Ecosystems exist as conceptual models in the minds of ecologists. They can be transferred into written form (as a descriptive model), or more commonly nowadays as a series of mathematical algorithms for use in a computer programme. The value of the ecosystem model can be found in the way in which it can mimic the relationships between the living and non-living components of a habitat.

Whereas the ecosystem is a model, the ecosystem components (plants, animals, soil and climate) exist in real life. They can be measured, captured and studied in detail. When treated in this way a detailed ecosystem description can be made but there would be little by way of understanding the means by which the components were interrelated.

The ecosystem concept is an attempt to allow the ecologist a method of studying and hence understanding the relationship between the living and non-living components of a habitat. This is a difficult task and is possible only for relatively simple habitats. Chorley & Kennedy (1971) have suggested that ecosystem study can be made at three different levels:

(a) ‘Black-box’ level in which system inputs and outputs can be identified and measured but the inter-relationships which take place between components are known; this is probably the most common situation in habitat studies.

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(b) ‘Grey-box’ level: some of the connections between components are understood through substantial areas are still poorly understood. This level of understanding can best be seen in habitats which have been constructed (or repaired) by man, e.g., agricultural systems, reforestation, reclaimed land, sand done stabilisation.

(c) ‘White-box level of understanding implies a complete knowledge of the habitat relationships. Habitat behaviour can be predicted with accuracy.

This situation is still uncommon and is confined to the simplest of habitat situations. The work of Teal (1957) and Duvigneand (1971) illustrates the complexity of this level of understanding. Computer models and simulation studies (May, 1974; Horn, 1981), have enabled progressive advances to be made in recent years.