Gaia, and particularly strong Gaia, is not lacking its critics. Of the many objections raised, these seem the most constructive and interesting: Gaia is not new; Daisyworld fails to prove Gaia; it contradicts evolutionary theory; it is not testable. I shall consider these points in turn.
Critics chastised Lovelock for failing to credit James Hutton with being the first to recognize a connection between life and its environment. In fairness to Lovelock, there is no reason why he should have been au fait with Hutton's writings, and he was happy to rectify the omission having had it pointed out. Moreover, the history of relationships between life and the physical environment is far richer and more complicated that perhaps some of critics realize (p. 9).
It is probably true that Daisyworld does not prove the Gaia hypothesis, and that it is misleading to read too much into Daisyworld simulations. Governing equations in simple mathematical models are apt to reflect the subjective judgment of the modeller rather than represent objective reality (Lapenis 2002). The Daisyworld model fails to incorporate the possible adaptation of black daisies to higher temperatures, the evolution of grey daisies or daises of other colours, and so forth. Daisyworld regulates planetary climate because it specifies a fixed set-point, namely, the optimum temperature for daisy growth (Weber 2001). In its original and overtly ecological form, this assumption seems reasonable. However, what happens if daisies can mutate and adapt. Under these circumstances - a Darwinian Daisyworld - the evolving daisies shift their optimal growth temperature to match their local environment and the biological regulation observed in the solely ecological models breaks down (Robertson and Robinson 1998; see also Lansing et al. 1998). Therefore, the model demonstrates how Gaia might work under a particular set of assumptions, but it does not vindicate the Gaia hypothesis. Of course, refinements to Daisyworld allow more realism. Some versions include grey daisies, daisies of several hues, herbivores, and carnivores. These more realistic additions to the Daisyworld's biosphere seem to increase the stability of the system, but the results still fail to prove the Gaia hypothesis. Daisyworld simulations suggest that system-dependent selection acting alone will not perforce bestow ecosystems with negative feedbacks; other homeostatic mechanisms must be going on (Knoll 2003). However, as an allegory, Daisyworld leads to two reasonable conclusions (Knoll 2003). First, when applied at the right scale, which population genetics and physical interactions require being local and not global (Weber 2001), Daisyworld hints that biological variation acts as a partial buffer against externally driven environmental change. Second, it suggests that the buffering capacity has it limits, so that when external environmental forcing crosses a certain threshold, homeostasis fails and the system moves to a new state, as during ice ages.
W. Ford Doolittle (1981) and Richard Dawkins (1986) objected to Gaia because, they argued, Nature cannot think ahead or behave in any kind of purposeful manner - it just behaves. Additionally, they could not accept the Earth as a living entity because it neither reproduces nor evolves in the traditional Darwinian sense. What would Gaia compete with for planetary natural selection to operate - Mars and Venus? In short, global self-regulation could never have evolved - the organism is the unit of selection, not the biosphere. Lovelock came to realize the truth of that fact, but still felt something keeps the Earth habitable (Lovelock 2003). His main purpose in building Daisyworld was to show that Gaia is consistent with natural selection (that it is a Darwinian world), and not as a facsimile of the Earth. Lovelock (2003) boldly countered the Dawkins-Doolittle censure by contending that Gaia theory does not contradict Darwinism, but extends it to include evolutionary biology and evolutionary geology as a single science. He explained that:
In Gaia theory, organisms change their material environment as well as adapt to it. Selection favours the improvers, and the expansion of favourable traits extends local improvement and can make it global. Inevitably there will be extinctions and losers, winners may gain in the short term, but the only long-term beneficiary is life itself. Its persistence for over three billion years in spite of numerous catastrophes, internal or external, lends support to the theory. I have never intended the powerful metaphor of 'the living Earth' more seriously than the metaphor of 'the selfish gene'. I have used it, along with my neologism geophysiology, to draw attention to the similarity between Gaian and physiological regulation.
(Lovelock 2003, 770)
A seemingly damning condemnation of the Gaia hypothesis is that it is not in a form suitable for testing (e.g. Kirchner 1989). Lovelock (2003) objects to this criticism, arguing that Gaia theory is fruitful and makes successful or useful predictions, some of which are confirmed, some still under test, and some still controversial. Confirmed predictions include atmospheric evidence suggesting that Mars is lifeless (at least currently) and the role of biogenic gases in transferring elements from the ocean to land. Predictions under test include the dominant role of methane in Archaean atmospheric chemistry and stable atmospheric oxygen levels for the past 200 million years. Still controversial is the prediction that the present interglacial is an example of a system failure in a physiological sense.
Despite the brickbats thrown at it, the Gaia hypothesis has stimulated a wealth of interdisciplinary research (e.g. Charlson et al. 1987; Schneider and Boston 1991; Schneider et al. 2004). It remains the subject of heated debate (e.g. Kleidon 2002, 2004; Lenton 2002; Lenton and Wilkinson 2003; Volk 1998, 2002, 2003a, 2003b; Kirchner 2002, 2003; Lovelock 2003). While the outcome of this debate is yet inconclusive, it is nevertheless important to note that much of the disagreement is attributable to a difference in perspective, with the planetary standpoint promoted by the Gaia hypothesis sharing many similarities with a viewpoint of non-equilibrium statistical mechanics and maximum entropy production. Recent developments move towards a testable model of biotic interactions with the Earth system.
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