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Daisyworld

The daisyworld model has a short and slightly controversial history, being intimately connected with Gaia Theory. Here we take the system on its merits and attempt in clude all the possible effects reported thus far in the literature.

Background

There have been numerous articles written on Daisyworld over the years, so if the reader is interested in a general background I would suggest reading something written by someone more articulate than I. Typical examples may be found here, here or here, but this is far from exhaustive.

In summary the daisyworld parable consists of a model planet that orbits some distance from an imaginary sun, but is nonetheless similar to our own. We model this world in an incredibly simplified way, firstly we assume that the living things on this planet consists of daisies which appear in one of two types – white or black. If a space on the planet is not populated by on of these endemic species it will be bare. The daisies (often called biota) will grow according to prescribed (replicator) growth equations and both types distribute their "seed" perfectly. The planetary dynamics are simplified by assuming that the total amount of heat absorbed by this world is only a function of the average colour or albedo (white has albedo greater than one half, black has albedo less than one half). White daisies reflect more of the suns energy back into space, black daisies absorb more. The planetary temperature is supposed to equilibrate fast so that it is in balance and that the different patches of coloured daisies transfer heat between each other. The model has no spatial component in its original guise.

This model then exhibits the property of homoeostasis or regulation as predicted by the Gaia hypothesis. More specifically the relative proportions of the black and white daisies adjust so that the overall planetary temperature is maintained at the optimal for growth.

The simplifications in the model are extreme, and many convincing arguments have levelled against it. To date however, the model survives intact and criticisms against it have failed to stick. Indeed most generalisations of model have continued to preserve the central property of homoeostasis, despite even increasing complexity. The model we introduce below is no exception.

Daisyworld models

Non-spatial models

The model for daisyworld was introduced by Andrew Watson and James Lovelock in 1983 and published in the journal Tellus B [1] (there is an unusual story of how it wasn't published in nature). The model is a system of differential equations in a series of variables which describe the situation described above.

System of differential equations describing original daisyworld model

These eight equations can be solved in closed form, the original solution for which was presented by Saunders [2]. This solution demonstrates the homoeostatic property of the daisies. Despite this success there are many critiques of the model, including its failure to include evolution, the possibility of chaotic behaviours and its pure simplicity. Here we shall concentrate on the former the criticisms.

Mentioned in the paper by Saunders and later developed by Robinson and Robertson [4] is the idea that the daisies should, instead of altering there albedo, alter their optimal temperature for growing, which is held fixed, at the same value, for both daisy types. Would it not be more likely that instead of voluntarily giving up space on an overly hot prototype world, the black daisies would simply adapt to the hotter temperatures? This effect was found to destabilised the regulation on daisyworld. Lenton and Lovelock [5] later countered this by arguing that there need to be some limits on this behaviour, life cannot simply smoothly adjust to frozen cells for example – there are necessarily physical bounds on adaptability. Introducing this effect restabilises the world.

Spatial models

An important extension to the basic daisyworld model is the explicit introduction of space, including a spatially dependent temperature field. The model by von Bloh et. al. [3] is the first example of this. Here the replication of daisies is now achieved by stochastic growth and death rules, rather than differential equations. Crucially the model thus developed displays even better regulation than the simple daisyworld model.

A model with two kinds of evolution

The model we have developed is essentially a sum of the component parts mentioned above. We have a fully spatial temperature field with a thermal diffusion and heat capacity. In addition our daisies evolve and mutate on the two-dimension periodic surface of the "planet" with stochastic growth rules. Importantly we allow the daisies to evolve both there albedo and their preferred growth temperature independently. This leads to number of effects, most striking of which is the oscillatory nature of the resultant system. This work is described in a recent publication submitted to the Journal of Theoretical Biology [6].

Daisyworld Simulator Applet

The applet below uses the same underlying code as our simulation runs, but the size of the system, for convenience, has been fixed at 100 sites per edge, so we have a potential population of 10000 daisies. Our ideal temperature is fixed at 22.5 degrees Celsius and the bracketing width of both the growth and bounding functions is set at 17.5 degrees Celsius. The other parameters are:

  • Loop - number of complete iterations of the system between GUI updates.
  • Cycles - number of loops in a run.
  • Insolation - solar drive. 1 indicates it is driven at the ideal growth temperature.
  • Death rate - probability that a live site will die in a given time step.
  • T mutation - maximum amount which the growth temperature can change between generations.
  • A mutation - maximum amount which the albedo can change between generations.

The panels to the right indicate the different view-ports and data one can monitor during a run. The panel at the top gives a continuous readout of the albedo (white), average planetary temperature (red) and the ideal temperature (green). Not that changing the loop number will alter the scale of this axis. Enjoy playing, or you can look at one of our walkthroughs to look at some specific effects.

    Relevant literature

    ^[1] Watson A. J. and Lovelock J. E. (1983)
    Biological Homeostasis of the global environment - the parable of daisyworld
    Tellus B 35 284
    The original daisyworld model.

    ^[2] Saunders P. (1994).
    Evolution without natural selection - further implications of the daisyworld parable.
    Journal of Theoretical Biology 166, 365.
    Exact analysis of Daisyworld.

    ^[3] VonBloh W. , Block A. , Schellnhuber H. J. (1997)
    Self-stabilization of the biosphere under global change: a tutorial geophysiological approach.
    Tellus B 49 249.
    First Spatial Daisyworld.

    ^[4] Robertson D. ,Robinson, J. (1998).
    Darwinian Daisyworld.
    Journal of Theoretical Biology 195, 129.
    Allowing the evolution of optimal temperature.

    ^[5] Lenton T. M. , Lovelock J. E. (2000).
    Daisyworld is Darwinian: Constraints on adaptation are important for planetary self-regulation.
    Journal of Theoretical Biology 206, 109.
    Placing a physically motivated bound on the temperature adaption.

    ^[6] Wood A. J., Ackland G. J., Lenton T. M. (2006)
    Mutation of albedo and growth response produces oscillations in a spatial Daisyworld.
    Journal of Theoretical Biology, 242