Energy is everywhere. If there is one principle we hope you will remember and pass on to every person you meet, it is this: to a consumer, such as man, only net energy counts, and this applies to all energy, food and fuel. Gross energy, as a potential, is often very impressive in quantity, but must always be evaluated in terms of the amount that can be converted into the desired work. And thermodynamic costs must be less than the net energy obtained if the conversion is to be a long – term benefit. We have seen that the gross energy of solar radiation is huge, but the net energy of food is very small. Likewise, ‘’proven reserves’’ of oil and coal (‘’enough to last many centuries’’ you may be told) are gross energy. The relevant question is how much will be left to power your car and run your city and what will be the price? If mining, extracting, shipping, and processing oil from under the sea, or that locked in shale rock, requires more energy than the final product is worth, there may be no net energy. Likewise for agriculture; already the energy value of some crops is less than the fuel energy required to produce them. Likewise with atomic energy. The potential energy in the atom is fantastic, but so are the costs of converting it. As with most scientists I was once more enthusiastic than I am today about atomic energy replacing fossil fuels within this century. Costs and difficulties have been greater than our best minds predicted, and we may have invested in the wrong kind of atomic energy, because of our preoccupation with atomic bombs (for more on the kinds of atomic energy). We must continue a massive research effort but it is going to take more time to prove out various possibilities for harnessing atomic energy on the scale we now extract energy from fossil fuels. Which means that we should ‘’power down’’ and be fore efficient in the use of ‘’proven net energy’’ at least for a while.
Most convenient and worthwhile uses of energy

        To put some of this in perspective let us consider the fuel – energy budget of the united states as of the early 1970s. from the large world reserves of fossil fuel energy the united states receives about 16.1010¹⁵ kcal/year, but only about half (50 percent) is actually converted into useful work. Thus, the ‘’net’’ is 8, not 16.10¹⁵, not quantities in the ground. Cost of extracting and processing goes up as gross supplies dwindle and we have to turn to lower quality materials. Two of our most convenient and worthwhile uses of energy are automobiles and electricity. But are about 30 percent efficient which contributes to the low overall efficiency. It seems likely that the 1970s will see us give up some convenience to improve efficiency and thus stretch out fuel energy supplies as they become more expensive to convert from gross reserves. We have already spoken of need for diversification in situations such as this.

        There is a parallel of sorts in economies. For many years the gross national product (GNP) has been considered a good index of economic well – being. Now, many economists are suggesting that net economic worth (NEW) would be a much better measure. In computing NEW the ‘’bads’’ (pollution costs, and so on), as well as the ‘’goods’’ (manufactured product, and so on) are considered, and maintenance work. Such as the work of the housewife, is included. In recent years the GNP of most nations has been going up the NEW for the united states has leveled off, indicating that the real economic situation has been improved by ever larger production of hard goods.

      
In the preceding posts important principles and some orders of magnitude regarding energy flow within ecosystems were discussed. As already emphasized, the movement of materials in the ecosystem is an equally important consideration. The more or less circular paths of the chemical elements passing back and forth between organisms and environment are known as biogeochemical energy cycles. ‘’bio’’ refers to living organisms and ‘’geo’’ to the rocks, soil, air, and water of the earth. Geochemistry is an important physical science, concerned with the chemical composition of the earth and the exchange of elements between different parts of the earth’s crust energy and its oceans rivers, and so on. Biogeochemistry energy is thus the study of the exchange (that is, back and forth movement) of chemical materials between living and nonliving components of the biosphere.  plified  energy – flow diagdram to show the interrelation of the two basic processes, and to reemphasize the point already made, namely that energy is required to drive the cycling of materials. Vital elements in nature are never, or almost never, homogeneously distributed or present in the same chemical form throughout an ecosystem. Rather, materials exist in compartments or pools, with varying rates of exchange between them. From the ecological standpoint it is advantageous to distinguish between a large, slow – moving nonbiological pool and a smaller but more active pool that is exchanging rapidly with organisms. In figure 4 -1 the large reservoir is the box labeled ‘’pool’’; and the rapidly cycling material is represented by the stippled circle going from autotrophs to heterotrophs and back again. Sometimes the reservoir portion is called the unavailable pool and the cycling portion the available pool; such a designation is permissible provided it is clearly understood that the terms are relative. An atom in the reservoir pool is not necessarily permanently unavailable to organisms but only relatively so; in comparison, an atom in the cycling pool is instantly available. Almost always there is a slow movement of atoms between the unavailable and the available pools.
biogeochemical   Decomposition

       Decomposition not only releases nutrients energy but the organic energy by products may also increase the availability of minerals energy for uptake by autotrophs. On way this occurs is by a process known as chelation in which organic molecules ‘’grasp’’ or form complexes with, calcium, magnesium, iron, copper, zinc, and other ions. Such chelated minerals are more soluble and less toxic than some of the inorganic salts of the element, especially in the case of metals. Chelators are nearly added to cultures and microcosms to enhance the availability of nutrients.

     
   Ecosystems maintain themselves by cycling energy and nutrients obtained from external sources.Assuming that adapted organisms are present in an area of the biosphere, the number and diversity of organisms and the rate at which they live depends not only on the magnitude of available ecosystem energy and resources, geographical position, evolutionary history, but also on the manner in which energy flows through the biological part of the system and on the rate at which materials circulate within the system and/ or are exchanged with adjacent systems. It is important to emphasize that nonenergy – yielding materials circulate, but ecosystem energy does not. Nitrogen, carbon, water, and other materials of which living organisms are composed may circulate many times between living and nonliving entites; that is, any given atom of material may be used over and over again. On the other hand, energy is used once by a given organism or population, is converted into heat; in this degraded form it can no longer power life processes and is soon lost from the ecosystem. The food you ate for breakfast is no longer available to you when it you when it has been respired; you must go to the store and buy more for tomorrow. Likewise, water, paper, and metals in the city can be recycled, but not the energy that powers the city. All living organisms and all machines are alike in that they are going by the continuous inflow of ecosystem energy from the outside.

     We would do well to close the general discussion of the study of natural ecosystems with an example that illustrates the value of studying the whole ecosystem as well as the component parts, even the system is much more complex than a small fish pond or field. A tropical biotic reef represents one of the most beautiful and well – adapted ecosystems to be found in the world. Corals, small animals with hard calcareous calcereous skeletons, and calcareous algae build up the reef substrate which is the home of numerous organisms. Embedded in the tissues of the coral, and also in and on the skeleton of many animals and the general calcareous substrate are numerous algae. If supplied with abundant zooplankton food, some coral species can be maintained in ecosystem laboratory tanks without the algal associates. However, when the metabolism of a whole reef is measured (as for example, by measuring diurnal changes in oxygen as water passes over the reef – a modification of the method just described for assaying the metabolism of a pond), the input – output budget indicates not enough animal food suspended in the water to completely support the corals. In such a situation there must be supplemental sources of food, perhaps that produced by algal associates. ecosystem Tracer experiments have shown that exchanges of organic matter between plant and animal tissues within the colony do occur. Also, it has been clearly demonstrated that mineral nutrients are recycled back and forth between animal plant components so the colony does not require a high rate of fertilization from without. These discoveries indicate that, in nature, coral animals and algae are metabolically linked and dependent on one another. The history of recent research on coral reefs bears out the point we have already emphasized: the behavior of an isolated component (coral in a tank) may not be the same as the behavior of the same component in its intact ecosystem (the reef) where a available sources and nutrient constraints may be quite different. And the corollary to this: to understand the ecosystem, the whole as will as the part must be studied.