An ecosystem, as a unit of study, must be a bounded system, yet the scale can range from a puddle, to a lake, to a watershed, to a biome. Indeed, ecosystem scale is defined more by the functioning ofthe system than by any checklist of constituent parts, and the scale of analysis should be determined by the problem being addressed. Whereas individuals perish over time and even populations cannot survive indefinitely - none can fix their own energy and process their own wastes - every ecosystem contains the ecological community necessary for sustaining life: primary producers, consumers, and decomposers, and the physical environment for oikos (Figure 1 shows a simple ecosystem model). It is this feature of ecosystems, that they are the basic unit for sustaining life over the long-term, which provides one of the main reasons for studying them for environmental management and conservation. The two main features of the ecosystem, energy flow and nutrient biogeochemical cycling, comprise the major areas of ecosystem ecology research.
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