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  One well-known definition is that of E.P. Odum (1969) who defined an ecosystem as \'any unit that includes all of the organisms in a given area interacting with the physical environment so that a flow of energy leads to … exchange of materials between living and non-living parts of the system\'. The term was introduced by A.G. Tansley in 1935, but the idea is much older. It derives from Haeckel\'s use of the term ecology, which itself echoes the ideas of Alexander von Humboldt who wrote on plant geography in 1807 that \'in the great chain of causes and effects no thing and no activity should be regarded in isolation\' (Major, 1969, p. 11). Tansley linked ecology and system as ecosystem.

Identifying an ecosystem is challenging because of overlapping ecosystems and because of the issue of scale, which may range from the micro-organisms within a drop of water, through to the entire planet Earth. In practice, the term is generally reserved for units below the scale of the major world units, the biomes. However, the term has been used outside traditional understandings of ecology, as in Mardh\'s 1991 article, \'The vaginal ecosystem\', a use of the word that is compatible with a definition of ecosystems as being interactions and interdependence between living organisms and their physical, chemical and biological environment, regardless of scale.

An ecosystem refers to a community of organisms interacting with each other and their physical surroundings. Ecosystems may be defined by their dominant features (e.g. tropical rainforest ecosystem), but are more likely to be defined on the basis of scientific and economic interest. Setting boundaries for ecosystems implicitly involves value judgements about what is considered important, and what is considered \'outside\' the ecosystem. Sometimes ecosystems are studied without regard to the influence of humans, but those impacts need to be considered as part of ecosystems in ways other than as \'interference\' or \'disturbance\'. This may be represented through terms such as \'agro-ecosystem\' and \'urban ecosystem\' to describe the major modifications of previous ecosystems and the functioning of new ecosystems respectively.

While it is possible to characterize ecosystems by an inventory of their living and nonliving components, the essential feature of an ecosystem is the dynamic relationship between its components. These relationships cannot be understood from knowledge of the constituent parts. The relationship between component parts of an ecosystem involves the flow of energy which originates from the sun, without which life on earth in its present form would not be possible.

The study of functional relationships in ecosystems has often concentrated on phenomena that can be measured, e.g. energy, water and mineral nutrients (see Likens and Bormann, 1995). The temporal dimensions of the system are also studied, e.g. by tracing changes in population numbers through time. For most species each ecosystem has a carrying capacity which may be a simple number or subject to fluctuation due to seasonal and other factors. This optimum level is where the number of members of a species within the ecosystem does not change significantly. The temporal dimensions of ecosystems also includes the idea of a succession of species, which is said to reach a mature or climax ecosystem when there is no possibility for change from within the ecosystem. However, factors such as climate change (including global warming) and sea level change have ensured that so-called climax ecosystems are subjected to major changes over time, and the theory of climax is no longer treated seriously by many scientific ecologists.

Recognition that these changes have resulted from past human activity (e.g. clearing of forests in the Mediterranean region and in Scotland, followed by the introduction of goats and sheep respectively), and are being increasingly generated by contemporary human activities, has been important for geography. It links with work in human ecology, urban ecology and environmental impact assessment. It links geography with systems theory, systems analysis and environmentalism. Geographers have been important in introducing considerations of culture and political economy to overcome the reductionist tendencies of quantifying nature that is found in some work on ecosystems. Increasingly this work is highlighting the importance of indigenous people as part of the ecosystem (Willems-Braun, 1997).

Ecosystems are gaining increased recognition as part of sustainable development. One way of moving towards sustainability is to maintain or promote \'ecosystem health\'. The \'ecosystem approach\' is being increasingly used in urban and regional planning, and other applied fields. \'Ecosystem diversity\', i.e. differences between groups or communities of different species, also fits in with environmentalism. The idea of a \'web of life\' depicts humans as one component of ecosystems, rather than the final consumer of materials and energy in a linear chain. (PM)

References Likens, G. and Bormann, H. 1995: Biogeochemistry of a forested ecosystem, 2nd edn. New York: Springer-Verlag. Major, J. 1969: Historical development of the ecosystem concept. In G. Van Dyne, ed., The ecosystem concept in natural resource management. New York and London: Academic Press, 9-22. Mardh, P. 1991: The vaginal ecosystem. The American Journal of Obstetrics and Gynaecology 165 (4-2): 1163-8. Odum, E. 1969: The strategy of ecosystem development. Science 164: 262-70. Willems-Braun, B. 1997: Buried epistemologies: the politics of nature in (post)colonial British Columbia. Annals of the Association of American Geographers 87 (1): 3-31.

Suggested Reading McDonnell, M. and Pickett, S., eds, 1993: Humans as components of ecosystems: the ecology of subtle human effects and populated areas. New York: Springer-Verlag. Barkham, J. 1995: Ecosystem management and environmental ethics. In T. O\'Riordan, ed., Environmental science for environmental management. Hanlow: Longman, 80-104.



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