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There are many definitions for ecosystem. The definition of Christopherson (1997) seems quite workable, but so are quite a few others. Ecosystem definition: An ecosystem is a natural system consisting of all plants, animals and microorganisms (biotic factors) in an area functioning together with all the non-living physical (abiotic) factors of the environment (Christopherson 1997). 

The term ecosystem was coined in 1930 by Roy Clapham, to denote the physical and biological components of an environment considered in relation to each other as a unit. British ecologist Arthur Tansley later refined the term, describing it as the interactive system established between biocoenosis (a group of living creatures) and their biotope (the environment in which they live). Central to the ecosystem concept is the idea that living organisms are continually engaged in a set of relationships with every other element constituting the environment in which they exist. Ecosystems can be bounded and discussed with tremendous variety of scope, and describe any situation where there is relationship between organisms and their environment.

The term ecosystem (a contraction of ecological system) is generally understood as to the entire assemblage of organisms (plant, animal and other living beings—also referred to as a biotic community or biocoenosis) living together in a certain space with their environment (or biotope), functioning as a loose unit. Together, these components and their interactions with and relationships to each other form a dynamic and complex new whole, functioning as an "ecological unit", with additional characteristics that can't be found in the individual components. Nor could any organism live completely on its own without involving any other species of organism.  

There are no conceptual restrictions on how large or small a space or an area must be to host an ecosystem, nor on the minimum numbers species or individual organisms to be present.

Early conceptions of an ecosystem were as a structured functional unit in equilibrium of energy and matter flows among constituent elements. Some considered this vision limited, and preferred to view an ecosystem in terms of cybernetics, which, like any other type of system, is governed by the rules of systems science and cybernetics, as applied specifically to collections of organisms and relevant abiotic components. The branch of ecology that gave rise to this view has become known as systems ecology. 

Politically, the concept has become important, since the Convention on Biological Diversity, (CBD), signed by almost 200 nations. The CBD formulates the concept in the following definition: "Ecosystem" means a dynamic complex of plant, animal and micro-organism communities and their non-living environment interacting as a functional unit" (Convention on the Biological Diversity, 1992).

With the need of protecting ecosystems, the political need arose to describe and identify them within a reasonable time and cost-effectively. The IUCN task force on Protected Areas System Composition and Monitoring (Vreugdenhil et al 2003) argued that this could most effectively be achieved using a physiognomic-ecological classification systems, as they are easily recognizable in the field as well as on satellite images. They argued that the structural characteristics - such as forest, savannah, bush-like, prairie vegetations, seasonality of the vegetation and leaf-morphology - complemented with geophysical data - such as elevation, humidity, drainage, salinity of water, characteristics of water bodies - each are determining modifiers that separate partially distinct sets of species. They argued that this is true not only for plant species, but also for species of animals, fungi and bacteria. The degree of ecosystem distinction is subject to the physiognomic classifiers or modifiers that can be recognized on a satellite image and/or in the field. Based on that principle, they developed a methodology for ecosystem mapping, which you can find here. The principle is that physiognomic ecological vegetation classes also represent ecosystem classes, as those vegetation classes represent ecological conditions with partially distinct assemblages of both plants and animal species. 

Several physiognomic-ecological are available: the UNESCO system: Physiognomic-Ecological Classification of Plant Formations of the Earth, Mueller Dombois and Ellenberg, 1974, and its derivatives, developed by the United States Vegetation Committee USVCS and the FAO developed Land Cover Classification System, LCCS. Several systems aquatic systems are available and an effort is being made by the USGS to design yet another ecosystem classification system that covers both terrestrial and aquatic ecosystems, but it is not clear how that has been progressing. The UNESCO system related classification variants all allow fairly to rather detailed (depending on the use of floristic elements) distinction of biounits with a reasonable degree of geographical consistency. From the previous analysis of modifiers, it may be clear that these classification systems not only provide information that leads to definitions of the vegetation types, but about conditions that determine the suitability of that location to representatives of any taxon - including fauna - particularly when complemented with additional faunal characteristics when appropriate. 

From the previous consideration, it may be deducted that different recombinations of modifiers most likely lead to partial different assemblages of species. Particularly by incorporating an aquatic “formation”, sets or assemblages of ecosystems and species can be be added that were not considered in the original design of the UNESCO classification system. The different ecosystem classification systems and definitions are reviewed here.

Source: The IUCN task force on Protected Areas System Composition and Monitoring (Vreugdenhil et al 2003)

Key words: ecosystem, concept, definition

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