Daisyworld

Purpose and impact

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 selection 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 sought to demonstrate the stability of Daisyworld by making its sun 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.

Criticism

Daisyworld was designed to refute the idea that there was something 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.

Wood and colleagues noted in 2008 that a key element in the hypothetical construct of Daisyworld was that the species of focus,

"the daisies alter the same environmental variable (temperature) in the same direction at the local level and the global level. Hence what is selected for at the individual level is directly linked to its global effects. This makes the original model a special case (and it is one that is not particularly prevalent in the real world). Evolutionary biologists often criticize the original model for this reason."

The Gaia hypothesis has otherwise attracted a substantial amount of criticism from scientists, e.g., 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. 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. Others countered the criticism that some "secret consensus" would be required for planetary regulation, suggesting that thermoregulation of a planet beneficial to the two species arises naturally.

Later criticism of Daisyworld centers on the fact that although it is often used as an analogy for Earth, the original simulation leaves out many important details of the true Earth system. For example, the hypothetical system requires an ad-hoc death rate (γ) to sustain homeostasis, and 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 the system to become unstable, making it a false analogy. These criticisms were countered by Timothy Lenton and James Lovelock in 2001, who argued that including further factors can improve climate regulation on later versions of Daisyworld.

Subsequent research

Later versions of Daisyworld, identifying the research area as "tutorial modelling of geosphere–biosphere interactions", introduced a range of grey daisies, as well as populations of grazers and predators, and found that these further increased the stability of the homeostasis.

More recently, other research, modeling real biochemical cycles of Earth, and using various types of organisms (e.g. photosynthesisers, decomposers, herbivores and primary and secondary carnivores) also argues to have produced Daisyworld-like regulation and stability, in support of ideas related to 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. For instance, research on the Redfield ratio of nitrogen to phosphorus suggests that local biotic processes might regulate global systems.

Later extension of the Daisyworld simulations which included rabbits, foxes and other species, led to the proposal that the larger the number of species, the greater thermoregulartory improvement for the entire planet, results suggesting that such a hypothetical system was robust and stable even when perturbed. Daisyworld simulations where environments were stable gradually became less diverse over time; in contrast gentle perturbations led to bursts of species richness, lending support to the idea that biodiversity is valuable.

This finding was supported by a 1994 primary research report on species composition, dynamics, and diversity in successional and native grasslands in Minnesota by David Tilman and John A. Downing, which concluded that "primary productivity in more diverse plant communities is more resistant to, and recovers more fully from, a major drought". They go on to add that their "results support the diversity stability hypothesis but not the alternative hypothesis that most species are functionally redundant".

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.