daisies in the third phase, no daisies are able to grow because the surface of barren, gray Daisyworld is still far too hot. Eventually, the sun's rays decrease in power to a more comfortable level which allows white daisies to grow, who begin cooling the planet.
One prediction of the simulation is that the biosphere works to regulate the climate, making it habitable over a wide range of solar luminosity. Many examples of these regulatory systems have been found on Earth.
s, fox
es and other species, led to a surprising finding that the larger the number of species, the greater the improving effects on the entire planet (i.e., the temperature regulation was improved). It also showed that the system was robust and stable even when perturbed. Daisyworld simulations where environmental changes were stable gradually became less diverse over time; in contrast gentle perturbations led to bursts of species richness. These findings lent support to the idea that biodiversity is valuable.
Daisyworld was designed to refute the idea that there was anything inherently mystical about the Gaia hypothesis that Earth's surface displays homeostatic and homeorhetic properties similar to those of a living organism. Specifically, thermoregulation was addressed. The Gaia hypothesis had attracted a substantial amount of criticism from scientists such as Richard Dawkins, who argued that planet-level thermoregulation was impossible without planetary natural selection, which might involve evidence of dead planets that did not thermoregulate. Dr. W. Ford Doolittle rejected the notion of planetary regulation because it seemed to require a "secret consensus" among organisms, thus some sort of inexplicable purpose on a planetary scale. Both neoDarwinians discarded the large body of evidence for planetary regulation based on absence of a mechanism to account for it. Lovelock's model countered such criticism by showing how regulation arises naturally from the assumption of growth within a temperature range. Neither conscious purpose nor natural selection is required to account for the thermoregulation of Daisyworld.
Later criticism of Daisyworld itself centers around the fact that although it is often used as an analogy for Earth, the original simulations leaves out many important details of the true Earth
system. For example, the system requires an ad-hoc death rate (γ) to sustain homeostasis, and it does not take into account the difference between species-level phenomena and individual level phenomena. Detractors of the simulation believed inclusion of these details would cause it to become unstable, and therefore, false. Many of these issues are addressed in a more recent paper by Timothy Lenton and James Lovelock in 2001. In this paper it is shown that inclusion of these factors actually improves Daisyworld's ability to regulate its climate.
Computer simulation
A computer simulation, a computer model, or a computational model is a computer program, or network of computers, that attempts to simulate an abstract model of a particular system...
, is a hypothetical world orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
ing a star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
whose radiant energy is slowly increasing. It is meant to mimic important elements of the Earth-Sun system, and was introduced by James Lovelock
James Lovelock
James Lovelock, CH, CBE, FRS is an independent scientist, environmentalist and futurologist who lives in Devon, England. He is best known for proposing the Gaia hypothesis, which postulates that the biosphere is a self-regulating entity with the capacity to keep our planet healthy by controlling...
and Andrew Watson
Andrew Watson (scientist)
Andrew James Watson FRS is a British marine and atmospheric scientist and an expert in processes affecting atmospheric carbon dioxide and oxygen concentrations...
in a paper published in 1983 to illustrate the plausibility of the Gaia hypothesis
Gaia hypothesis
The Gaia hypothesis, also known as Gaia theory or Gaia principle, proposes that all organisms and their inorganic surroundings on Earth are closely integrated to form a single and self-regulating complex system, maintaining the conditions for life on the planet.The scientific investigation of the...
. In the original 1983 version, Daisyworld is seeded with two varieties of daisy
Asteraceae
The Asteraceae or Compositae , is an exceedingly large and widespread family of vascular plants. The group has more than 22,750 currently accepted species, spread across 1620 genera and 12 subfamilies...
as its only life forms: black daisies and white daisies. White petaled daisies reflect light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
, while black petaled daisies absorb light. The simulation tracks the two daisy populations and the surface temperature of Daisyworld as the sun's rays grow more powerful. The surface temperature of Daisyworld remains almost constant over a broad range of solar output.
