Systems are said to be stable if their variables return to, or towards, their original states following disturbances. For example, the temperature of a room is stable because, when a door is opened (= an external or exogenous disturbance), the thermostatic control system returns the room to its previous temperature. Similarly, the sycamore aphid population also appears to be stable because, although it fluctuates considerably, it always returns towards its mean density following a disturbance. Hence, stable systems tend to persist in a state of balance in variable environments. They are said to be homeostatic or self-regulated. A necessary condition for stability is the presence of negative feedbacks which cause variables to return towards their original values. Negative feedbacks lead to equilibrium or balance in mechanical and ecological systems; i.e., they regulate or control the system. However, although negative feedback is a necessary condition for stability, it is not a guarantee of stability. To guarantee stability, the negative feedback must act rapidly and gently, otherwise the variables may oscillate to varying degrees around their equilibrium points.

Delays are caused in the action of negative feedback processes when many species are involved in the feedback loop. Time delays in negative feedback loops usually cause periodic oscillations cycles, such as the 9-10 year cycles of the larch budmoth.

Systems are said to be unstable if their variables continue to move away from their original positions following a disturbance. Instability usually results from positive feedback, the force behind population explosions, inflation spirals, and arms races. The human population, for instance, is currently exhibiting unstable dynamics because it is increasing continuously (births continually exceed deaths).

The feedback structure of an ecological system determines its stability properties, and this has a dominant influence on the patterns and regularities we observe in nature. Because of this, it is important that we understand how feedbacks are created, and how they can be manipulated to produce stable, self-sustaining forest ecosystems.

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© 1998 Alan A. Berryman