Dynamic 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 (= 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 regulated.

Negative feedbacks generally cause variables to return towards their original values and, therefore, they tend to act as stabilizing forces in dynamic systems. Hence, negative feedbacks tend to lead, with time, to equilibrium or balance in mechanical and ecological systems; i.e., they regulate or control the variables. However, although negative feedback is a necessary condition for stability, it is not sufficient to ensure 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 in the action of negative feedback processes are usually caused by the order of the feedback loop (how many dynamic variables are involved in the loop). Hence, it generally takes more time for the signal to pass through long -feedback loops involving many variables and, for this reason, systems with many variables are usually less stable than ones with few variables. Instabilities due to time delays in the feedback structure are usually manifested as cycles of increasing period and amplitude. Hence, the extreme 9-10 year cycles of the larch budmoth are probably due to feedback involving the budmoth and one or more other species in the community, species that are affected by budmoth numbers and which also affect budmoth numbers directly or indirectly.

Dynamic systems are said to be unstable if their variables continue to move away from their original states following a disturbance. The human population, for instance, is currently exhibiting unstable dynamics because it is increasing continuously, as may be the collapsing blue whale population. One of the main causes of instability in dynamics is positive feedback. In contrast to stabilizing negative feedback, +feedback accentuates or amplifies changes in state and is the force behind population explosions, inflation spirals, arms races, and organic evolution. Positive feedback can also create unstable breakpoints or thresholds in dynamic systems.

The feedback structure of dynamic systems determines its stability properties, which, in turn, have a dominant influence on the dynamic patterns and regularities we observe in the system. Because of this, it is important that we understand how feedback loops are created in ecological systems, and how to detect and manipulate those feedback loops to produce stable, self-sustaining systems.

The concepts of stability and instability can be illustrated by reference to a ball resting on different landscapes (see Figure)

A = Stable equilibrium. The ball moves back towards its equilibrium position when disturbed. If the valley is infinitely deep the system is said to be globally stable.

B = Unstable equilibrium. The ball moves away from its equilibrium when disturbed.

C = Neutrally stable equilibrium. The ball stays wherever it is placed.

D = Metastable equilibrium. The ball returns towards equilibrium as long as disturbances are not too large. The equilibrium is not globally stable because once the ball has crossed the unstable equilibrium (the peak), it is unlikely to return to the stable point.

E = Metastable equilibria with multiple stable states. The ball returns towards one or the other equilibrium depending on which side of the unstable equilibrium it is. The system is globally stable.

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