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Energy budgets can be developed from measurements of available solar energy, primary productivity, secondary productivity, decomposition, and respiration. Comparison of budgets and conversion efficiencies among ecosystems can indicate factors affecting energy flow and contributions to global energy budget. Development of energy budgets for agricultural ecosystems can be used to evaluate the efficiency of human resource production.

Lindeman (1942) was the first to demonstrate that ecosystem function can be represented by energy flow through a trophic pyramid or food web. He accounted for the energy stored in each trophic level, transferred between each pair of trophic levels, and lost through respiration. H. Odum (1957) and Teal (1957,1962) calculated energy storage and rates of energy flow among trophic levels in several aquatic and wetland ecosystems (Fig. 11.5). E. Odum and Smalley (1959) and Smalley (1960) calculated energy flow through consumer populations. The International Biological Programme (IBP) focused attention on the energy budgets of various ecosystems (e.g., Bormann and Likens 1979, Misra 1968, E. Odum

The Energy Flow Mosquito

| Energy flow (kcal m-2 yr-1) in the Silver Springs ecosystem. H, herbivores; C, predators; TC, top predators; D, decomposers. From H. Odum (1957) with permission from the Ecological Society of America.

1969, Petrusewicz 1967, Sims and Singh 1978), including energy flow through insect populations (Kaczmarek and Wasilewski 1977, McNeill and Lawton 1970, Reichle and Crossley 1967).

More recently, the energy budgets of agricultural ecosystems have been evaluated from the standpoint of energetic efficiency and sustainability. Whereas the energy available to natural communities comes from the sun, additional energy inputs are necessary to maintain agricultural productivity. These include energy from fossil fuels (used to produce fertilizers and pesticides and to power machinery) and from human and animal labor (Bayliss-Smith 1990, Schroll 1994). These additional inputs of energy have been difficult to quantify (Bayliss-Smith 1990). Although the amount and value of food production is well-known, the efficiency of food production (energy content of food produced per unit of energy input) is poorly known but critical to sustainability and economic development (Patnaik and Ramakrishnan 1989). Promotion of predaceous insects to control pests, as an alternative to energy-expensive pesticides, and of soil organisms (including insects) to reduce loss of soil organic matter, as an alternative to fertilizers, has been proposed as a means to increase efficiency of agricultural production (Elliott et al. 1984, Ostrom et al. 1997).

Costanza et al. (1997), Daily (1997), N. Myers (1996), and H. Odum (1996) attempted to account for all energy used to produce and maintain the goods and services that support human culture. In addition to the market and energy value of current ecosystem resources, energy was expended in the past to produce those resources. The energy inputs, over time, that produced biomass must be included in the energy value of the system. When forests are harvested, the energy or resources derived from the timber can be replaced only by cumulative inputs of solar energy to replace the harvested biomass. Additional energy is expended for transportation of resources to population centers and development of societal infrastructures. Solar energy also generates tides and evaporates water necessary for maintenance of intertidal and terrestrial ecosystems and their resources.

H. Odum (1996) proposed the term emergy to denote the total amount of energy used to produce resources and cultural infrastructures. Costanza et al. (1997), Daily (1997), and H. Odum (1996) note that ecosystems provide a variety of "free" services, such as filtration of air and water, pollination, and fertilization of floodplains, with energy derived from the sun and from topographic gradients, that must be replaced at the cost of fossil fuel expenditure when these services are lost as a result of environmental degradation (e.g., channelization and impoundment of streams). Sustainability of systems based on ecosystem resources thus depends on the energy derived from the ecosystem relative to the total emergy required to produce the resources. Consequently, many small-scale subsistence agricultural systems are far more efficient and sustainable than are larger-scale, industrial agricultural systems that could not be sustained without massive inputs from nonrenewable energy sources. Unfortunately, these more sustainable agroecosystems may not provide sufficient production to feed the growing world population.

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