A mathematical model to sustain the Gaia hypothesis
The purpose of the model is to demonstrate that feedback mechanisms can evolve from the actions or activities of self-interested organisms, rather than through classic group selectionGroup selection
In evolutionary biology, group selection refers to the idea that alleles can become fixed or spread in a population because of the benefits they bestow on groups, regardless of the alleles' effect on the fitness of individuals within that group....
mechanisms. Daisyworld examines the energy budget
Earth's energy budget
The Earth can be considered as a physical system with an energy budget that includes all gains of incoming energy and all losses of outgoing energy. The planet is approximately in equilibrium, so the sum of the gains is approximately equal to the sum of the losses.Note on accompanying images:...
of a planet populated by two different types of plants, black daisies and white daisies. The colour of the daisies influences the albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
of the planet such that black daisies absorb light and warm the planet, while white daisies reflect light and cool the planet. Competition between the daisies (based on temperature-effects on growth rates) leads to a balance of populations that tends to favour a planetary temperature close to the optimum for daisy growth. Lovelock and Watson demonstrated the stability of Daisyworld by forcing the sun
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
that it orbits to evolve along the main sequence
Main sequence
The main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Hertzsprung and Henry Norris Russell...
, taking it from low to high solar constant. This perturbation of Daisyworld's receipt of solar radiation caused the balance of daisies to gradually shift from black to white but the planetary temperature was always regulated back to this optimum (except at the extreme ends of solar evolution). This situation is very different from the corresponding abiotic world, where temperature is unregulated and rises linearly with solar output. Later versions of Daisyworld introduced a range of grey daisies and populations of grazers
Grazing
Grazing generally describes a type of feeding, in which a herbivore feeds on plants , and also on other multicellular autotrophs...
and predators
Predation
In ecology, predation describes a biological interaction where a predator feeds on its prey . Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of its prey and the eventual absorption of the prey's tissue through consumption...
, and found that these further increased the stability of the homeostasis
Homeostasis
Homeostasis is the property of a system that regulates its internal environment and tends to maintain a stable, constant condition of properties like temperature or pH...
. More recently other research, modelling the real biochemical cycles of Earth, and using various "guilds" of life (e.g. photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...
ers, decomposers
Decomposition
Decomposition is the process by which organic material is broken down into simpler forms of matter. The process is essential for recycling the finite matter that occupies physical space in the biome. Bodies of living organisms begin to decompose shortly after death...
, herbivores and primary and secondary carnivores) has also been shown to produce Daisyworld-like regulation and stability, which helps to explain planetary biological diversity
Biodiversity
Biodiversity is the degree of variation of life forms within a given ecosystem, biome, or an entire planet. Biodiversity is a measure of the health of ecosystems. Biodiversity is in part a function of climate. In terrestrial habitats, tropical regions are typically rich whereas polar regions...
.
This enables nutrient recycling
Recycling
Recycling is processing used materials into new products to prevent waste of potentially useful materials, reduce the consumption of fresh raw materials, reduce energy usage, reduce air pollution and water pollution by reducing the need for "conventional" waste disposal, and lower greenhouse...
within a regulatory framework derived by natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....
amongst species
Species
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
, where one being's harmful waste becomes low energy food for members of another guild. This research on the Redfield ratio
Redfield ratio
Redfield ratio or Redfield stoichiometry is the molecular ratio of carbon, nitrogen and phosphorus in plankton. This empirically developed stoichiometric ratio is found to be C:N:P = 106:16:1. This term is named after the American oceanographer Alfred C. Redfield, who first described this ratio in...
of nitrogen to phosphorus shows that local biotic processes can regulate global systems (See Keith Downing & Peter Zvirinsky, The Simulated Evolution of Biochemical Guilds: Reconciling Gaia Theory with Natural Selection).
Original 1983 simulation synopsis
At the beginning of the simulation, the sun's rays are weak and Daisyworld is too cold to support any life. Its surface is barren, and gray. As the luminosityLuminosity
Luminosity is a measurement of brightness.-In photometry and color imaging:In photometry, luminosity is sometimes incorrectly used to refer to luminance, which is the density of luminous intensity in a given direction. The SI unit for luminance is candela per square metre.The luminosity function...
of the sun's rays increases, germination of black daisies becomes possible. Because black daisies absorb more of the sun's radiant energy, they are able to increase their individual temperatures to healthy levels on the still cool surface of Daisyworld. As a result they thrive and the population soon grows large enough to increase the average surface temperature of Daisyworld.
As the surface heats up, it becomes more habitable for white daisies, whose competing population grows to rival the black daisy population. As the two populations reach equilibrium, so too does the surface temperature of Daisyworld, which settles on a value most comfortable for both populations.
In this first phase of the simulation we see that black daisies have warmed Daisyworld so that it is habitable over a wider range of solar luminosity than would have been possible on a barren, gray planet. This allowed growth of the white daisy population, and the two populations of daisies are now working together to regulate the surface temperature.
