The systems of nature that depend largely or entirely on the direct rays of the sun can be designated as unsubsidized solar – powered ecosystems. They are unsubsidized in the sense there is little, if any, available auxiliary source of energy to enhance or supplement solar radiation. The open oceans, large tracts of upland forests and grasslands, and large deep lakes are examples of relatively unsubsidized solar – powered ecosystems. Frequently, they are subjected to other limitations as well, as, for example, a shortage of nut re = ients or water. Consequently, ecosystems in this broad category vary widely, but are generally low powered and have a low productivity, or capacity to do work. Organisms that populate such systems have evolved remarkable adaptations for living on, and efficiently using, scarce energy and other resources.
Major sources of energy Ecosystem

     Although the ‘’power density’’ of natural ecosystem in this first category is not very impressive, nor could such ecosystems by themselves support a high density of people, they are none the less extremely important because of their huge extent (the oceans alone cover almost 70% of the globe). From the human interest standpoint the aggregate of solar – powered, natural ecosystem can be thought of, and the certainly should be highly valued, as the basic life – support module which provide desirable stability and homeostatic control for spaceship earth. It is here that large volumes of air are purified daily, water recycled, climates controlled, weather moderated, and much other useful work accomplished. A portion of man’s  food and fiber needs are also produced as a by – product without economic cost or management effort by man. This evaluation, of course, does not include the priclude the priceless aesthetic values inherent in a sweeping view of the ocean, or the grandeur of an unman aged forest, or the cultural desirability of green open space.

      Where auxiliary sources of energy can be utilized to augment solar radiation the power density can be considerably, perhaps an order of magnitude. In this frame of reference an energy subsidy is an auxiliary energy source that reduces the unit cost of self – maintenance of the ecosystem, and thereby increases the amount of solar energy that can be converted to organic production. In other words, solar energy is augmented by non solar energy freeing it for organic production. Such subsidies can be either natural or man – made (or, of course, both). For the purpose of our simplified classification we have listed naturally subsidized and man – subsidized solar – powered ecosystems.
ecosystems energy example

       A coastal estuary is a good example of a natural ecosystem subsidized by the energy of tides, waves, and currents. Since the back and forth flow of water does prt of the necessary work of recycling mineral nutrients and transporting food and wastes, the organisms in an estuary are able to concentrate their efforts, so to speak , on more efficient conversion of sun energy to organic matter. In a very real sense, organisms in the estuary are adapted to utilize tidal power. Consequently, estuaries tend to be more fertile than, say, an adjacent land area or pond which receives the same solar input, but does not have the benefit of the tidal and other water flow energy subsidy. Subsidies that enhance productivity can take many other forms, as for example, wind and rain in a tropical rain forest, the flowing water of a stream, or imported organic matter and nutrients received by a small lake from it watershed.

        Man, of course, learned early how to modify and subsidize nature for his direct benefit, and he has become increasingly skillful in not only productivity, but more especially in channeling that productivity into food and fiber materials that are easily harvested, processed, and used. Agriculture (land culture) and aquaculture (water culture) are the man subsidized solar – power ecosystems. High yields of food are maintained by large inputs of fuel (and in more primitive agriculture, human and animal labor) involved in cultivation, irrigation, fertilization, genetic selection, and pest control. Thus, tractor fuel, as will as animal or human labor, is just as much an energy input in aggro – ecosystems as sunlight, and it can be measured as calories or horsepower expended,not only in the field, but also in processing and transporting food to the supermarket. As H. T. Odom (1971) was so aptly expressed it, the bread, rice, orn, and potatoes which feed the masses of people are partly made of oil. This is why fuel, or some comparable auxiliary energy, is vital to food production for man.

        It is very important to note that recent increases in crop yield, the so – called ‘’green revolution,’’ has resulted from genetic selection of plants, not so much for their ability to utilize solar energy as for their ability to benefit from fuel subsidies. Thus, what has been called in the popular press ‘’miracle’’ rice and wheat are dwarf plants with small root systems and just enough leaves and stem to capture a maximum of usable solar radiation. Since man’s fuel and chemicals do most of the work of protection and maintenance that a wild plant would have to do with an expenditure of its own energies, the crop plant is able to convert more of the sun energy into grain. It can do this because it is highly selected (that is, genetically programmed) to produce grain at the expense of non edible tissue. Pouring the fertilizer, or other subsidies, on a wild rice plant would not have such a great effect on grain yield since the wild plant would be programmed to use the additional resources for stalks and leaves as well as grain. Man’s skill in augmenting the natural conversion of sun energy into food in this fashion parallels nature’s own design and has, at least temporarily, staved off starvation in some parts of the world. However, the fuel subsidized ogre – ecosystem is not without its economic and pollution costs resulting from the heavy energy consumption the he high degree of genetic specialization produces an inherent vulnerability to disease. Whether fuel subsidized food production and rising per capita. 

