The relationship between gross production (pd) and total community respiration is important in the understanding of the function of the ecosystem and in predicting future events. One kind of ecological ‘’steady – state’’ exists if the annual production of organic matters equals total consumption (P/R =1) and if exports and imports of organic matter are either nonexistent or equal. In a mature tropical rain forest the balance may be almost a day – by – day affair, whereas in mature temperature forests an autotrophic regime in summer is balanced by a heterotrophic regime in winter. Another type of steady – state exists if gross production plus imports equal total respiration, as in some types of stream ecosystems, or if gross production equals respiration plus exports, as in stable agriculture.

      Seasonal fluctuations and annual shifts related to short – term meteorological or other cycles in the physical environment occur in almost all ecosystems, but overall structure and species composition of steady – state communities tend to remain the same, although it is not yet certain that this is always true. If primary production and heterotrophic utilization are not equal (P/R greater or less than I), with the result that organic matter either accumulates or is depleted, we may expect the community to change by the process of ecological succession. Succession may proceed either from an extremely autotrophic condition (P>R) or from the extremely heterotrophic condition (P<R) toward a steady – state condition in which P equals R. organic development in a new pond, or the development of a forest on a fallow field are examples of the first kind of succession. In these situations the kind of organisms change rapidly from year to year and organic matter accumulates. Changes in a stream polluted with a large amount of organic sewage is an example of the other type of succession in which organic matter is used up faster than it is produced. Ecological succession will be discussed in greater detail.

        The ratio of biomass energy to rate of energy flow is an important property of ecosystems as is the P/R ratio. In the ecosystem the ratio of total community respiration to total community biomass (R/B) can be considered to be a thermodynamic order function, for reasons already made clear. The larger the biomass the larger the respiration, of course, but if the size of the biomass units is large and the structure diverse and well ordered, the respiratory maintenance cost per unit of biomass can be decreased. Nature’s seems to be to reduce the R/B ratio (or increase the B/R efficiency if you prefer) while man’s strategy has tended to be the opposite, since he has been preoccupied with harvesting as much as possible and leaving as little structure and diversity on the landscape as possible.      

         The transfer of food energy from the source in plants through a series of organisms with repeated stages of eating and being eating and being eaten is known as the food chain. In complex natural communities, organisms whose food is obtained from plants by the same number of steps are said to belong to the same trophic level (troph =nourishment, the same root as in autotrophic and heterctrophic). Thus, green plants occupy the first trophic level (the producer level); plant eaters (herbivores, and so on) the second level (the primary consumer level); carnivores that eat the herbivores the third level (secondary consumers), and perhaps even a fourth level (tertiary consumers). It should be emphasized that this trophic classification is one of function, and not of species as such; a given spccupy one, or more than one, trophic level according to the source of energy actually assimilated. We have already called attention to certain algae that may depend in part on their own food and in part on food made by other algae; or the populations of men who utilize food from both plant and animal sources.

       Figure 3 -2 is an energy flow diagram of a food chain. This is a more detailed rendition of the last three modules in figure 3 -1. In this model we introduce the biological terminology used to describe come munity metabolism – and the quantities shown are on a daily, rather than annual, basis. The boxes represent the population mass or biomass, and the pipes depict the flow of energy between the living units. As already indicated, about half of the average sunlight impinging upon green plants (that is, producers) is absorbed by leaves and about 1 to 5 percent is converted into food energy by productive vegetation. The total assimilation rate of producers in an ecosystem is designated as primary production or primary productivity (pg or A in figure3 – 2). It is the total amount of organic matter fixed, including that used up by plant respiration during the measurement period. Net primary productivity (P n ) is the organic matter stored in plant tissues in excess of respiration during the period of measurement. Net production represents food potentially available to heterotrophy. When plants are growing rapidly under favorable light and temperature conditions, plant respiration may require as little as 10 percent of gross production so that net production may be 90 percent of gross. However, under most conditions in nature net production is a smaller percentage of gross, usually about 50 percent, as shown in the lower line of figure 3 – 2.

