Breadth of the Terms "Ecological Stability" and "Ecological Equilibrium"
The many definitions of ecological stability point to special, as well as many common, aspects. Generally, these definitions are linked to the idea of a system able to maintain its structures and functions in the long term. Sizes of populations of species may fluctuate around a mean value and the range is understood as ecological equilibrium. Changes of species are possible to a limited extent, but not that of life forms or functional groups. However, it is actually the latter that provide functional relations, the many different niches and feed-back mechanisms in a system. Theoretically, stability may be defined as the sum of all system connections.
Stability is a dynamic concept, despite all continuity in the face of changing environmental conditions, as the stability of processes is included. Stable systems react to disturbance and return to the initial stage by (self) regulation. Ecological stability includes cyclic changes, but not successions. A labile system is not able to balance changing external influences. Ultimately, stability may not be equated with constancy, as the latter excludes any change.
According to Schaefer (1992), four basic types of stability may be distinguished which could be applied to phytocenoses:
• constancy (the system is not disturbed and does not change from within);
• cyclicity (the system changes from within, but returns to the initial stage);
• resistance (the system is disturbed, but does not change);
• resilience (the system changes with disturbance, but returns to the initial stage).
These interpretations of stability consider the stress concept and were developed again by Pimm (1984) and Gigon (1984). Whilst stability is often taken as the basic precondition for the functioning of systems, occasionally stability is considered irrelevant (as not existing in any form), because no such order is recognisable in ecosystems (chaos theory).
Closely linked to ecological stability is ecological balance, which does not describe constant relations, but a dynamic system. According to Bick (1993) this implies two aspects. One of these is the export and import of nutrients and water, and the formation and degradation of organic substances in an ecosystem (equilibrium). In agriculture the attempt is made to recreate this balance after harvesting by fertilisation. The others are biocenotic equilibria, species-to-species relations, and those of plants to herbivores or of partners in symbiosis. The (self) regulation of ecosystems is regarded as the most important factor for the maintenance of such dynamic equilibria.
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