Kangas - Ecological Engineering - Principles and Practice - Chapter 4 doc

117 4 Microcosmology INTRODUCTION Microecosystems or microcosms are relatively small, closed or semi-closed ecosys- tems 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 engineer- ing 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 ecolog- ical 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 118 Ecological Engineering: Principles and Practice 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 meso- cosms, several ideas have been published. Lasserre (1990) suggests a practical though arbitrary limit of 1 m 3 (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. 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.) Natural system Whole system Mesocosm 10 10 Century 10 9 Decade 10 8 Year 10 7 Month 10 6 Week 10 5 Day 10 4 Hour 10 −2 10 0 10 2 Volume (m 3 ) Time (s) 10 4 10 6 10 8 10 10 10 12 Microcosm Microcosmology 119 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 dis- turbance 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- TABLE 4.1 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. 120 Ecological Engineering: Principles and Practice 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. STRATEGY OF THE CHAPTER This chapter reviews the uses of microcosms and mesocosms as experimental eco- systems. Numerous excellent reviews have been published on this topic, and many are cited for further reading throughout the text. An effort is made to focus on elements of relevance to both the engineering side and the ecological side of appli- cations. In relation to engineering, design aspects of microcosms are covered, includ- ing scaling, energy signatures, and complexity. The controversy between ecologists and engineers over the role of microcosms in research on space travel life support systems is given special attention as a case study in ecological engineering. In relation to ecology, aspects of the new systems that have emerged from microcosm 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.) Real world Microcosm Microcosmology 121 research are highlighted. The new qualities show up in (1) examples of micrcocosm replication, and (2) when microcosms are compared with real analog ecosystems. MICROCOSMS FOR DEVELOPING ECOLOGICAL THEORY Microcosms have a long tradition of use for developing theories about most of the hierarchical levels covered by ecology: organism, population, community, and eco- system. While some of this work has been descriptive, most has relied on experi- ments. In the experimental approach, replicate microcosms are developed and par- titioned into groups with some being held as controls and others being treated in some fashion. The experiment is analyzed by statistically comparing the control group with the treated group(s) after a given period of time. Such an experiment can be a challenge to carry out in nature due to the difficulty in establishing replicates and the difficulty in changing only one factor per treatment group. On the other hand, it is easy to carry out this kind of controlled experiment with microcosms, which allows them to be used as valuable tools in ecology. The earliest microcosm work was done on species change during succession of microbial communities (Eddy, 1928; Woodruff, 1912), but most research using microcosms dates after the 1950s. Uses of microcosms for developing ecological theory generally fall into two groups: one in which the ecosystem itself is of interest (ecosystem scale) and the other in which the ecosystem provides a background context and population dynamics or interactions between species are of interest (community or population scale). In both cases, microcosms often are used in a complementary fashion with basic field studies and mathematical models as part of an overall research strategy. Many of the important figures in modern ecology used microcosms in early studies of ecosystems including Margalef (1967), Whittaker (1961), and H. T. Odum (Armstrong and H. T. Odum, 1964; H. T. Odum and Hoskin, 1957; H. T. Odum et al., 1963a). Robert Beyers, H. T. Odum’s first doctoral student, also was an early proponent of microcosms (1963a, 1963b, 1964) and, together with H. T. Odum, co- authored probably the most comprehensive text on the subject (Beyers and H. T. Odum, 1993). The early studies outlined the basic processes of energy flow (primary production and community respiration) and biogeochemistry (nutrient cycling), which are the foundations of ecosystem science today. One example of the contri- bution of microcosms to ecosystem science can be seen in papers by E. P. Odum and his associates on succession (Cooke, 1967, 1968; Gordon et al., 1969). These papers described ecosystem development under both autotrophic (initial conditions of high nutrients and low biomass) and heterotrophic (initial conditions of low nutrients and high biomass) pathways in laboratory microcosms. These studies directly contributed to E. P. Odum’s development of a tabular model of ecological succession (see Chapter 5) as can be seen by comparing their summary tables [Table 2 in Cooke (1967) and Table 12 in Gordon et al. (1967)] to E. P. Odum’s tabular model [Table 1 in E. P. Odum (1969) and Table 9.1 in E. P. Odum (1971)]. E. P. Odum’s model compares trends expected through succession for 24 ecosystem 122 Ecological Engineering: Principles and Practice attributes and is an