In the ecosystem, autotrophic organisms (chiefly the green plants) use the energy of solar radiation to produce organic matter, which is used by all the living organisms, including autotrophic organisms themselves, in running their life activities. The organic matter is used by the living organisms through their respiratory activity. Out the total organic matter produced by photosynthetic activity of autotrophic organisms, a certain portion is consumed by these organisms themselves and the remaining organic matter is available in the ecosystem as the net organic matter production of autotrophic organisms. A relatively very small part of this net organic matter production in the ecosystem is directly transformed into mineral substances. This takes place without the participation of any other living organisms through the processes such as forest and prairie fires during which organic matter is transformed into carbon dioxide, water vapor and certain mineral compounds. Further, a still smaller portion of organic matter is deposited in the upper layers of lithosphere and at the bottom of water bodies in the form of coal, peat and other organic compounds. The remaining organic matter is now passed on to heterotrophic organisms in the ecosystem through various food chains. All the living organisms of a particular type in the ecosystem that receive organic matter as food in a particular manner constitute a trophic level. The organic matter received by a trophic level undergoes three fates:
A portion is consumed by that that trophic level itself though respiration in that trophic level
A certain other portion is passed on to next higher trophic level as organic food and
Remaining organic matter is stored in the trophic level as increase in the biomass of that trophic level (i.e. increase in the number of organisms of that trophic level).
From the point of view of ecosystem energetics, the organic matter that is received, passed on to next higher trophic level or stored by a trophic level represents the amount of energy received, passed on or stored by that trophic level. It is obvious that in a dynamically stable ecosystem, there can not be any storage of energy (i.e. organic matter) in any of its trophic levels. Therefore, in the dynamically stable global ecosystem, a very small portion of the net production of organic matter by autotrophs is stored in the abiotic components of the environment (i.e. lithosphere and hydrosphere) while major portion is consumed by heterotrophic organisms through their respiration.
The consumption of organic matter in a trophic level (including autotrophic organisms themselves) through the respiration in that level represents the loss of energy in that trophic level. It is a feature of global ecosystem that the flow of energy (represented by flow of organic matter as food) between trophic levels is associated with large losses of energy at each trophic level. The ratio of the amount of energy passed on from a trophic level to its next higher trophic level (n) and the amount of energy received by that trophic level from its previous trophic level (n1) is termed ecological efficiency () of that trophic level i.e.
Ecological efficiency () = n/n-1
The ecological efficiency of trophic levels, in general, is estimated to range between 10-20%. Such small general value of ecological efficiency indicates that biomass in each successively higher trophic level in the ecosystem is bound to be substantially reduced. Since ecological efficiency of a trophic level depends on the respiration of that level, smaller the value of ecological efficiency of a trophic level, greater is the consumption of organic matter through respiration (i.e. loss of energy) at that trophic level. As a result, there is greater reduction of biomass in that trophic level and in the next higher trophic level.
Nature of organisms and transformation of organic matter
Since intensity of metabolism per unit mass of a live organism usually increases with decrease in the size of organism, the biomass present at a specific trophic level in the food chain depends on the size of organisms of that trophic level. One of the causes of this relationship is that the metabolism depends substantially on the ratio of the rate of diffusion of gases through the surface and the mass of organism. This ratio increases as the size of organism decreases. Thus the rate of metabolism of a given unit weight of microorganisms is many times greater than that of macro-organisms. Further, metabolism also depends on the nature of physiological processes within the tissues of organisms. In wood of plants, the metabolism is usually much slower than in vertebrate tissue of similar size. These general principals largely determine the total biomass of various types of organisms in the global ecosystem.
The largest proportion of forests in the overall biomass of living organisms is due to the fact that autotrophic trees are located at the first link in the food chains and also due to the large size of individual trees. Together with specific properties of the wood, this feature substantially reduces the rate of metabolism per unit biomass in forests. Though the productivity of ocean phytoplankton is comparable with forests, small size of individual plankton organisms intensifies their metabolism per unit weight so much that the total mass of plankton on Earth is negligible in comparison with that of forests.
About 95% of the total biomass on Earth belongs to plants and rest to the animals. Biomass of aquatic organisms is substantially less than that of terrestrial organisms. Therefore, the distribution global biomass is largely determined by the distribution of terrestrial plant cover i.e. by the forest cover on continents. Considering that total biomass on Earth (global biomass) is approximately 3×1012tonnes and total productivity of plants on continents is approximately 140×109tonnes, the time period of one cycle of organic matter for the plants on Earth comes to be approximately 20 years. This average figure relates to forests that constitute major portion of the biomass of plants on Earth. In other natural zones on continents, the duration of one cycle of plant organic matter is much shorter. The duration of this cycle in the oceans having phytoplankton is still shorter and appears to be only a few days.
The total biomass of animals is assumed to be approximately 1011 tonnes. Assuming that the animals assimilate about 10% of the total productivity of plants, the average duration of one cycle of animal organic matter comes to be several years. However, the actual length of life of one generation varies widely in animal kingdom and the nature of the distribution of biomass among different animal groups is still not much clear.
Invertebrates are the largest components of animal biomass and among them, most important are organisms living in soil. The zoological mass of large animals per unit area on Earth is relatively quite low. Calculations of Huxley (1962) show that while in African savannahs, the biomass of large wild animals may be 15-25 tonnes/km2, this figure is only about 1.0 ton/km2 in middle latitudes, 0.8 ton/km2 in tundra and 0.35 ton/km2 in semi-desert areas.
Man occupies topmost position in the food chain on the Earth and consumes both the primary production of autotrophic plants and the biomass produced by many herbivorous and carnivorous animals. For the present size of human population of over 4.0 billion, its biomass is approximately 0.2×109 tonnes. Assuming that each human being expends on average about 2.5×103 kcal of energy per day, the total energy consumption of human population comes to be about 1.8×1015 kcal/year. Thus, the human population consumes about 0.2% of the total production of Earth’s organic world.