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May, 1973). In reference to the problems with this relationship, Paine (1971) spoke of it as "the ecologist's Oedipus complex" in a book review of the first symposium on the topic. Semantics is a factor in developing generalities about diversity and stability, especially in relation to the latter concept. For example, Grimm and Wissel (1997) review the use of stability in ecology and find 163 definitions of 70 different concepts (see also Figure 4.4 in Peters, 1991). Holling's (1973, 1996) classification of two main types of stability seems to be accepted by most ecologists. Resilience is the extent to which a system returns to a previous state after perturbation, while resistance is the extent to which a system maintains itself without change during perturbation. These two concepts cover the ability of an ecosystem to withstand (resistance) and to recover from (resilience) from a perturbation. Other related stability concepts, using analogies from engineering, have been introduced, including strain (Deevey, 1984; Kersting, 1984) and elasticity (Cairns, 1976). Recently, emphasis seems to have shifted away from the diversity-stability relationship toward more general relationships between biodiversity and ecosystem function (Hart et al., 2001; Naeem et al., 1999; Risser, 1995b; Tilman et al., 1997).

Microcosm work has played a role in the quest for a valid relationship between diversity and stability (Hairston et al., 1968; Van Voris et al., 1980 as examples), but the evidence remains inconclusive. This is not to deny the importance of diversity in its own right as an ecological characteristic for describing microcosm designs. While most microcosms support low diversities due to various factors, some designs support higher levels. For example, Small et al. (1998) found 534 species supported by a 5 m2 microcosm of a Caribbean coral reef, with another 30% suspected to be present. The diversity of this microcosm was used as a basis of estimating global diversity of coral reefs, and according to their analysis, the authors suggested that existing global estimates are three times too low!

FIGURE 4.20 The species-area relationship from ecology.

In general a designer has three strategies available to increase diversity in microcosms: (1) continually add species to balance extinctions, (2) add refuges (physical complexity), and (3) increase the size. The first strategy is based on the species equilibrium model of island biogeography mentioned earlier. Some microcosms which rely on flow-through water systems have the potential to continually introduce planktonic species with the turnover of water. However, most microcosm experiments are more static because the seeding of species is stopped at some early point in time. Adding refuges can also increase diversity by providing spaces within the microcosm where inferior competitors or vulnerable prey may escape and be sustained. Early work on microcosms (Gause, 1934; Huffaker, 1958) demonstrated this role for refuges and it contributed to the increase in awareness by ecologists of the importance of spatial heterogeneity in ecosystems (Hastings, 1977, 1978; Roff, 1974; Wiens, 1976). Finally, increasing the size of a microcosm increases the number of species that can be supported according to the species-area relationship (Figure 4.20). This is probably the most robust relationship in ecology in that it has been found to apply to all examples that have been studied. Early work on species-area curves was done for the practical purpose to study the optimal sample areas needed to describe a plant community (Arrhenius, 1921; Gleason, 1922). Later, the relationship was used to describe diversity on islands of different sizes in studies of island biogeography (MacArthur and Wilson, 1967). The relationship is actually a general expression of the organization of ecological communities by combining aspects of the species-abundance relation and the spatial pattern of habitats (H. T. Odum, 1983; Pielou, 1969). McGuinness (1984) reviews the history and alternative forms of the species-area curve. Size or area is always a constraint in microcosm design, but as a rule of thumb, bigger is better in terms of the amount of diversity that can be supported. The largest "microcosm" ever built is Biosphere 2, which covers 1.25 ha (3.0 acres), and is described in the next section.

Closed Microcosms

One of the most interesting microcosm experiments has been the construction of closed systems. These systems are actually open to energy exchange in terms of sunlight as an input and heat as an output but closure applies to all other materials. Thus, a system is seeded with biota, an atmosphere, media, and any other structure and then sealed shut. The microecosystem then self-organizes into a stable system

FIGURE 4.21 Some of Folsome's original microcosms on display at Biosphere 2 in Arizona.

of biotic and abiotic components. These closed systems provide models of the whole biosphere, which makes them of special interest (Jones 1996).