intellectual benchmark in the synthesis of ecosystem science. E. P. Odum (1971) also used data from Cooke’s (1967) work to illustrate the generality of certain metabolic patterns of succession by comparing small-scale microcosm results with field-scale results (Figure 4.3). This figure is particularly interesting in showing a kind of self-similarity or scaling coefficient on the order of days for the microcosm and years for the forest. Although many other examples could be cited, Hurlbert’s studies of pond microcosms (Hurlbert and Mulla, 1981; Hurlbert et al., 1972a, b) are especially detailed examples of ecosystems comparing effects of fish predation and insecticides on ecosystem structure and function. For another line of research, the microcosm provides only a context for studies of population dynamics or species interactions. Recent reviews of this work are given by Drake et al. (1996), Lawler (1998), and Lawton (1995). Included here are some of the fundamental studies of ecology such as those by Gause (1934) and Park (1948). G. F. Gause was a Russian scientist who studied interactions among proto- zoan populations in glass vials. He is credited with the first expression of the competitive exclusion principle which states that when two species use similar resources (or occupy the same niche), one species will inevitably be more efficient and will drive the other extinct under limiting conditions (see Chapter 1). He also conducted laboratory experiments on predator–prey relations such as shown in Figure 4.4. Paramecium caudatum was the prey population in these laboratory FIGURE 4.3 Comparison of the development of a forest ecosystem with a microcosm. The time patterns are similar but the time scaling is different. P G = gross production; P N = net production; R = total community respiration; B = total biomass. (From Odum, E. P. 1971. Fundamentals of Ecology, 3rd ed. W. B. Saunders, Philadelphia, PA. With permission.) 20 40 60 80 200 Days 40 60 80 100 B B R R Forest Succession Microcosm Succession Years P G P G P N P N Microcosmology 123 cultures, which was supported on an undefined set of bacteria at the base of the food chain, and Didinium nasutum was the predator population. Much work was required to design an effective growth media for all of the species (Gause, 1934). Three conditions were demonstrated by the experiments. With no special additions, the predator consumed all of the prey and they both went extinct (Figure 4.5 A). When sediment was placed in the bottom of the vials, it acted as a refuge for the prey to escape the predator. In this case the predator eventually went extinct and the prey population grew after being released from predation pressure (Figure 4.5B). Finally, when periodic additions of both prey and predator were used to simulate immigra- tion, the oscillations characteristic of simple mathematical equations were found (Figure 4.5C). Thomas Park also studied basic population dynamics and competition with laboratory cultures of flour beetles (Figure 4.6). More than 100 papers were produced by Park and his students over a 30-year period on this extremely simple ecological system, which laid the foundation for important population theory. The microcosm consisted of small glass vials filled with a medium of 95% sifted whole-wheat flour and 5% Brewers’ yeast. A known number of adult beetles of one or two species (depending on the experiment) in equal sex ratios were added to the media and were incubated in a growth chamber for 30 days. At that time the media were replaced and the beetles were censused and returned to the vials. This procedure was followed for up to 48 censuses (1,440 days), which was “roughly the equivalent of 1,200 years in terms of human population history” when scaled to human dimensions (Park, 1954)! Obviously, the engineering involved in these microcosms was minimal but elegant in providing such a powerful experimental tool for the time period. Also, the flour beetles themselves were preadapted for use in the microcosms because they spend their entire life cycle in flour. The focus of Park’s work was on the population rather than the ecosystem, though it did simulate a natural analog of food storage and pests (Sinha, 1991). Park (1962) described the experimental system with a machine analogy as follows: FIGURE 4.4 Energy circuit diagram of Gause’s classic microcosm. Note the series connec- tions characteristic of predator–prey relations. Inocula Media Microcosm Bacteria X X X Paramecium Sediment Didinium 124 Ecological Engineering: Principles and Practice Let us begin with two seemingly unrelated words: beetles and competition. We identify competition as a widespread biological phenomenon and assume (for present purposes at least) that it interests us. We view the beetles as an instrument: an organic machine which, at our bidding, can be set in motion and instructed to yield relevant information. If the machine can be properly managed and if it is one appropriate to the problem, FIGURE 4.5 Outcomes of Gause’s experiments on the role of predation. (A) Result of experiment with no sediment or species additions. (B) Result of experiment with the addition of sediment which acts as a refuge for the prey Paramecium. (C) Result of experiment with periodic additions of both the predator Didinium and the prey Paramecium resulting in oscillations of population sizes. (Adapted from Gause, G. F. 