Ecosystems have a rather delicate balance of inputs and outputs, but this balance is often not sufficient to avoid instability. Ecosystem diversity generally accompanies its physical stability, and ecosystems seem to have some regulatory or homoeostatic machanisms. An essential feature of such regulatory mechanisms is the process of feedback operating both at the level of the individual and the entire system.
Homoeostasis actually means constancy maintained by negative feedback, and the individuals of any community seem interrelated by means of homoeostatic mechanisms. The best illustration of this mechanism is probably constituted by predator-prey interactions.
The members of any complex community are involved in the cycling of materials and flow of energy. These members are interlocked by feedback loops and are hence adapted to the prevailing conditions. Each species has multiple relations with other species in the community.
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It has been commonly believed that a causal relationship exists between the numbers of prey and their food although it is agreed that oscillations do exist. It is now questionable whether in all cases the numbers of predators merely go up and down with fluctuations in the numbers of the prey.
The progress of organic evolution- leads to increasing complexity of order, with succession being directed towards a highly diverse ecological community in dynamic equilibrium with its environment; this kind of community is termed the climax community. With increase in the energy flux, entropy increases, many homoeostatic devices are lost or destroyed and a simplified community with lesser stability and higher productivity results.
Largely as a consequence of human activities, e.g., through energy dissipation, land, water and air pollution is caused and the ecosystems become loaded with diverse pollutants. They then tend to shift towards an increased entropy status, remote from complex groups of specialized species towards the generalists, remote from species diversity to species monotype, and remote from tight and stable nutrient cycles towards loose and unstable nutrient cycles.
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Increased production of industrial materials frequently leads to acceleration of hydrogeochemical cycles, disturbance of input-output balances, accumulation of toxic substances such as hydrocarbons, metals and gases, over-production or depletion of certain essential substances, and eutrophication.
This simplification of ecosystems involves or results in disordering and disintegration of the biosphere, shortening of food webs, decreases in species diversity and counteraction of forces of natural selection and organic evolution. Ecologists strongly feel that our future concern should be with the fact that most of the energy dissipation by the industrial society eventually causes an undesirable oversimplification of the ecosystem, specifically a curtailment of the food web. Our objective should be to prevent this and to strive for the maintenance of more highly complex natural ecosystems which are not so easily upset by perturbations and man-made catastrophes.
Energy and pollutants affect ecosystems where they may cause more rapid growth of algae or bacteria, damage species, or alter the entire ecological structure. Since an ecosystem is extremely complex, it is a difficult task to predict the environmental effects that a disturbance (e.g., energy, emission, pollutant) will have.
It is here that the use of suitable models can help. With sound ecological knowledge it is possible to extract the features of the ecosystem that are involved in the pollution problem under consideration and to form the basis of the relevant ecological model. As shown the model resulting from this can be used to select the environmental technology best suited for the solution of specific environmental problems, or legislation reducing or eliminating the emission set up.
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Patten (1974) developed a theory of ecosystem based on energy-matter constraints in living systems. Some central features of this theory are:
(1) Ecosystems have a zero state trending tendency, pertinent to stability.
The concept of stability incorporates two ideas: (a) resistance to change, and (b) restoration to the near original state after the change has occurred.
(2) Curtailment of energy and material inputs tends to lead to decay or extinction of ecosystems to zero state; such decay is guaranteed by the second law of thermodynamics (the principle of maximum entropy).
(3) Ecosystems have only one free (unforced) equilibrium, the zero state.
(4) Ecosystems tend to revert to nominal, non-equilibrium dynamics when perturbed by uniformly vanishing disturbances.
(5) Ecosystems are structurally stable.
(6) Ecosystems have only forced steady state.
(7) Ecosystems adapt and evolve in small degree by parameter variation within fixed structure; and
(8) Ecosystems adapt and evolve in large degrees by structure variation. The conclusion drawn by Patten (1974) is that ecosystems are stable because their states are limited and drawn toward zero. However, reaching the zero state is equivalent to ecological extinction which is an adaptive alternative to instability.