Robert J. Beyers

Ponds and Pools

Ponds are small bodies of water, usually much wider than they are deep, and usually containing self-organizing ecosystems. Ponds have been called microcosms by many writers in literature, such as Thoreau in his description of Walden pond in Massachusetts. Forbes (1887) and Hutchinson (1964) called ponds microcosms in the scientific sense of being a miniature demonstration of the processes occurring on a larger scale in lakes and oceans. In our terminology, most ponds are actually mesocosms, being larger than the small experimental laboratory microcosms.

Succession and Self-organization

The self-organization process by which ecosystems develop structure, functions, and diversity from available energies and matter is called succession. These processes occur in microcosms, often rapidly, when the components are small with short turnover time. What actually occurs depends on the initial seeding of matter and available species. On a longer scale of time, the mature states which follow initial growth may oscillate, alternating production, consumption, and diversity. The simulation models in Figures 2.12 and 2.13 show how growth of biomass is related to initial conditions, available nutrients, and organic matter. However, succession is more than growth; it includes species additions and change. Figure 3.1 is a simulation minimodel that includes species seeding and diversity, and their contribution to production efficiency. In Figure 3.1b, the simulation first develops biomass, which supports more species, in turn, increasing efficiency, but additional species put additional demands on the biomass. The simulation with a small overshoot produces results which parallel what is observed in many microcosms, and suggest some consistency between model simulations and theory. Figures 3.2–3.9 show sequences of species in microcosm succession.

Introduction to Microcosmology

Ecological microcosms are small ecosystems held in containers. Starting originally as a way to bring the beauty and complexity of nature into schoolrooms and living rooms the world over, these small “worlds” have become a major research tool. They are useful for studying the way ecosystems work and for the very practical purpose of determining what happens to toxic substances in ecosystems. Microcosms are important because they are a way to study whole, simplified ecosystems, and because they can be replicated for experimental studies at reasonable cost. This book is a review of the extensive but scattered papers and reports on microcosms, with emphasis on the concepts of systems ecology that emerge from the hypotheses and experimental tests.

Metabolism and Homeostasis

The processes occurring in microcosms are the same as those found in ecosystems, but they are simplified since the system is enclosed and isolated. We consider in Chapter 2 the metabolic basis of microecosystems. The main processes are photosynthetic production, consumption, and the recycling of nutrients. Oxygen and carbon dioxide are produced, consumed, and exchanged with the atmosphere. Organic biomass is produced, accumulated, or used up, depending on conditions. There is a self-regulating homeostasis in the coupling of production and consumption, which makes microcosms remarkably stable. In this chapter, using models and simulations, we examine the metabolism of various types of microcosms and the patterns of diurnal variation, succession, and temperature change. Special emphasis is placed on the internal responses to external sources.

Reefs and Benthic Microcosms

The ecosystems of underwater surfaces are important, often conspicuous, intensively metabolic, and some are the most organized and diverse in the biosphere. Water motions relative to the fixed surface help to maintain the attached organisms and high rates of metabolism. We have already considered stream microcosms where the water flow is unidirectional. This chapter concerns microcosms which mimic the reefs, bottom communities, and encrusting ecosystems in lakes and seas where water motions are multidirectional.

Chemical Cycles and Limiting Factors

The circulation of materials is a necessary part of any system, and the biogeochemical cycling of the chemical elements is a major part of ecosystems, including the macrocosm of the biosphere. When a microcosm is enclosed, the chemicals fall into a pattern of circulation within the small container. If material is neither added nor lost, the principle of conservation of matter applies. An important part of ecosystem studies is chemical cycling, evaluating the storages of material, evaluating the main flows, and determining the processes controlling each pathway.

Terraria and Soil Microcosms

Terraria, the terrestrial equivalent of aquaria, are small containers usually containing soil materials, terrestrial plants, animals, and air. Terraria are found by the millions, self-organizing in schoolrooms, offices, and homes. Often open to external air exchange, these microcosms may dry out and become water-limited. Oxygen and carbon dioxide are maintained at external levels by air exchange. Humans occasionally add water. The simulation in Figure 2.11 represents conditions typical of those found in houses and schoolrooms. However, serious studies of typical terraria are scarce. On the other hand, considerable scientific literature exists on transplanted and recently assembled terrestrial microcosms. Gillet and Witt (1978) reviewed earlier work, especially that concerned with study of the way such.terrestrial ecosystems process chemical substances.

Food Microcosms and Mesocosms

Systems of food production for the world’s people are sometimes studied in microcosms, and some self-organizing, ecological mesocosms actually yield food products. Agriculture, fisheries, and aquaculture supply humanity its food. Some of these sources are intensively managed agro-ecosystems and others less-managed environmental systems. Agricultural and aquacultural microcosms have been used for controlled experiments to study larger food production systems under controlled conditions. The processes of food production and systems of waste decomposition are basic to human existence. Just as photosynthesis and respiratory consumption were coupled in a balanced aquarium (Chapter 2), the equivalents of these processes in the human economy are also coupled with the circular, causal symbiosis shown in Figure 17.1.

Stress, Toxicity, and Adaptation

In the biosphere, there are large and long-interval phenomena that shock ecosystems with infrequent destructive pulses. Examples are storms, floods, volcanic actions, fires, or surges of long-period consumers, such as snowy owls. When conditions change, ecosystems reorganize When outside influences cause severe disruption, new succession follows. To better understand these processes of stress, environmental impact, and recovery, agencies of environmental protection have supported extensive research on the effects of chemical, physical, and biological factors on microcosms.

Thermal and Brine Microcosms

Hot springs, brines, anaerobic waters, temporary ponds, tide pools, frigid water, and other extreme conditions are recurring in the biosphere. They are accompanied by characteristic ecosystems composed of adapted organisms. Often occurring in small volumes as natural microcosms (Duke 1967; Ganning and Wulff 1970), these ecosystems can be readily duplicated in experimental microcosms.