Components and Connections

The measurement and description of community attributes (see Communities) help to outline the framework of local ecosystems, but they reveal little about how such entities work. Instead of picturing the components as entries in an inventory of species that live together in the same place at the same time, the working parts of ecosystems are seen as energy-materials processors.

Interactions between population systems (primary consumption, predation and parasitism, commensalism and mutualism) are the channels that connect the processors, often in complex networks that change along environmental gradients and over time. Such systems also contain pools, or reservoirs, that temporarily store energy and chemicals. The component population systems undergo changes owing to natality-mortality and emigration-immigration dynamics, controlled both by intrinsic factors (such as the amount of primary production, competitive interactions, grazing/predation, and the products of habitat-altering organisms that re-engineer proximal environments) and extrinsic forcing or control (climate, hydrography, geology, dis turbance regime, imported resources, and invasions). Action at each processor includes intake, metabolism/respiration/waste produc-

Figure 2 a

Howard Odum's Energy Symbols for Biotic and Abiotic Components






Note: The working parts of ecosystems are idealized as energy-materials processors. The components include organisms that function as producers and as consumers and also abiotic storage compartments.

Figure 2b

Generalized Static Model of Energy and Element Flows and Cycles in Ecosystems



Note: Solid lines = flow of materials (A). Dotted lines = flow of energy (B). N = nutrient pool. P = primary producers. C = Consumers. D = decomposers.

Source: Pomery, Lawrence R., and James J. Alberts, eds. 1988. Concepts of Ecosystem Ecology. New York: Springer-Verlag. figs 3.2 and 3.3, p. 45. (Reprinted with permission)

Mountains, forest, and pond in the American West. (USQS)

tion, leakage into the surrounding environment, and output to other processors or sinks. In terms of energy capture and transfer, ecosystems can be supported by either photosyn-thetic or chemosynthetic primary producers, or by detrital material imported from an adjacent system. In terms of chemical flow, ecol-ogists usually focus on measurement of the most essential nutrients (C, N, P, Ca, K, and Fe in terrestrial systems, which have been studied extensively).

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