       As shown in figure 3 – 2 part of net primary production may be stored or exported, and part may beome an energy source for heterotrophy. NU and I, are apportioned within different kinds of ecosystems. Some portion of food ingested by consumers is usually not digestible or assimilable, so some of the energy is likely to be egested unused (NA in the diagram); this component may be stored, exported, or consumed by microorganisms or other heterotrophy. Consumers as well as producers must respire (R) a large part of the energy assimilated (A) so as to maintain structure and function (represented by the boxes in the diagram). Respiratory degradation of energy ‘’pumps out’’, that is, reduces entropy as is required to maintain a high level of organization. The R flows in figure 3 – 2 represent heat losses from biological components that were shown by the more generalized heat sink symbol in figure 3 – 1. Assimilated energy not respired is available for production (Pn P2 P3 …….) which can take the from of growth of new tissue and reproduction and growth of individuals (population growth). Production at heterotrophic levels is often known as secondary production to distinguish it from the primary production of plants. Any auxiliary energy or nutrients that reduces the cost of maintenance (respiration) enhances the rate of production. If the autotrophic level is subsidized, the increased production may be passed along the chain; if a consumer level is augmented, the ‘’downstream’’ links will be mostly affected, although secondary production can ‘’loop back’’ upstream so that even plants might benefit from organic matter not used by animals.

       Roughly speaking, the reduction of available energy with each link in the food chain (as required by the second law of thermodynamics ) is about two orders of magnitude at the first or primary trophic level, about one order of magnitude thereafter. By order of magnitude we mean by a factor of 10. If an average of 1500 kcal of light energy were absorbed by green plants per square meter per day, we might expect 15 to end up at net plant production, 1.5 to be reconstituted as primary consumers (herbivores), and 0.3 as secondary consumers (carnivores) – provided, of course, that there are adapted organisms present that can fully utilize these resources. Efficiencies in terms of percent energy transfer are thus on the order of 1 percent at the first level and 10 – 20 percent at the heterotrophic levels. Since meat is generally of higher nutritional quality, percent transfer tends to be higher at meat – eating levels. Since so much energy is lost at each transfer the amount of food remaining after two or three successive transfers is so small that few organisms could be supported if supported if they had to depend entirely on food available at the end of a long food chain. For all practical purposes, then, the food chain is limited to three or four ‘’links’’. The shorter the food chain, or the nearer the organism to the beginning of the food chain, the greater the available food energy. About ten times more people can be supported by 100 acres of corn of they function as primary rather than secondary consumers, that is, if they eat the corn directly rather than feed if to animals and then eat meat. If you prefer to eat meat then you must plan not to let the population become so dense as to preclude the meat – eating option. In debating man’s’’ world food problem’’ we must consider the quality as well as the amount that might be obtained from a given area of land or water.

        It should be emphasized that the scheme in figure 3 – 2 is a model useful for making comparisons with the real thing. Somewhat larger, and frequently quite a bit smaller, percentages may actually be involved under different conditions. Much remains to be learned, not only about the orders of magnitude in  different ecosystems but also about the upper limits. Since the efficiencies of transfer seem low in terms of man – made machines, man has often thought that he could improve on nature by increasing the percent of transfer of light to food, and food to consumer. However, when we consider the low quality of available energy, the fact that organisms unlike machines are self – maintaining, and the need for storage and diversity for future survival than it turns that nature’s efficiencies are just about optimum. (for further discussion of this important point, see odum and pinkerton 1955; H.T.odum, 1971.

