Daisyworld revisited: quantifying biological effects on planetary self-regulation

Daisyworld demonstrates that self-regulation of the global environment can emerge from organisms altering their local environment in different ways. In Daisyworld, natural selection is directly linked to environmental effects such that what is selected for at the individual level is beneficial to the global environment. The model has been modified and extended in many studies that have highlighted the effect of biological processes on system self-regulation. Here we better quantify their effects and present new variants of the model in an attempt to resolve outstanding debates. The results confirm that Daisyworld is a remarkably robust self-regulating system and they offer some general lessons about systems where life has a strong effect on the environment, which we think are relevant to the Earth. As forcing becomes extreme, such systems can exhibit co-existing stable states with and without life (bi-stability), and rapid transitions From one to the other that are difficult to reverse. The growth response of organisms to the environment has a role in determining the range of forcing over which a system can regulate. Density-dependent ecological interactions improve Daisyworld's regulatory properties, although increased inter-species competition destabilises the environment in one interval. Self-regulation is little affected by introducing organisms that "cheat" by not altering their local environment and in so doing gain a growth advantage. Increased variation in an environment-altering trait (albedo) can weaken the negative feedback it provides on the environment. However, random mutation of this trait and subsequent natural selection can generate and extend the range of temperature regulation and improve the system's response to perturbation with time. Internal adaptation of organisms toward prevailing environmental conditions, or to tolerate extremes, can also extend the range of forcing over which life persists.