An ecosystem is an assemblage of organisms living together and interacting with each other and their environment. An element of biodiversity and biogeochemical

Case Study: Coral Reef Microcosm Case Study: Florida Everglades Mesocosm Case Study: Biosphere 2 Further Reading time stability is implied, although dimensions are optional, ranging from the biosphere subset, the biome, to perhaps a field or pond. Ecosystems with their complex food webs and biotic physical/chemical relationships are self-organizing due to the genetic information existing in the genome of each species. Even when spatially well bounded, ecosystems are not closed. At the very least, they are subject to energy input and energy and materials exchange with adjacent ecosystems. Often, ecosystems demonstrate biotic exchange with adjacent ecosystems that can be complex and include reproductive and seasonal phases.

The development of an ecosystem in a greenhouse implies that the ecosystem is solar driven, and thus no deeper than the photic zone of the ocean, although the basic principles discussed could generally apply to deep ocean ecosystems. Greenhouse placement generally requires spatial limitation, and scaling of model to analog is necessary for many physical and biotic factors. In some cases, it is intuitive, and in others must be empirically demonstrated by trial and error in comparison with analog function. All of the following case studies demonstrate aspects of this necessary scaling exercise. Normal, biogeochemical, and biotic exchange with adjacent ecosystems must also be simulated.

The reasons for placing or developing ecosystems within greenhouses for research or educational purposes have varied enormously and have ranged from the strongly funded, multiscientist research endeavors, to the classroom aquarium or terrarium. Even the naming of the research field of endeavor has varied widely: to name a few, synthetic ecology, ecological engineering, controlled ecology, closed systems ecology, ecosystem modeling, etc. The systems themselves are called living systems models, microcosms, mesocosms, macrocosms, ecotaria, living machines, closed ecological life support systems (CELSS), etc.

In this article, we limit our discussions to those serious research efforts in which significant effort is expended to match biodiversity, food web and symbiotic relationships, as well as biogeochemical function to analog wild ecosystems. By definition, such systems cannot be closed; however, the known interchanges, biotic and biogeochemical, with adjacent ecosystems must be known, studied in the wild, and be simulated so that the essential functional characteristic of the analog ecosystem can be maintained.

Hundreds, perhaps thousands, of microcosm studies of liter or few liter dimensions of a very limited biodiversity have been undertaken to elucidate component ecosystem function, often related to toxic compound effect. Rarely could these studies be regarded as the modeling of an ecosystem. At the other extreme, perhaps the most complex ecosystem modeling effort ever undertaken was the Biosphere II project in Arizona during the 1980s and 1990s. Biosphere II was an ecologically well-conceived collection of interacting terrestrial marine and freshwater ecosystems. However, it was intentionally operated as a closed system because of its planned space station future. Several decades ago, it was widely regarded among ecologists that even though greenhouse enclosure provided a critical element of control over variables, the difficulties inherent in enclosure and operation were too great to allow ecosystem modeling. As the examples we provide below show, this judgment was only minimally correct and perhaps no more severe than the breakup of wild ecosystems by development or farming expansion. Human expansion and perturbation has severely altered many of the ecosystems on Earth, and has altered all ecosystems in at least minor ways. The entire biosphere has been in effect placed in a poorly operated greenhouse, with the atmosphere serving as its upper 'glass' roof. There can be no valid argument against greenhouse enclosure of ecosystems for research and education. It is simply one end of a complex spectrum of interacting biota and biogeochemistry that we seek to understand. Indeed, in many ways, such model ecosystems may be 'purer' than their wild counterparts.

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