The second phase of the simulation documents what happens as the sun's luminosity continues to increase, heating the surface of Daisyworld beyond a comfortable range for the daisies. This temperature increase causes white daisies, who are better able to stay cool because of their high albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
or ability to reflect sunlight, to gain a selective advantage over the black daisies. White daisies begin replacing black daisies, which has a cooling effect on Daisyworld. The result is that Daisyworld's surface temperature remains habitable - in fact almost constant - even as the luminosity of the sun continues to increase.
In the third phase of the simulation, the sun's rays have grown so powerful that soon even the white daisies can no longer survive. At a certain luminosity their population crashes, and the barren, gray surface of Daisyworld, no longer able to reflect the sun's rays, rapidly heats up.
At this point in the simulation solar luminosity is programmed to decline, retracing its original path to its initial value. Even as it declines to levels that previously supported vast populations of daisies in the third phase, no daisies are able to grow because the surface of barren, gray Daisyworld is still far too hot. Eventually, the sun's rays decrease in power to a more comfortable level which allows white daisies to grow, who begin cooling the planet.
Relevance to Earth
Because Daisyworld is so simplistic, having for example, no atmosphere, no animals, only one species of plant life, and only the most basic population growth and death models, it should not be directly compared to Earth. This was stated very clearly by the original authors. Even so, it provided a number of useful predictions of how Earth's biosphere may respond to, for example, human interference. Later adaptations of Daisyworld (discussed below), which added many layers of complexity, still showed the same basic trends of the original model.One prediction of the simulation is that the biosphere works to regulate the climate, making it habitable over a wide range of solar luminosity. Many examples of these regulatory systems have been found on Earth.
Modifications to the original simulation
Later extensions of the Daisyworld simulation which included rabbitRabbit
Rabbits are small mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world...
s, fox
Fox
Fox is a common name for many species of omnivorous mammals belonging to the Canidae family. Foxes are small to medium-sized canids , characterized by possessing a long narrow snout, and a bushy tail .Members of about 37 species are referred to as foxes, of which only 12 species actually belong to...
es and other species, led to a surprising finding that the larger the number of species, the greater the improving effects on the entire planet (i.e., the temperature regulation was improved). It also showed that the system was robust and stable even when perturbed. Daisyworld simulations where environmental changes were stable gradually became less diverse over time; in contrast gentle perturbations led to bursts of species richness. These findings lent support to the idea that biodiversity is valuable.
Daisyworld was designed to refute the idea that there was anything inherently mystical about the Gaia hypothesis that Earth's surface displays homeostatic and homeorhetic properties similar to those of a living organism. Specifically, thermoregulation was addressed. The Gaia hypothesis had attracted a substantial amount of criticism from scientists such as Richard Dawkins, who argued that planet-level thermoregulation was impossible without planetary natural selection, which might involve evidence of dead planets that did not thermoregulate. Dr. W. Ford Doolittle rejected the notion of planetary regulation because it seemed to require a "secret consensus" among organisms, thus some sort of inexplicable purpose on a planetary scale. Both neoDarwinians discarded the large body of evidence for planetary regulation based on absence of a mechanism to account for it. Lovelock's model countered such criticism by showing how regulation arises naturally from the assumption of growth within a temperature range. Neither conscious purpose nor natural selection is required to account for the thermoregulation of Daisyworld.
Later criticism of Daisyworld itself centers around the fact that although it is often used as an analogy for Earth, the original simulations leaves out many important details of the true Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
system. For example, the system requires an ad-hoc death rate (γ) to sustain homeostasis, and it does not take into account the difference between species-level phenomena and individual level phenomena. Detractors of the simulation believed inclusion of these details would cause it to become unstable, and therefore, false. Many of these issues are addressed in a more recent paper by Timothy Lenton and James Lovelock in 2001. In this paper it is shown that inclusion of these factors actually improves Daisyworld's ability to regulate its climate.
External links
- Online DaisyWorld simulator, with many options (Flash)
- Java Applet for daisyworld on a 2D space
- Spatial Daisyworld Model Java Applet and explanation of Daisyworld with evolution
- A Unix/X11 simulation of Daisyworld.
- Daisyworld Flash Animation Student friendly flash animation illustrating the Daisyworld Computer model found at ThinkQuest.org
- Modeling the Gaia Hypothesis: DaisyWorld A test applet of a basic Daisyworld model using a 2D cellular automata.
- Card, O.S., Xenocide (science fiction novel, sequel to Ender's Game and Speaker for the Dead, Tor, Aug 1991)