1934. The Struggle for Existence. Williams & Wilkens, Baltimore, MD.) Homogeneous Microcosm without Immigrations Bacteria Paramecia Didinium Heterogeneous Microcosm without Immigrations Bacteria Paramecia Didinium Homogeneous Microcosm with Immigrations Bacteria Paramecia Didinium Prey Prey Prey Predator Predator Predator Number of Individuals A B C Microcosmology 125 we are able to increase our knowledge of the phenomenon. … Obviously, there exists an intimate marriage between machine, its operator, and the phenomenon. Ideally, this marriage is practical, intellectual, and esthetic: practical in that it often, though not immediately, contributes to human welfare; intellectual in that it involves abstract reasoning and empirical observation; esthetic in that it has, of itself, an intrinsic beauty. Perhaps these rather pretentious reflections seem far removed from the original words — beetles and competition. But I do not think this is the case. Basic scientific research on populations and communities at the mesocosm scale began with the work of Hall et al. (1970) on freshwater pond systems. Historically, most mesocosm studies have been directed at applied studies of ecotoxicology but, as noted by Steele (1979), this work almost always also yields insights on general ecological principles. One of the best examples of basic mesocosm research may be the work of Wilbur (1987, 1997) and his students on interactions among amphib- ians in temporary pond mesocosms. These studies of life history dynamics, compe- tition, and predation have led to a detailed understanding of the community structure of this special biota. The mesocosms consist of simple metal tanks, and an interesting dialogue on Wilbur’s experimental approach is given in a set of papers in the journal Herpetologia (Jaeger and Walls, 1989; Hairston, 1989; Wilbur, 1989; and Morin, 1989). Much discussion has been recorded on the trade-offs between realism and precision in this type of research (see, for example, Diamond, 1986), and Morin (1998) describes mesocosms as hybrid experiments at a scale between the laboratory and the field with an optimal balance between the two extremes of experimental design. MICROCOSMS IN ECOTOXICOLOGY Microcosms are important as research tools in ecotoxicology for understanding the effect of pollutants on ecosystems. Experiments in which treatments are various concentrations of pollutant chemicals can be conducted in microcosms with repli- cation and with containment of environmental impacts due to isolation from the FIGURE 4.6 Energy circuit diagram of Park’s classic microcosm. Note the parallel connec- tions of competition between the two Tribolium species. Source of Flour Flour Microcosm Tribolium castaneum Tribolium confusum 126 Ecological Engineering: Principles and Practice environment. Although this role for microcosms in ecotoxicological research is well established, their potential role within formal regulatory testing or screening proto- cols in risk assessment is controversial. Challenges for ecological engineering include the design and operation of microcosms that are effective for both research and risk assessment in ecotoxicology. Uses for risk assessment will be emphasized in this section owing to the controversial debate about the role of microcosms and the wide potential applications of microcosm technology that are involved. Testing or screening of chemicals is regulated by the Environmental Protection Agency (EPA) in the U.S. This regulation is necessary because of the tremendous number of new chemicals that are produced each year for industrial and commercial purposes. Many of these chemicals are xenobiotic or man-made, whose potential environmental effects are unknown. Thus, uncertainty arises because natural eco- systems have never been exposed to them and species have not adapted to them. Special concern is needed for pesticides because they are intentionally released into the environment and are intended to be toxic, at least to target organisms. The primary examples of legislation covering regulatory testing and screening of chemicals are the Toxic Substances Control Act and The Federal Insecticide, Fungicide, and Rodenticide Act, along with several others (Harwell, 1989). An interaction has developed among the EPA, the chemical industry, environmental consulting firms, and academic researchers in relation to risk assessment of new chemicals, which has in turn created opportunities for applications of ecologically engineered micro- cosm technology. EPA’s risk assessment approach for chemicals (Norton et al., 1995) has evolved over time since early work in the 1940s on methods for measuring the effects of pollutants. The purpose of risk assessment is to evaluate potential hazards in order to prevent damage to the environment and human health. The basis for testing or screening is a hierarchical (tiered) protocol of sequential tests. Physical and chemical properties are tested at the lowest tier, and acute and chronic toxicity data along with estimated exposure data are gathered for several aquatic species at intermediate tiers, followed at least in principle by simulated field testing at the highest tier (Hushon et al., 1979). The intention is to minimize the number of tests required to assess a chemical’s hazard and at the same time to include a comprehensive range of tests. Each tier level can trigger testing at