       At this point it would be well to make clear a few points about units, ratios, and efficiencies. Too often our thinking is clouded by people who, unintentionally or intentionally, use units that are not comparable in forming ratios and percentages. Biomass varies in energy value, both quantitatively and qualitatively. Plant biomass runs about 4.5 kcal per ash – free dry gram, and animal close to 5.5 where energy is being stored, as in seeds or in bodies os migrating or hibernating animals; the values approach 7 pr 8 kcal. If plant matter is mostly cellulose and lignin (wood), its energy (4 kcal/g) is unavailable to most animals. The main point to remember about ratios and percentages when used as estimates of efficiency is that they should be dimensionless; that is, the same unit should be used for both the denominator and numerator of the ratio. This, it should be calories/ calories not calories/grams. Otherwise comparisons may not be valid. For example, poultry or catfish farmers may tell you they are getting one pound of meat for every two pounds of feed, implying a 50 percent efficiency of transfer. However, since the food Is highly concentrated, dry material worth 5 or more kcal/g, and the meat s ‘’wet weight,’’ worth only about 2 kcal/g, then the real energy out/ energy in efficiency is more on the order of 20 percent.

The relationship between the ‘’boxes’’ and the ‘’pipes’’ that is, between standing crops and the energy flows P, A, or I is of great interest and importance. As we have seen, the energy flow must always decrease with c=each successive trophic level. Likewise, in many situations, the standing crop also decreases. However, standing crop biomass is much influenced by the size of the individual organisms making up the trophic graphic group in question. In general, the smaller the organism the greater the rate of metabolism per gram of weight. This trend is often known as the inverse size – metabolic rate ‘’law’’. Consequently, if the producers of an ecosystem are composed largely of very small organisms and the consumers are large, the standing crop biomass of consumers may be greater than that of the producers even though, of course, the energy flow of the latter must average greater (assuming that food used by consumers is not being ‘’imported’’ from another ecosystem). Such a situation often exists in marine environments where the water is moderately deep: bottom – dwelling invertebrate consumers (clams crustaceans, echinoderms, and so on) and fish often outweigh the microscopic phytoplankton on which they depend. By harvesting at frequent intervals, man (as well as the clam) may obtain as much food (net production) from mass cultures of small algae as he obtains from a grain crop harvested after a long interval of time. However, the standing crop of algae at any one time would be much less than that of a mature grain crop.

To reiterate, standing crop is a measure of the amount of living material present at a particular time. Productivity is a rate to be expressed as energy flow per unit area per unit time. As indicated by the examples, these two quantities should not be confused; the relationship between the two depends on the kind of organisms involved. Standing crop can be used as an index of productivity only if production accumulates unused, as in a crop where harvest is deferred until the end of the season. If growth is used as fast as it is produced (as in a grazed pasture). Then standing crop cannot be used to estimate productivity.

       We are all aware that the kinds of organisms to be found in both rural and urban areas in a particular part of the world depend not only on the local conditions of existence – that is, hot or cold, wet or dry – but also on geography. Each major land mass as well as the major oceans have their own special fauna and flora. Thus, we expect to see kangaroos in Australia but not elsewhere; or hummingbirds and cacti in the new world but not in the old world. And the different continents are the original home of different races of human beings and different kinds of domesticated plants and animals. The fascinating story of adaptive radiation is considered in more detail in other volumes of the modern biology series that deal with animal and plant diversity. From the standpoint of the overall structure and function of ecosystems, it is important only that we realize that the biological units available for incorporation into system vary with the geographical region. The word taxa is a good term to use in this connection when we wish to speak of orders, families, genera, and species without wishing to designate a particular taxonomic category. Thus, we can say that both local environment and geography play a part in determining the taxa of an ecosystem. As already indicated, the type and level of energy plays an important role in determining the kinds as well as the numbers of organisms present. As will be discussed later, the biotic community itself may play an important role in this regard.

