Introduction

Microecosystems or microcosms are relatively small, closed or semi-closed ecosystems used primarily for experimental purposes. As such, they are living tools used by scientists to understand nature. Microcosms literally means "small world," and it is their small size and isolation which make them useful tools for studying larger systems or issues. However, although they are small, as noted by Lawler (1998), microcosms should share enough features with larger, more natural systems so that studying them can provide insight into processes acting at larger scales, or better yet, into general processes acting at most scales. Of course, some processes may operate only at large scales, and big, long-lived organisms may possess qualities that are distinct from those of small organisms (and vice versa). Because large and small organisms differ biologically, it will not be feasible to study some questions using microcosms. However, to the extent that some ecological principles transcend scale, microcosms can be a valuable investigative tool.

Microcosm, as a term, was originally used in ecology as a metaphor to imagine a systems concept (Forbes, 1887; see also Hutchinson, 1964). More recently, Ewel and Hogberg (1995) and Roughgarden (1995) used microcosm as a metaphor for islands, which have been used profitably as experimental units in ecology (Klopfer, 1981; MacArthur and Wilson, 1967). Microcosms are a part of ecological engineering because (1) technical aspects of their creation and operation (often referred to as boundary conditions) require traditional engineering and design, and (2) they are new ecological systems developed for the service of humans.

A large literature exists on the uses of microcosms primarily to develop ecological theory and to test effects of stresses, such as toxic chemicals, on ecosystem structure and function. This literature demonstrates a high degree of creativity in design of experimental systems as surveyed in the book length reviews by Adey and Loveland (1998), Beyers and H. T. Odum (1993), and Giesy (1980). Adey (1995) graphs microcosm-based publications/year from 1950 through 1990, showing a steady increase in literature production over time. Lawler (1998) suggests that production is about 80 microcosm-based publications/year, while Fraser and Keddy (1997) find more than 100 per year for the mid-1990s. Microcosm research covers a tremendous range from gnotobiotic systems composed of a few known species carefully added together (Nixon, 1969; Taub, 1969b) to large mesocosms composed of thousands of species seeded from natural systems, such as Biosphere 2 [see Table 1 in Pilson and Nixon (1980) for an example of the variety of microcosms used in ecological research]. Some are artificially constructed systems kept under controlled environmental conditions while others are simple field enclosures exposed to the natural environment. Philosophies of microcosm use vary across these kinds of

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FIGURE 4.1 Comparisons of time and space scales showing the appropriate dimensions for use of microcosms and mesocosms. (From Cooper, S. D. and L. A. Barmuta. 1993. Freshwater Biomonitoring and Benthic Macroinvertebrates. D. M. Rosenberg and V. H. Resh (eds.). Chapman & Hall, New York. With permission.)

experimental gradients, which makes this a rich and interesting subdiscipline of ecological engineering.

One useful size distinction occurs between microcosms and mesocosms, with microcosms being smaller and mesocosms being larger experimental systems. Although there is no consensus on the size break between microcosms and mesocosms, several ideas have been published. Lasserre (1990) suggests a practical though arbitrary limit of 1 m3 (264 gal) volume to distinguish laboratory-scale microcosms from larger-scale mesocosms used outside the laboratory. Lawler (1998) prefers to base the distinction on the scale of the system being modelled:

Whether the term "microcosm" or "mesocosm" applies should depend on how much the experimental unit is reduced in scale from the system(s) or processes it is meant to represent. A microcosm represents a scale reduction of several orders of magnitude, while a mesocosm represents a reduction of about two orders of magnitude or less ... The distinction between terms is admittedly rough, but I hope it is preferable to an anthropocentric view where a microcosm is anything small on a human scale (smaller than a breadbox?) and mesocosms are somewhat larger.

Cooper and Barmuta (1993) combine time and space scales in a diagrammatic view that portrays overall experimental systems used in ecology (Figure 4.1). Taub (1984) suggests that microcosms and mesocosms serve different purposes and answer different questions in ecology (Table 4.1). Clearly, by their relatively larger size, mesocosms contain greater complexity and exist at different scales of space and time compared with typical laboratory-scale microcosms (Kangas and Adey, 1996; E. P. Odum, 1984). However, both microcosms and mesocosms share the aspects of ecological engineering noted earlier and are treated together in this chapter.

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FIGURE 4.1 Comparisons of time and space scales showing the appropriate dimensions for use of microcosms and mesocosms. (From Cooper, S. D. and L. A. Barmuta. 1993. Freshwater Biomonitoring and Benthic Macroinvertebrates. D. M. Rosenberg and V. H. Resh (eds.). Chapman & Hall, New York. With permission.)

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