The first modern closed microcosms were constructed and studied by Clair Folsome and his students and collaborators (Folsome and Hansen, 1986). Folsome had the title of Director of the Exobiology Laboratory at the University of Hawaii. He was a microbiologist with an interest in the origin of life (Folsome, 1979) and a commitment to microcosm research. His systems consisted of vials or flasks which were filled with defined media and seeded with either gnotobiotic assemblages or mixed cultures from the environment, including bacteria, algae, other microbes, and at least one metazoan or nonmicrobe, the crustacean Halocaridina rubra. Folsome started enclosing microcosms in 1967 and most of the research on them was published in the journal BioSystems (Kearns and Folsome, 1981; Obenhuber and Folsome, 1984, 1988; Takano et al., 1983; Wright et al., 1985). Some of Folsome's oldest systems have maintained microbial activity for more than 30 years, demonstrating that self-organization can result in stable systems (Figure 4.21).

More recently, interest in global change as a result of the buildup of CO2 and other atmospheric changes due to human activity has led to renewed research on closed systems. Most of this work consists of short-term studies of the effects of elevated CO2 levels. For example, Korner and Arnone (1992) built small (17 m3 or 600 ft3) closed greenhouses with tropical plant communities to study the effects of CO2 on various measures of system structure and function. Their microcosms were run for 3 months after an initial stabilization period of 1 month, and they developed many patterns similar to tropical forest communities, such as representative light extinction curves. Much more of this line of research can be expected with several national initiatives focused on understanding expected global changes.

The great challenge is the design of a closed system that contains a human. Some details have been worked out for living underwater (Miller and Koblick, 1995), but the challenge remains for manned space flight and the long-term occupation of extraterrestrial environments such as space stations. For this purpose a life support

Mass of Total Regeneration -System

Mass of Air Regeneration -System

Mass of H2O Regeneration -System

Mass of Basic System

Mass of Total Regeneration -System

Mass of Air Regeneration -System

Mass of H2O Regeneration -System

Mass of Basic System

Time

FIGURE 4.22 Scaling relationships of life support alternatives. Nonregenerative storage is best when mission time is short and total regenerative systems are best when mission time is long. (Adapted from Myers, J. 1963. American Biology Teacher 26:409-411.)

Time

FIGURE 4.22 Scaling relationships of life support alternatives. Nonregenerative storage is best when mission time is short and total regenerative systems are best when mission time is long. (Adapted from Myers, J. 1963. American Biology Teacher 26:409-411.)

system is required that will supply at least the minimum human needs (oxygen, water, and food) and eliminate human wastes (carbon dioxide, urine, feces, and heat). Size and weight are obvious considerations of such a life support system as are many other concerns. Figure 4.22 illustrates different life support alternatives and how the choice between them changes with the mission duration of a space flight. This curve was introduced in the 1960s (Myers, 1963) but remains unquan-tified (Eckart, 1994). Stored supply is the best choice for short durations followed by systems which recycle water at intermediate durations. Water recycling is a technical problem whose engineering was worked out long ago. Only the longest durations will require a totally regenerative system, and this is the design challenge that has not been solved after nearly 50 years of intensive research funded by government agencies interested in space travel. The totally regenerative system must maintain an atmosphere, provide food, and recycle wastes for the human occupants of the life support system. Most agree that this kind of life support system must include biological species that maintain biogeochemical cycles, and therefore these systems have been referred to as bioregenerative.

The history of research on bioregenerative life support systems is fascinating and includes an element of great relevance to ecological engineering, that is, the design of a multispecies microcosm in which a human is a component (and of course the most important) species. This history is outlined here especially because it contains a dialogue between a small group of ecologists who advocated the multi-species approach and the majority of researchers from engineering and physiology who advocated an approach that emphasized a mechanical system with one or a few species. Taub (1974) provides an excellent review of the first two decades of research on bioregenerative life support systems. This early period was a time of creative searching of many lines of design. Table 4.4, derived from Taub's (1974) review,

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