Daisyworld

Abstract: Organisms can greatly affect their environments, and the feedback coupling between organisms and their environments can shape the evolution of both. Beyond these generally accepted facts, the Gaia hypothesis advances three central propositions: (1) that biologically mediated feedbacks contribute to environmental homeostasis, (2) that they make the environment more suitable for life, and (3) that such feedbacks should arise by Darwinian natural selection. These three propositions do not fare well under close scrutiny. (1) Biologically mediated feedbacks are not intrinsically homeostatic. Many of the biological mechanisms that affect global climate are destabilizing, and it is likely that the net effect of biological feedbacks will be to amplify, not dampen, global warming. (2) Nor do biologically mediated feedbacks necessarily enhance the environment, although it will often appear as if this were the case, simply because natural selection will favor organisms that do well in their environments – which means doing wellunder the conditions that they and their co-occurring species have created. (3) Finally, Gaian feedbacks can evolve by natural selection, but so can anti-Gaian feedbacks. Daisyworld models evolve Gaian feedback because they assume that any trait that improves the environment will also give a reproductive advantage to its carriers (over other organisms that share the same environment). In the real world, by contrast, natural selection favors any trait that gives its carriers a reproductive advantage over its non-carriers, whether it improves or degrades the environment (and thereby benefits or hinders its carriers and non-carriers alike). Thus Gaian and anti-Gaian feedbacks are both likely to evolve.

Abstract: We define the Gaia system of life and its environment on Earth, review the status of the Gaia theory, introduce potentially relevant concepts from complexity theory, then try to apply them to Gaia. We consider whether Gaia is a complex adaptive system (CAS) in terms of its behaviour and suggest that the system is self–organizing but does not reside in a critical state. Gaia has supported abundant life for most of the last 3.8 Gyr. Large perturbations have occasionally suppressed life but the system has always recovered without losing the capacity for large–scale free energy capture and recycling of essential elements. To illustrate how complexity theory can help us understand the emergence of planetary–scale order, we present a simple cellular automata (CA) model of the imaginary planet Daisyworld. This exhibits emergent self–regulation as a consequence of feedback coupling between life and its environment. Local spatial interaction, which was absent from the original model, can destabilize the system by generating bifurcation regimes. Variation and natural selection tend to remove this instability. With mutation in the model system, it exhibits self–organizing adaptive behaviour in its response to forcing. We close by suggesting how artificial life (‘Alife’) techniques may enable more comprehensive feasibility tests of Gaia.

Abstract: A numerical model is presented based on Alfred Lotka's notion that the mathematics of evolutionary models would be simpler, not more complex, if the evolution of the organisms and of their physical environment was considered as a single process. The model is concerned with the climate and the community ecology of an imaginary planet, Daisyworld, in orbit around a star like the Sun. The stabilizing influence of environmental feedback is used to enable experiments in community ecology that might otherwise be impossible. The model planetary ecosystem is populated with numerous types of organism distributed in up to three trophic levels. There follows a preliminary report of an exploration of this model planet and an account of its biodiversity and species richness.