Lindeman (1942) was the first to propose the community energetic approach (otherwise called the tropic-dynamic model) to ecology which enables an investigator to compare the relative rates at which different kinds of ecosystems convert solar energy into chemical form. Useful information concerning energy flow through forest ecosystems has been obtained by the application of this kind of approach, e.g.
, by comparing ratios of leaf fall to litter deposition on the forest floor. It has been found that the rates of leaf production are higher and those of litter accumulation lower, in the tropics than at higher latitudes. In order to understand the trophic structure of any community it is necessary to trace the pathways that energy follows through the food-web. The basic unit of the trophic structure is really the individual link of the food- web. The food ingested by an organism follows the various pathways shown. Some parts of the ingested food, e.g., cellulose and lignin, or hairs, feathers and connective tissue cannot be digested and hence are ejected or discarded either as such or after only partial digestion.
The food energy actually assimilated is mostly available for metabolic and other needs of the organism but some part of it is usually always lost as heat. Significant amounts of proteins eaten by animals may also be excreted by them in the form of ammonia, urea or uric acid. Such excretion products can then be utilized by certain detritus feeders. That part of the ingested energy not lost through excretion and respiration is consumed in the synthesis of biomass during growth and reproduction. Of course, some part of this new biomass of a population can again be lost through the accidental death of individuals and the remainder may be eaten by predators.
The dead bodies upon decay can in turn contribute to the detritus pathways, and most detritus feeders are themselves subject to predation by other organisms whereby some transfer of energy from the detritus web to the predator web occurs. Both in terrestrial and aquatic ecosystems, the movement of animals aids the spatial redistribution of nutrients and influences the growth and performance of other organisms. Grazing animals may feed in one terrestrial ecosystem and defecate in another, resulting in transference of nutrients from one ecosystem to another. A similar process is known to operate in coral reefs and probably occurs in the flow of energy through a community obviously depends on the production efficiencies of its biota. It is also determined by the efficiency with which production at each trophic level can be converted into production at the next higher trophic level. However, in order to understand this so-called ecological efficiency of the food-web, the web must be broken up into its component links (see Fig.
4.3). Ecosystem production characteristics depend inter alia on its carbon metabolism. Woodwell and Botkin (1970) have defined these various characteristics as follows: Gross Primary production, GPP = Total photosynthetic C fixation Autotrophic Respiration, RA = GPP—NPP Net Primary Production, NPP = GPP—RA Heterotrophic Respiration, RH = Respiration of consumers and decomposers Ecosystem Respiration, Rg= RA+RH Net Ecosystem Production, NEP = GPP—RE The three major steps in energy flow correspond to (a) exploitation efficiency; (b) assimilation efficiency, and (c) net production efficiency. Gross production efficiency is given by the product of the assimilation and the net production efficiencies, i.e., by the fraction of the eaten material eventually transformed into consumer biomass. Similarly, the entire food-web may be taken to be the product of the gross production efficiency and the exploitation efficiency.
The various kinds of energetic efficiencies can be defined as follows: Exploitation efficiency = Ingestion of food/prey production; Assimilation efficiency = Assimilation/ingestion; Net production efficiency = Production/assimilation; Ecological efficiency = Exploitation efficiency x Assimilation efficiency x Net production efficiency; = Consumer production/prey production; Gross production efficiency = Production/ingestion. In animals, rate of production appears to depend on body mass. Per unit body mass, small animals are more productive than big animals. Also invertebrates are less productive than mammals. Molluscs, annelids, isopods, and insects ace invertebrates of intermediate size between copepods and echinoids. Determination of inter level energy transfer efficiencies in plankton food chain is of great value in understanding the dynamics and energetic of aquatic ecosystems.
Studies done under the IBP has indicated that there is no quantitative relationship between the production of a certain trophic level and the production of the next lower trophic level (both in calorific terms) except for the very high or very low values of the former. This is applicable to the “phytoplankton-filter feeders” as well as “filtrators-invertebrate predators” trophic links in the plankton food chain. The utilization of primary production in pelagic zone very often depends on the nature of dominant species of producers and consumers. Thus in a system containing nannoplanktonic algae-macroconsumers (e.g.
, calanoids, cladocerans), effective utilization occurs mostly via grazing. On the other hand, in the case of larger algae (colonial forms, dinoflagellates, cyanophytes) and smaller consumers, primary production is mainly utilized via bacterial- detritus medium. It has also been observed that the energy transfer efficiency from the filtrators trophic level to their invertebrate predators is often higher than from phytoplankton to filtrators. Plankton eating fishes have been found to be generally less efficient convenors of energy than the invertebrate predators and it seems that the fish are unable to completely utilize total biomass production of zooplankton. Of course, fishes do play a significant role in the trophic dynamics of aquatic ecosystems. In specialized cases of pond culture, the correlation between primary production and fish yield can be quite high, but it is often much lower in natural waters and reservoirs. In pond culture, greater species diversity commonly results in more effective utilization of the primary-secondary food supply for fish production. In many tropical lakes, Tilapia has been reported to be an efficient consumer of phytoplankton but this and other tropical fishes are also frequently omnivorous and opportunists in food habits and this factor may also be responsible for the observed efficiency at least in part.
Another general attribute of most fishes is their unusual growth plasticity; this confers some adaptive advantage in relation to food scarcity. Energy flow patterns are influenced by types and abundances of consumer organisms in aquatic and terrestrial ecosystems. Herbivorous and detritivorous consumers can often exert greater influence on ecosystem dynamics through transformation and translocation of nutrients than by transformation of energy (Chew, 1974). In grasslands and forests, detritivores frequently are quite important for the recycling of nutrients from decomposing litter (Woodmansee and Duncan, 1980). Herbivores appear to stimulate primary production through enhancement of nutrient cycling in grasslands, forests, and lakes.
Comparable studies in streams have shown both stimulation (Murphy, 1984) and reduction (Summer and Mclntire, 1982) of primary production per unit chlorophyll due to grazing.