Figure 12.6 Negative feedback, stability and instability. a: Gain and phase of the transfer functions are matched properly: deviations from the "desired" value are small. b: Gain of the transfer function is too high (driver over-corrects for deviations): the system goes into an unstable oscillation. c: Phase response of the transfer function is too slow (driver's reaction time is delayed): the system also is unstable.
Figure 12.7 A simple symbiosis consisting of a heterotroph (H), photoautotroph (P), and a carbon sink (S). a: Possible movements of carbon between the heterotroph, the photoautotroph, and the sink. b: The transfer functions governing the symbiosis operate in a feedback loop.
gus). Energy enters in the form of light, which is captured by P and stored in chemical bonds in glucose. The energy in the glucose is transferred to H, which breaks it back down to carbon dioxide and water. The carbon dioxide is then fed back to P to be incorporated again into glucose. Carbon in this system can exist in three energy states. The highest is glucose, boosted there by the capture of light energy. Lower is carbon dioxide, produced by the heterotroph. Lowest of all is a carbon sink, which could be some stable mineral salt of carbon, like calcite. Let us say that neither P nor H can mobilize carbon from this sink—once it is there it is lost to both forever. The cycling of carbon between
P and H therefore form a feedback loop, governed by two transfer functions: for the flow of car bon through the heterotroph and back to the photoautotroph, and for the flow of carbon through the photoautotroph to the heterotroph. There is also a third transfer function, which governs the flow of carbon between the heterotroph and the sink (s).
Is there anything about this association that would make it tend toward homeostasis? Let us follow the flow of energy and matter through this system. The symbiosis is successful only when carbon cycles perpetually between the photoautotroph and the heterotroph. If, for some reason, P or H "drop the ball" and allow the carbon to leave the loop that joins them, the carbon falls into the carbon sink and is lost to both. The symbiosis can survive only if it keeps the carbon "in play." That is to say, it will survive only if there is homeostasis. We can state this more explicitly: successful symbiosis results only if the transfer functions and are matched. If they are not, carbon, and the capacity for work it carries with it, will be lost. Thus, the continued success of the symbiosis depends crucially on the control of one symbiont's transfer function with respect to the other's. This type of operation implies feedback, cooperation, and control. What is truly interesting, though, is that benefits arise only in the context of the assemblage and not in the context of its individual members. There is no intrinsic value to either P or H being individually homeostatic—regu-lating their internal pH, temperature, water balance, whatever. Homeostasis only has value if there is physiological coordination between the members of the assemblage.
Homeostasis, Symbiosis, and Fitness
When homeostasis is viewed in this light, Gaia begins to make just a little more sense. It does not yet make complete sense, though, because the real world is not composed of nice cooperative symbioses. Rather, it is full of organisms that have a vital interest in forcing existing assemblages to "drop the ball"—in short, there is competition in the real world. If Gaia is to be credible, it must somehow explain how dog-eat-dog competition between organisms nevertheless results in the cooperation and coordination of living entities that global homeostasis seems to require.
In fact, it may not be as great a leap as it seems. Let us admit competition into our model symbiosis. Assume again a single photoautotroph, P,but now there are two heterotrophs, HA and HB (Fig. 12.8). The cycling of carbon through this system is now governed by six transfer functions, two each for the carbon flowing between P and each H, and two more for carbon lost to the sink. There now is competition for carbon, but it is only superficially between the two heterotrophs: the real competition is between the two
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