Comparisons between Microcosms and Mesocosms

Microcosms Smaller, with more replicates

Usually used in the laboratory with greater environmental control

More easily analyzed for test purposes

Often focus on certain components or processes

Mesocosms Larger, with fewer replicates

Often used outdoors with ambient temperature and light conditions Realistic scaling of environmental factors

Give maximum confidence in extrapolating back to large-scale systems Provide greater realism by incorporating more large-scale processes

Source: Adapted from Taub, F. B. 1984. Concepts in Marine Pollution Measurements. H.

H. White (ed.). Sea Grant Publ., University of Maryland, College Park, MD.

Several authors have almost playfully referred to the use of microcosms in ecology as microcosmology, implying a special world view (Beyers and H. T. Odum, 1993; Giesy and E. P. Odum, 1980; Leffler, 1980). Adey (1995) has also hinted at this kind of extensive view by suggesting the term synthetic ecology for the use of microcosms. The issue is one of epistemology, or how we come to gain knowledge, and the suggestion seems to be that microcosms provide a unique, holistic view of nature perhaps by reducing the scale difference between the experimental ecosystem and the human observer. In this way a special insight is conferred on the scientist from use of microcosms or at least it is easier to achieve than when dealing with ecosystems of much greater scale than the human observer.

Perhaps the most important philosophical aspect of the use of microcosms is their relationship to real ecosystems. Are they only models of analogous real systems or are they real systems themselves? Leffler (1980) provided a Venn diagram which shows that microcosms overlap with real systems but also have unique properties (Figure 4.2). Likewise, the real-world systems have unique properties such as disturbance regimes and top predators that are too large to include in even the largest mesocosm. Clearly, there are situations when a microcosm is primarily used as a model of a real system. For example, it is obviously advantageous to test the effect of a potentially toxic chemical on a microcosm and be able to extrapolate to a real ecosystem rather than to test the effect on the real system itself and risk actual environmental impact. When a microcosm is meant to be a model of a particular ecosystem, the design challenge is to create engineered boundary conditions that allow for the microcosm biota to match the analogous real system with some significant degree of overlap in ecological structure and function. While this use may be the most important role of microcosms, there are situations when the microcosm need not model any particular real system, such as their use for studying general ecological phenomena (i.e., succession) or their direct functional use as in wastewater treatment or in life support for remote living conditions. Natural micro-

FIGURE 4.2 Venn diagram of the philosophical bases of microcosmology. (Adapted from Leffler, J. W. 1980. Microcosms in Ecological Research. J. P. Giesy, Jr. (ed.). U.S. Dept. of Energy, Washington, DC.)

cosms, such as phytotelmata (Kitching, 2000; Maguire, 1971), depressions in rock outcrops (Platt and McCormick, 1964), and tide pools (Bovbjerg and Glynn, 1960), demonstrate that systems on the scale of even the smallest microcosm are real systems whose study can yield insights as valid as from any other real-world system. In fact, there may be value in purposefully creating microcosm designs that do not match with any existing real ecosystem in order to study the ability of systems to adapt to new conditions that have never existed previously. In this case the portion of the microcosm set outside the zone of overlap with the real world in Figure 4.2 is of great interest. This sense is somewhat analogous to the use of islands in ecology mentioned earlier. In classic island biogeography, the islands are not necessarily meant to be models of continents but rather natural experiments of different ages, sizes, and distances from continents. Therefore, the position taken in this chapter is that microcosms are real systems themselves, but they may or may not be models of larger ecosystems depending on the nature of the experiment being undertaken. See Shugart (1984) for a similar discussion about the relationship of ecological computer simulation models and real ecosystems, which includes a Venn diagram similar to Figure 4.2.

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