higher levels by comparison of test results to established end points which determine whether or not the chemical is considered to be toxic or hazardous. Choice of end points is important because they are the criteria for determining regulatory action. Concern exists at all levels about tests that result in false negatives (results which indicate that a chemical is toxic when it is in fact not toxic) and false positives (results which indicate that a chemical is not toxic when it is in fact toxic). Cairns and Orvos (1989) suggest that the sequential arrangement of tests that were used from simple to the more complex possibly reflects, in a broad, general way, the historical development of the field. As a consequence, tests with which there is a long familiarity are placed early in the sequence and more recent and more sophisticated tests that are still in the experimental stage or development are placed last. [...]... useful paradigm for understanding ecological organization (see Chapter 5) This theory is applicable to microcosms in terms of the number of species that can be supported by a closed system In most cases, seeding of a 146 Ecological Engineering: Principles and Practice Source of Species P Immigration k1(P-S) Extinction Island Species S k2S dS k = k1(P-S) - K 2S dt FIGURE 4. 18 Energy circuit diagram... collapse ecologically and be a short-term model, especially of food chain biomagnification Metcalf and his students studied more than 100 pesticides and other chemicals with this system mostly in the 1970s, and the microcosm was modified and used by other researchers (Gillett and Gile, 1976) 132 Ecological Engineering: Principles and Practice Frieda Taub developed a standardized aquatic microcosm (SAM)... to higher levels of ecological organization (Levin, 1998) Arguments against the ability to extrapolate have been provided by the Cornell University Ecosystems Research Center (Levin and Kimball, 19 84; Kimball and 128 Ecological Engineering: Principles and Practice Levin, 1985; Levin et al., 1989), by Taub in relation to her work on the standardized aquatic microcosm (Taub, 1997), and most strenuously,... days, and has been studied and verified to such a degree that it has been registered with the American Society for Testing and Materials as a standard method (ASTM E136 6-9 0) The system is especially significant in ecological engineering because it represents the culmination of several decades of research design by Taub and her co-workers The system is widely known and the chemically defined media and the... teaching laboratory workbook: 144 Ecological Engineering: Principles and Practice PREPARATION OF BROTH: Add 150 gm of dried grass to 2000 ml of distilled water, and boil for 15 minutes Cool and strain quickly, once through a double thickness of cheesecloth and once through a thick nest of glass wool in a large funnel Dilute with distilled water to a volume of 3000 ml, and store in a refrigerator until... valuable progress at bridging the gap between regulators and ecologists has been in the development of standardized microcosms Regulators value precision (low variance) and reproducibility (Soares and Calow, 1993), and these preferences have led some ecologists to design, build, and operate small, simple 130 Ecological Engineering: Principles and Practice System Response Treatment Introduced X ± 1 S.E... to standardize design for some purposes, the literature is filled with unique and ingenious microcosms that demonstrate a wide creativity for this subdiscipline of ecological engineering General design principles for microcosms are covered by Adey and Loveland (1998) and Beyers (19 64) Design of aquatic microcosms historically derives in part from the commercial aquarium hobby trade (Rehbock, 1980) and. .. effects and missing components The first aspect of wall effects is the composition of the walls of the container themselves A wide variety of wall materials has been used in microcosms Most are rigid (such as fiberglass), but flexible walls (such as plastic) are used for limnocorrals or other large in-situ enclosures Schelske (19 84) has covered possible chem- 136 Ecological Engineering: Principles and Practice. .. performed and their validity versus the big bag be assessed and reported.” Recognition of the problem also led to designs that generated turbulence in pelagic microcosms, including bubbling the water column with compressed air within floating bags (Sonntag and Parsons, 1979) and mechanical mixing with plungers or propellers in fixed tanks (Estrada et al., 1987; Nixon 140 Ecological Engineering: Principles and. .. 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 150 Ecological Engineering: Principles and Practice Mass and Cost Per Man Approaching Total Regeneration Mass of Total Regeneration System . (Levin and Kimball, 19 84; Kimball and FIGURE 4. 7 A typical dose–response curve from ecotoxicology. 100 Percentage Kill 50 0 LD 50 Log Dose of Poison 128 Ecological Engineering: Principles and Practice Levin,. (1969) and Table 9.1 in E. P. Odum (1971)]. E. P. Odum’s model compares trends expected through succession for 24 ecosystem 122 Ecological Engineering: Principles and Practice attributes and is. across these kinds of 118 Ecological Engineering: Principles and Practice experimental gradients, which makes this a rich and interesting subdiscipline of ecological engineering. One useful size