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 ecosystem. While some of this work has been descriptive, most has relied on experiments. In the experimental approach, replicate microcosms are developed and partitioned 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 contribution 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

Molde Para Faca

Days

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. PG = gross production; PN = 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.)

Days

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. PG = gross production; PN = 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.)

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 protozoan 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 4.4 Energy circuit diagram of Gause's classic microcosm. Note the series connections characteristic of predator-prey relations.

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 immigration, 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 4.5 Outcomes of Gause's experiments on the role of prédation. (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.)

FIGURE 4.5 Outcomes of Gause's experiments on the role of prédation. (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.)

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 4.6 Energy circuit diagram of Park's classic microcosm. Note the parallel connections of competition between the two Tribolium species.

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 amphibians in temporary pond mesocosms. These studies of life history dynamics, competition, 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.

Growing Soilless

Growing Soilless

This is an easy-to-follow, step-by-step guide to growing organic, healthy vegetable, herbs and house plants without soil. Clearly illustrated with black and white line drawings, the book covers every aspect of home hydroponic gardening.

Get My Free Ebook


Post a comment