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row). Trying to draw complex feedback diagrams with this notation would makes things
very cumbersome, so I ask you to remember that sensors and amplifiers like these are
'hidden' inside of many of the components of the Gaian feedback diagrams that we
will be working with. Life-mediated amplification makes the feedbacks in Gaian models
far more responsive and unpredictable than those of conventional models of the Earth,
which treat life as a simple 'black box' in which little sensing and amplification takes
place.
It is possible to generate very complex systems which include both positive and neg-
ative feedbacks, and this is what scientists charged with making models of future cli-
mate try to do on their supercomputers. Let's look at a more complex model now that
links both positive and negative feedbacks as an illustration
(Figure 12)
.
Figure 12: A tumour as a complex system with both positive and negative feedbacks.
This example makes the point that complex systems consist of veritable spaghetti junc-
tions of relationships between many components. What makes a tumour so potentially
deadly is the positive feedback, which can lead to runaway exponential growth. But
there are two negative feedbacks which could restrain or even remove the tumour if they
are strong enough. If the tumour receives a fixed supply of nutrients from the surround-
ing tissues, more tumour cells will die of starvation whenever the tumour attempts to
grow, thereby reducing the tumour cell population. Likewise, if there is a limited but
constant capacity to remove toxins made by the tumour, then as this grows more of
its own cells will die of self-poisoning, which will reduce the size of the tumour. How