The ecosystems illustrated in figure 2-1 are contrasting types of sun powered ecosystems, and thus emphasize basic similarities and differences. A terrestrial ecosystem (illustrated by the field shown on the left) and an open – water aquatic system (illustrated by a lake or the sea as shown on the right) are populated by entirely different kinds of organisms, with the possible exceptions of a few kinds of bacteria that may be able to live permanently in either situation. Yet the same basic ecological components are present and function in much the same manner in both of ecosystem. On land, the autotrophy are usually rooted plants ranging in size from grasses and other herbs that occupy dry or recently denuded lands to very large forest trees adapted to moist lands. In deep water systems the autotrophs are microscopic suspended plants called phytoplankton ( phyto = plant; plankton = floating), which be long to several different classes of algae. The include: (1) the diatoms, tiny plants with silicon shells; (2) green flagellates that move about propelled by rapidly beating ship like flagella; (3)the green algae which may occur as single cells, colonies, or filaments of cells; and (4)the blue – green algae, some of which have gelatinous capsules and thrive on organic pollution, thus clogging public water supplies and creating nuisances in recreational lakes. As would be expected, shallow water ecosystems are occupied by mixtures of macroscopic plants and microscopic algae.

Because of size differences in plants, the bimass, or standing crop, of terrestrial and aquatic ecosystems may be widely different. Plant biomass in terms of grams of dry matter per square meter may be 10000 or more in a forest in contrast to less than 5 in a pond, lake, or ocean. Despite the size discrepancy, 5 g of tiny plants are capable of manufacturing as much food in a given period of time as are 10,000 g of large plants given the same quantity of light, minerals, and energy subsidies. This is because the rate of metabolism of small organisms is very much greater per unit of weight than that large organisms. Furthermore, large land plants are mostly composed of woody tissues that are relatively inactive; only the leaves are active in photosynthesis, and in forest leaves comprise only about 1 to 5 persent of the total plant biomass.

This is a good place to introduce the concept of turnover as a first step in relating structure to function in an ecosystem. We can think of turnover as the ratio of the standing state (that is, amount present) of biotic or abiotic components to the rate of replacement of the standing state. For example, if the biomass of a forest is 20,000grams per square meter (g/m) and the annual growth increment is 1000 g, then the ratio 20/1 can be expressed as a turnover time or replacement time of 20 wears. The reciprocal, that is, 1/20 = 0.05, is the turnover rate. In a pond the turnover time for phytoplankton would be measured in days rather than years.

It is particularly important to have information on turnover rates between biotic compartments when it comes to evaluating the impact of mineral nutrients or other chemical components in an ecosystem. It is more important to know how fast materials are moving along the pathways between organisms and environment than it is to know the total amount present. This, a soil might contain a large amount of phosphorus, but if it is not available to organisms, perhaps because it is in an insoluble form, then it might as well not be there. We have already made of man’s tendency to extract materials from the environment and return them to the environment in poisonous forms; man also, often inadvertently, returns them in unusable form. Then it is like the marooned in the middle of the sea – there is ‘’water, water everywhere, but not a drop to drink.’’  

Food and energy for man is very important in the world.  Approximate yields of major food crops at three levels of auxiliary energy, and three levels of protein content are shown in table 3-2. Millions of people are not getting enough calories of food; millions of other are not getting enough protein, especially children who have a higher requirement for growth. Thus, it is important to consider both food for man and protein quality. From table 3 – 2 we see that world average yield is two to three times less than that of high yields in affluent countries because most of the world’s cultivated acres do not have the benefit of high – energy subsidy. Average yields are, in fact, very close to the bottom; that is, world  averages are little better than that reported from the very poorest countries. To double food yield in the latter requires a tenfold increase in fuel, fertilizers, and pesticides. Those who think that food for man we can upgrade agricultural production in ‘’undeveloped’’ countries simply by sending seeds of new varieties and a few ‘’agricultural advisors’’ are tragically naïve. Crops highly selected for industrial agriculture must be accompanied by many calories of fuel which, of course, the developed countries are hoarding for themselves a second point to note from table 3 -2 is that, as always, quality comes at the expense of quantity. Yields of a high protein crop, such as soybeans, average one half to one third less than that of low or moderate protein content crops. Getting one’s protein from meat, however desirable from the nutritional standpoint, does not help the per – unit – area – yield problem since, as we have seen in our discussion of food chain dynamics, food for man there is at least an 80 percent loss in transfer from grain to meat.

