A more or less periodical spurt in the behavior of a system or a process.
Fluctuations of the main variables are essential for the equilibrium of any system.
There are however two basic, and quite opposed types of fluctuations. In I. PRIGOGINE's words: "Fluctuations on a sufficiently small scale are always damped by the medium. Conversely, once a fluctuation attains a size beyond a critical dimension, it triggers an instability"(1976, p.119).
The first type corresponds to systems in dynamic equilibrium. In such cases, fluctuations happen in a more or less periodic way and with limited amplitude (i.e. tending to damping). Indeed, the main condition of dynamic stability is that no value of any variable should cross any defined critical threshold. Various types of fluctuations may interfere with each others, producing for the observer confusing and quite unpredictable trajectories. However, the global behavior of the system remains regulated or controlled.
The second type of fluctuations is dominated by positive feedback, thus supplied with continuously increasing energy inputs from the environment, sometimes motored by the system itself. In such cases, the system runs away from its homeostatic trajectory, as the fluctuations become ever increasing until a critical threshold is crossed. A discontinuous transition is then triggered through a last infinitesimal increase of the fluctuation. This may lead either to the demise of the system, as megafluctuations can be very destructive, or to a process of bifurcations and dissipative structuration that carry the system towards a higher level of complexity. (PRIGOGINE's "order through fluctuations"). This author observes: "Unlike equilibrium systems, nonequilibrium systems do not have a general prescription, like the EINSTEIN formula, to describe the fluctuations. Nonequilibrium fluctuations are highly specific" (1984, p.58).
An intermediate situation is however possible when the stability domain of the system is complex, consisting of various domains of attraction which correspond to different internal or external conditions. In such cases, the crossing of a local stability threshold implies only a change of domain, while global dynamic stability is sustained. As emphasized by C. HOLLING this type of fluctuations enhances dynamic stability, notably in complex ecosystems, whose general resilience is thus increased (1976, p.81-4).
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Bertalanffy Center for the Study of Systems Science (2020). Title of the entry. In Charles François (Ed.), International Encyclopedia of Systems and Cybernetics (2). Retrieved from www.systemspedia.org/[full/url]
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