Figure 12.8 Competition between two heterotrophs, Ha and Hb, for carbon cycling through a photoautotroph, P a:The movement of carbon forms two feedback loops. b: The competition is actually between the loops and is only secondarily between the heterotrophs.

Figure 12.8 Competition between two heterotrophs, Ha and Hb, for carbon cycling through a photoautotroph, P a:The movement of carbon forms two feedback loops. b: The competition is actually between the loops and is only secondarily between the heterotrophs.

loops through which carbon can flow. The presence of competition does not negate or otherwise minimize the necessity for coordination between the component members of the loops. There are certainly many possible outcomes to the competition: the extinction of either HA or HB or some stable association of the two. But no matter who the survivors are, all the members of the assemblage still must keep that carbon moving, and the only way they can do that is to coordinate their respective transfer functions.

This line of argument can be extended to a physiological definition of evolutionary fitness, which is crucial if Gaia is to be credible. The conventional theory of natural selection looks upon fitness as the likelihood of passing on a gene. To the physiologist, though, fitness is a matter of energetics. Reproduction is, at root, making copies of yourself, which in turn requires the imposition of orderliness on otherwise inanimate matter. The genes might provide the template, but without the energy, the genes are useless. The rate of reproduction depends directly upon the rate at which this energy can be mobilized—in other words, on the reproductive power. In any collection of self-reproducing or autocatalytic entities, those that work at high power will produce more copies of themselves and be more fit than those that work at lower power.

Now we are in a position to see the real value of ho-meostasis. Let us return to our simple assemblage of one photoautotroph and two competing heterotrophs (Fig. 12.8). Focusing on carbon as I did is actually a bit of legerdemain. What is really powering the assemblage is a flow of electrons: the atoms are important only as electron carriers. For work to be done, the electrons must make a controlled migration from bonds in glucose to lower-energy bonds in carbon dioxide and water. More powerful electron currents will perform more order-producing work than will less powerful currents, but the current will never flow unless something provides electrons that can be boosted into the high-energy bonds in glucose. What provides them are the lower-energy bonds of carbon dioxide and water. Thus, the individual organisms in an assemblage are secondary: an organism has a high level of fitness only if its metabolism is coordinated with that of other members of the symbiosis.

This energetic perspective on fitness offers a natural check to unrestrained competition by organisms and ties fitness to coordination and cooperation. Suppose, for example, HA attempted to maximize its fitness with respect to P,which it could easily do by diverting its carbon dioxide directly to an insoluble mineral like calcite. This move would make selfish sense for HA, but it would break the loop for electron flow and soon HA would no longer be provided with the electron carriers it needs to power its own physiology. Its only hope for survival, in fact, is to cooperate metabolically with P.

Homeostasis and Telesymbiosis

Perhaps you are persuaded by now that a successful symbiosis is a "well-tuned" symbiosis—one in which matter and energy flow between the symbionts with a sort of harmony. But does this model get us to the global physiology of the type posited by Gaia? I would suggest it does. The requirement for "tuning" a symbiosis follows from elementary principles of conservation of mass and energy. Even organisms far removed from one another are subject to these constraints, and it is conceivable that they could engage in a kind of telesymbiosis (literally "symbiosis at a distance"). The challenge for Gaia is to explain how telesymbiosis can work. If telesymbiosis is credible, perhaps a global physiology, one that involves the entire Earth—the oceans, crust, and atmosphere—and not simply the organisms in it, is not so far-fetched an idea.

In fact, many natural ecosystems do exhibit a degree of physiological coordination that might qualify as telesymbiosis. The possibility may be assessed by comparing how readily matter cycles between organisms— is kept "in play"—against how readily it drops into a sink and is re-mobilized. The two can be combined into what Tyler Volk has called the cycling ratio. For carbon, for example, the cycling ratio is about 200 to 1, which means that carbon atoms on average cycle between heterotrophs and photoautotrophs about 199

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