In many ecosystems that have been studied carefully, the networks of interconnected population systems are able to maintain the functional identity of the overall system during times of disturbance, as long as disturbances do not exceed some threshold beyond which a radical reorganization results. Holding the line against disruption is called homeostasis if systems are at equilibrium prior to the insult, or homeorhesis if systems were tracking a developmental trajectory instead of being in true
dynamic equilibrium. Systems consisting of interconnected dynamic subsystems (for example, populations of living organisms that form nodes in interaction networks) can maintain stability through what is called resilience, if they are capable of bouncing back to the original state or homing in again on their previous developmental trajectory. This is like a ping-pong ball being pushed below the surface of water in a tub, then being released to pop back up to the surface again. The rate of such a recovery is a quantitative estimate of the stability of a dynamic system; this is the kind of stability that most ecologists think about when they consider the durability of ecosystems.
Other systems display what is referred to as persistence stability by holding out for a long interval before finally succumbing to disruption, possibly owing to the special adaptations of key components of the system (body armor, spe cial physiologic devices, versatile behavior, use of special structures as domiciles). This is like the strong wooden door of a fort that gives way to the battering ram only after application of repeated, energetic assaults. And still others, having developed an internal network of population systems that includes a kind of defense-in-depth resulting from a modular organization of functional parts around dominant species, show resistance stability by being able to sacrifice subunits but still maintain significant aspects of overall functioning. This is like a very complicated machine or information system that can partially break down but continue to perform its vital functions.
In any case, if the ability of an ecosystem to persist or rebound after disturbance is exceeded, the system degrades and collapses; it finally undergoes ecosystem replacement, as a new system is built up at the same location either from the durable remnants of the previous system (the physiologically robust species, resource general-ists, organisms with resting stages) or from invading species able to exploit or tolerate the new environmental factors.
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