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Each ecosystem has spatial and temporal dimensions and perturbation is now considered as an integral component of ecosystem functioning.
All ecosystems are subjected to a variety of perturbations, which contribute towards their resilience and sustainability.
Ecosystem’s response to perturbation results in ecosystem fragility. Some ecosystems are more fragile than others. According to Ramakrishna (2001), the ecosystem of Cherrapunji forest is extremely fragile.
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In a balanced condition, ecosystem functioning is self- regulating and self-sustaining. This dynamic nature of ecosystem is dependent upon a number of factors including flow of energy, cycling of materials and perturbations, both intrinsic and extrinsic. Ecosystem is now recognized as a dynamic concept with structural heterogeneity based on at least four functional compartments or phases (Rolling, 1995).
A rapid ‘release phase’ (Fig. 5.2) consisting of tightly bound resources is replaced by a ‘reorganization phase’ followed by an ‘exploitative phase’, which is gradually transformed into ‘conservation phase’ or the so-called climax phase.
In conservation phase, ecosystem consolidates resources largely through slow growing species that conserve resources. During the slow change (as indicated in the figure by more distance between arrows) from exploitative phase to conservation phase, stability and connectedness increase with accumulation of biomass and nutrient pool. With tightly bound resources in climax phase, a small perturbation may bring about rapid changes, which may lead to release of resources in the biomass, and the disturbed part of the ecosystem then gradually returns to the conservation phase.
But larger perturbations between the exploitative and reorganization phase lead to breakdown of the ecosystem, as indicated by an arrow on the left side of (Fig. 5.2). The control of ecosystem function by nutrient flux and the conditions of the physical environment is called Bottom-up control. The regulation of ecosystem function via trophic interactions is called Top-down control.
The science of control (cybernetics) of ecosystems is quite complex. Both positive and negative feedbacks control ecosystems. De Angelis et al (1986) gave an account of positive feedback in natural ecosystems. For example, species populations grow exponentially until some control is imposed on them from outside. Negative feedback operates at sublevels in ecosystems. For example, when nutrient release exceeds a certain level, negative feedback inhibits further release or when a population attains enormous size, various events are started that control reproduction. Thus, ecosystems are self- regulating entities.
