Ecosystem, Concept of and the city provides many desirable cultural institutions, such as museums and symphonies, that are not available in rural areas In summary, cities are essentially parasites on the lowenergy countryside To call a city a parasite is not to belittle it, but to be realistic In undisturbed nature, parasites and hosts tend to coevolve for coexistence; otherwise, if the parasite takes too much from its host, both die if the parasite has only one host Currently humans have only one habitable hostFthe earth Especially threatening to the global life-support ecosystems is the explosive growth of mega-cities in the less-developed nations, caused in part by the increasing dominance of another techno-ecosystem, that of industrialized agriculture, with its often excessive consumption of water and use of toxic and enriching chemicals These systems produce more food products per unit of space, but in turn are prodigious polluters and by their economic might drive small farmers out of business worldwide, forcing them into cities that are unable to assimilate them This current situation illustrates what engineer and former president of MIT Paul Gray (1989) has written: ‘‘A paradox of our time is the mixed blessing of almost every technological development.’’ In other words, technology has its destructive as well as beneficial side To bring the natural and technical ecosystems into a mutualist relationship will be society’s greatest challenge in the twenty-first century 63 See also: Ecosystem Services Energy Flow and Ecosystems Human Impacts on Ecosystems: An Overview References Force JE and Maddie GE (1997) The human ecosystem Society and Natural Resources 10: 369–382 Gray PE (1989) The paradox of technological development In: Technology and the Environment, pp 192–205 Washington, DC: National Academy Press Lotka AJ (1925) Elements of Physical Biology Baltimore: Williams and Wilkins Machlis GE, Force JE, and Birch WR (1997) The human ecosystem as an organizing concept in ecosystem management Society and Natural Resources 10: 347–367 Naveh Z (1982) Landscape ecology as an emerging branch of human ecosystem science Adv Ecol Res 12: 189–237 Odum HW (1936) Southern Regions of the United States Chapel Hill: University of North Carolina Press Odum HW and Moore HE (1938) American Regionalism New York: Henry Holt Sukachev VN (1959) The correlation between the concepts ‘‘forest ecosystem’’ and ‘‘forest biogeocoenose’’ and their importance for classification of forests IX International Botanical Congress 2: 387 Tansley AG (1935) The use and abuse of vegetational concept terms Ecology 16: 284–307