     Table 3 -2 , annual yiels of edible food and estimated net primary production of major food at three levels of energy subsidy and three levels of protein content.

    W see from table 3 – 3 that in 1970 man harvested about 5.3. 10¹⁵  kcal of food, 99 percent from the land and 78 percent from plants. This would theoretically give the approximately 4 billion (4.10⁹) people in the world their minimum annual requirement of 1 million (10⁶ ) kcal even allowing for you unavoidable waste, if Food for man were evenly distributed. But, of course, it is not, and probably can never be because of the problems of distribution, transportation, economics, and so on.

The estimate of 5.10¹⁵ kcal harvested is about 1 percent of global net primary production and 0.5 percent f gross. It would seem that man is not yet making much of a dent in the phosynthetic capacity of the earth, but the real impact looks quite different when we consider the following.

     Perhaps the best way to view the food problem is to consider it from the per capita viewpoint. To provide the diet now consumed by an American, about 2.5 acres (1 hectare) are required when we consider land area required to produce meat, orange juice, and leafy vegetables along with staple grains. Another acre is required to produce fibers (wood, paper, cotton, and so on). As of 1970 there were only 10 acres of land per person average for the whole world. If population doubles in the next 50 years there will be only be 5 acres (2 hectares) per capita to provide all requirements – water, oxygen, waste treatment, fibers, living space, recreation, as well as food for man.

We cannot hope to do justice to the subject of ‘’food for man’’in this brief introduction. Relationships are extremely complex and there is much controversy. For further reading we recommend the three – volume treatise the world food problem and George borgstrom’s books (1967, 1969). But we must warn you that these are mot easy reading, and that th=ere are no easy ‘’quick – fix’’ solutions.

       The whole gambit of natural and cultivated net cultivated net primary production is summarized. When we look at it from the basic energy standpoint there is no difference between man’s crops and nature’s crops. Given sunlight, nutrients, water, and adapted plants, net production is a function of available supplemental energy – tides in case of salt marsh, and fuel in case of agriculture. Just because man does not harvest the net production of the marsh grass does not mean it is valueless to him. Useful work of waste assimilation and recycling worth many dollars is accomplished and food for man the seafood flowing off the end of the food chain is free except for the cost of harvesting and processing.   

        Nutrients for energy is very important in the world. Elements and dissolved salts essential to life may be conveniently termed biogenic salts or nutrients energy and divided into two groups, the macro nutrients and the micro nutrients. The former include energy elements and their compounds that have key roles in protoplasm and that are needed in relatively large quantities, as for example, carbon, hydrogen, oxygen, nitrogen, potassium, calcium, magnesium, sulfur, and phosphorus. The micronutrients include those elements and their compounds also necessary for the operation of living systems but are required only in very minute quantities; for example, iron, manganese, copper, zinc, boron, sodium, molybdenum, chlorine, vanadium, and cobalt. It should be emphasized at this point that elements that have no known biological function also circulate between organisms and environment. These may enter biogeochemical cycles linked with essential elements by reason of chemical affinity, or they may be simply carried along in the general energy – driven stream. Likewise, poisons produced by man, such as insecticides and radioactive strontium, all too often enter vital cycles and become lodged in the tissues of animals and man (more about this later). Although organisms do develop adaptive mechanisms to exclude harmful substances, there is no way that living membranes can function efficiently in the exchange of vital materials, and, at the same time be completely selective as to what is ‘’good’’ and what is ‘’bad’’ even if harmful substances are not lethal, an energy stress is placed on the organism since it must expend extra energy to sequester or ‘’pump out’’ the poison.
Nonessential and energy elements

        Nonessential and energy elements are, therefore, of great ecological importance if they occur in quantities or forms that are toxic, if they react to bind or make unavailable essential energy elements, or if they are radioactive. Thus, the ecologist is concerned with nearly all of the natural energy elements of the periodic table as well as with the newer man – made ones, such as plutonium.