Biomass and Gaia

The arguments I have developed above make a plausible case that life on a planet will be widespread and therefore have a substantial biomass (Chapter 3) and that because of this biomass the products of these organisms' physiology will play a large, and potentially regulating role, in the physiology of the planet (this chapter). It is clear that there is the potential for systems involving life to have a self-regulating character, but what about abiotic systems? The natural nuclear reactors which existed 2 billion years ago at what is now Oklo in Gabon, West Africa are a good illustration of a self-regulating natural system probably not having any significant biological involvement and hence showing that life is not necessary for regulation. These geological nuclear reactors comprised uranium ore in sandstone rocks overlying impermeable granite and were probably naturally moderated by water, if the reactors ran too fast this water turned to steam so reducing its ability to slow the neutrons and prevent them from being absorbed by naturally occurring 238U (Maynard Smith and Szathmary, 1995; Meshik, 2005). This controlled the rate of the nuclear reactions in a manner analogous to that used in nuclear power plants. In this case there is a possibility that bacteria, some of which can sequester uranium, may have been involved in the initial formation of these ore deposits (James Lovelock, pers. comm.); however, even if this was so the likelihood is that the regulation of these reactors was largely abiotic. Since regulation does not require biology, a key question is, are there any theoretical reasons to expect systems involving life to be more likely to be self-regulating than solely abiotic systems?

In the context of the Earth a plausible answer to this question is that Earth systems do appear to have regulated conditions within a life-friendly range over geological time; however, this is accidental, with no reason to expect the same to happen on any other planet with life. Several people raised this possibility in the context of Gaia theory at the end of the 1990s (Lenton, 1998; Watson, 1999; Wilkinson, 1999a); however, it is Andy Watson who has taken these arguments most seriously (Watson, 1999, 2004). The basic idea is easily explained and has much in common with the anthropic principle in astronomy, where the presence of astronomers clearly implies aspects of the nature of the Universe (Carr and Rees, 1979; Hoyle and Wickramasinghe, 1999b). As I have previously written, 'Any planet which is home to organisms as complex as James Lovelock and Lynn Margulis must have had a long period of time during which conditions were always suitable for life, and thus must give the impression of regulation for life-friendly conditions even if the persistence of life was purely a matter of chance' (Wilkinson, 2004b, p. 72).

As long as these arguments are based on a single example—the Earth—it is impossible to rule out an explanation based entirely on chance, however, such an explanation does seem rather unlikely. If life evolves very rarely—or just once—on a planetary surface then it is remarkably unlikely that chance would provide a self-regulating system. If one assembles biospheres in such a random way then it would seem likely that there would be many more ways of producing non-regulating systems than regulating ones. If we assume that life arises on planets reasonably frequently then there is the possibility of a selection process, so that only planets which develop self-regulating systems—even if this is 'by chance'—are likely to survive for long as a living planet. However, it seems likely that the full explanation involves special properties of planetary systems involving life.

The most important, and provocative, part of Gaia theory is that a planet's environment is regulated 'at a habitable state for whatever is the current biota' (Lovelock, 2003, p. 769). Imagine a planet where the climate and chemistry are currently in a suitable state for its prevailing life forms—an easy task as we live on such a planet. If the activities of life were tending to force this system towards an uninhabitable state, for example through the production of some metabolic by-product, then as the planet approached this uninhabitable state these organisms would increasingly struggle and so the forcing would be reduced (an alternative is that natural selection may produce something which consumes the by-product; for example, see Section 3.1 on lignin). These in-built regulating mechanisms, which by definition work to keep the system in life-friendly conditions, are not present with purely abiotic forcings. With the evolution of a significant biomass of more complex organisms the range over which this regulation happens will tend to decrease; for example, the upper temperature limit for prokaryotes is higher than for single-celled eukaryotes which are less temperature sensitive than vascular plants (Table 1.1). This is the significance of the phrase 'whatever the current biota', in Lovelock's definition of Gaia cited above. Just as Grime (1998) argues that the high biomass of dominant species largely controls the behaviour of a plant community—the mass ratio hypothesis—so in a similar manner the dominant life forms (by biomass, rather than by number of species or individuals) may control the properties of a planet's biosphere—at least until the rise of an intelligent species with an industrial economy. These Gaian mechanisms can be thought of as loading the dice in favour of the long-term survival of life once it has started on a planet. In the 'just chance' anthropic view the survival of life is like rolling a 6 with a fair dice (although the probability of 'success' is likely to be much less than 0.17), while Gaian mechanisms load the dice to make a 6 much more likely—although probably not inevitable. Therefore, key questions for a Gaian view of planetary ecology are what are the mechanisms? and how strongly do they load the dice?

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