SOC is a phenomenon wherein a system maintains itself in a critical or near-critical state. A classic example is the pattern of collapses in a growing pile of sand. Because information theory suggests that systems in critical states are most amenable to information processing and complexity, self-organized criticality has been proposed as a component of collective behavior in ant colonies, societies, ecosystems, and large-scale evolution. SOC is characterized by events whose size and frequency distributions follow an inverse power law. However, it is often difficult to distinguish genuine cases of SOC from simple cause and effect processes that exhibit similar distributions.
For example, ecosystems might tend toward critical states through the following mechanism. If new species or mutations appear in an ecosystem occasionally, then as the variation in the ecosystem increases over time, so does the probability of forming destabilizing positive feedback loops. Such destabilizing interactions could initiate avalanches of extinctions, and the probable size of such avalanches would be related to the preexisting connectivity of the system. In this way, mutation, migration, and extinction could keep the system near the critical region, as the addition of new variation drives the ecosystem out of subcriticality, while extinction avalanches prevent supercriticality. Proponents of this idea point to extinction events, whose distribution follows an inverse power law, as supporting evidence. However, other explanations of this pattern, such as cometary impacts, are also plausible.
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