       What is not always so well understood is that ecologically similar, or ecologically equivalent, species have evolved in different parts of the gloge where the physical environment is similar. The species of grasses in the temperate, semiarid part of Australia are largely different from those of a similar climatic region of north America, but they perform the same basic function as producers in the ecosystem. Likewise, the grazing kangaroos of the Australian grasslands are ecological equivalents of the grazing bison (or the cattle that have replaced them) on north American grasslands since they a similar functional position in the ecosystem in a similar habitat. Ecologists use the term habitat to mean the place where an organism lives, and the term ecological niche to mean the role that the organism plays in the ecosystem the habitat is the ‘’address’’ so to speak, and the niche is the ‘’progression’’. Thus, we can say that the kangaroo, bison, and cow, although not closely related taxonomically, occupy the same niche when present in grassland ecosystems.

       In recent years professional ecologists have become intensely interested in quantifying the ecological niche in terms of a set of conditions within which each kind of organism can operate (the fundamental niche) or does operate (the realized niche). In this manner ‘’niche width’’ and ‘’niche overlap’’ between two or more kinds of organisms can be compared. The reason for such interest stems from the discovery that the way in which taxa divide up available space, energy, and resources has a profound influence on the evolution of structure and behavior and on the origin and extinction of species. We will touch but briefly on these matters in this book, but if you wish to read more we suggest you start with the review by Whittaker, Levin, and root (1973) and the book entitled geographical ecology by MacArthur (1972)

Man, of course, has a considerable influence on the taxonomic composition of many ecosystems, not only urban but remote ones in which he may but a minor inhabitant. We might think of his efforts to remove or introduce species as a sort of ecosystem surgery; sometimes the surgery is planned, but too often it is accidental or inadvertent where the alteration involves the replacement of one species with another in the same niche, or the filling of an unoccupied niche, the overall effect on the function of the ecosystem may be neutral or beneficial. Thus, when Midwestern prairies were converted to agricultural fields the native prairie chicken was unable to adapt to the altered environment but the introduced ring – necked pheasant, which had become adapted to the agro – ecosystem in Europe (partly, at least, through artificial selection by man) has thrived in the altered in the altered landscape. As far as the hunter is concerned the ‘’game bird niche’’ has been more than adequately filled by the introduction. Too often, how – ever, the introduced species become pests, creating serious environmental problems. Especially grave problems often result with domesticated plants and animals ‘’ escape’’ back to nature and become severe pests because of the absence of both artificial or natural controls. Damage caused by weeds and feral2 animals to crops, watersheds, forests, and lakes can be extremely costly in terms of diverting energy away from human use. on some of the Hawaiian islands, feral goats have had a more severe impact on soil, and fauna that has man’s plow and bulldozers. Detrimental impact by man on his environment is not confined to industrialized societies nor to the twentieth century. overgrazing and other types of overexploitation of solar – powered nature have contributed to the downfall of many early civilizations.

        Species vary greatly in the rigidity of their niches. Same species may function differently that is , occupy different niches – indifferent  habitats  or  geographical  regions. the  case of the coral, as discussed in the previous section, is probably a good illustration. Man, himself, is another good example. In some regions man’s food niche is that of a carnivore (meat eater), while in other regions it is that of a herbivore (plant eater); in mosts cases man is omnivorous (mixed feeder). Man’s role in nature, as well as his whole way of life and cultural development can be quite different according to the major energy source on which he depends ofr food.

         Species vary, of course, in the breadth of their niche. Nature has its specialists and its generalists. There are insects, for example, that feed only on one special part of one species of plant, other species of insects may be able to live on dozens of different species of plants. Among the algae there are species that can species that can function either as autotrophy  or as heterotrophy; other species are obligate autotrophy only. Although more study is needed, it would seem that the specialists are often more efficient in the use of their resources and, therefore, often become very successful (that is, abundant) when their resources are in ample supply. On the other hand, the specialists are more vulnerable to changes, such as might result from marked environmental or biological upheavals or the exhaustion of the resource. Since the niche of nonspecialized species tends to be broader, they may be more adaptable to changes, even though never so locally abundant. Most natural ecosystems seemto have a variety of species, including both specialists and generalists.