Living and nonliving systems - weather - constantly interact and adjust to each other.  Again, in a sense, they are like a pump or seesaw and the rules of dynamic equilibrium and dynamic oscillation are effective.  They appear in balance only because we find it difficult to perceive the constant changes that take place between two systems.  We know that the environment [En] can effect living systems.  And, we have seen in our time how living systems can effect the environment - cut all the trees, and clouds that bring rain begin to decrease, and drought begins.  I see the host [H] and parasite [P] as interacting systems for the benefit of each so they can survive against the pressures on an ever-changing environment.   When a host system or its parts cannot maintain order because of decreasing energy, then another system - parasites, pathogens - begin to use the energy at lower amounts to maintain order in their system.  The energy that comes into living systems will be used.  When we think of pumps or seesaws, dynamic equilibrium and dynamic oscillation, and energy to maintain order, the subject becomes very simple and very clear (For more information see Tippett, J. T. and A. L. Shigo. 1981. Barrier zone formation: A mechanism of tree defense against vascular pathogens. IAWA Bull. Vol 2: 163-168).

SOURCE: Page 206 and Page 207 from MODERN ARBORICULTURE by DR SHIGO.