In many if not most cases, microcosms are intended to be models of some real ecosystems. In this context the model is usually compared with the real analog ecosystem in order to establish success of the design. If the model matches with the analog, then experiments done with the model can be extrapolated to the real world. Therefore, comparisons between microcosms and real ecosystems are important in the microcosm method; again, Pilson and Nixon (1980) provide useful insights on the issue. They state:
... the issue of correspondence between the living model and nature has emerged as a major question for those doing microcosm research and for those who hope to use the results from microcosms to develop management polices.
This issue of correspondence is a very difficult problem ... . there are no generally agreed upon qualitative or quantitative criteria for success. At this time it is not possible to give a general objective description of a successful microcosm. We do not know which parameters are most important to have in agreement or how similar they must be to be considered in agreement.
And later, they continue:
Although we admit that no microcosm can ever be an exact replica of nature, we still want them to be "not too abnormal."
Many kinds of measurements, such as nutrient concentrations, population densities, species diversity, biomass, or metabolism, can be compared between a microcosm and its analog ecosystem, and the choice of measurements has been subjective in practice. As an example, Figure 4.29 compares litterfall, which is a measure of aboveground net productivity, for mangroves inside Biosphere 2 and in Southwest Florida over the same annual cycle (Finn, 1996). In this particular case litterfall in the microcosm (Biosphere 2) follows a seasonal pattern similar to the real analog (Southwest Florida) though values are sometimes higher in the mesocosm. Gearing (1989) provides many other examples of this kind of exercise in a review of aquatic microcosm research.
The process of developing a realistic microcosm is itself a test of ecological knowledge as noted in the following quotes: ". the task of assembling, maintaining, and predicting the behavior of even moderately complex ecosystems in the laboratory
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