Food Chain, Energy Flow and Ecological Balance

The below mentioned article provides an overview on the food chain, energy flow and ecological balance.

Any interference with ecological principles upsets the ecological balance among the organisms which populate the earth. Ecological balance, and in fact, the conti­nuity of life, depends on the coupling of energy-producing and energy-consuming processes.

Energy production is restricted only to a group of autotrophic organisms, many of which are endowed with the capacity of converting light energy into chemical energy. Some non-photosynthetic organisms also can oxidise reduced inorganic com­pounds and utilize the chemical energy liberated in the process. The ultimate source of energy, in all cases, is the sun.

Biomass Production:

Every minute, each cm² of the earth’s surface receives about 1.34 K Cal of solar energy. The process of photosynthesis, on which life depends, however, can utilize no more than 2.5% of this energy. As light intensity decreases with increase in the depth of water or as sunlight filters through the canopy of plants, the net effect decreases still further. Nevertheless, about 10¹¹ tons of carbons are fixed from the atmosphere per year.

The photosynthetic bacteria which are usually present in anaerobic environ­ments in aquatic sediments, where light intensity is low, fix small amounts of CO_s and N₂ from the surrounding environment. In many lakes the contribution of the photo- synthetic sulphur bacteria is no more than 5% of the total organic matter production per unit time; however, in places where there is a good supply of H₂S, naturally or through discharges of industries, about 25% of photosynthesis can be attributed to them.

The non-sulphur photosynthetic bacteria also make significant contribution. The cyanobacteria or the blue green algae are much more efficient. Oscillatoria is a major component of the Caribbean Sea plankton, comprising about 60% of the total chlorophyll in the upper 50 metres and is responsible for about 20% of the primary production.

The Oscillatoria formation has been estimated to fix about 1.3 mg of N₂ per m² per day. Primary productivity of our oceans is about 1000 K Cal/m²/yr, the total gross production being 10¹⁶ K Cal/yr. In coastal zones and in the estuaries and reefs the primary productivity is 2,000-20,000 K Cal/m²/yr. About 18-36 million tons of N₂ are also fixed in the ocean/yr. The blue-green algae also fix large amounts of nitrogen in association with plants like Azolla. In Eastern India Azolla has been found to fix as much as 37 mg N₂/cm²/day.

Terrestrial primary production has been estimated to be 57.4 xlO¹⁶ K Cal/yr, about half of which come from the evergreen forests in the tropical and subtropical regions and about one-fifth is contributed by grasslands and pastures. Total primary production/yr. is about 10¹⁸ K Cal.

About 175 million tons of N₂ are fixed by micro­organisms independently or in association with other organisms, particularly plants; the permanent grasslands contribute about one-fourth of this amount and forests and woodlands about one-fifth. A part of the carbon fixed is lost in respiration; a small amount may also be lost as ammonia and excreta. The difference of the two is the net biomass.

The carbohydrates, lipids and proteins are the major organic constituents of living organisms. CO₂ and N₂ of our atmosphere, through a series of metabolic reactions, pass through these compounds, which in addition to being responsible for the structure and form of almost all organisms, also provide energy for activities of the living forms, depending on the level of reduction of carbon and nitrogen.

Non-photosynthetic microorganisms like the sulphur-reducing or methane-produ­cing bacteria produce these compounds as products of anaerobic metabolism. Several microorganisms can also utilize H₂. Their overall contribution to energy economy of the biosphere, however, is not very large. The anaerobic microorganisms derive energy by oxidation of reduced compounds using compounds other than O₂ as terminal electron acceptors e.g., nitrate, sulphide, sulphate etc. Organic compounds may also be reduced as in fermentation reactions.

Water was used as a source of electrons first about 2.6 billion years ago and since then the atmosphere is becoming richer in O₂. When oxygen accumulated in sufficient concentration, mechanisms were developed for utilization of O₂ as the terminal accep­tor of electrons. This resulted in aerobic respiration accompanied by oxidative phosphorylation. ATP production in aerobic respiration is much higher than that in anaerobic respiration.

Since green plants are capable of both photosynthetic and oxidative phosphorylation, the autotrophic forms were more efficient for biomass production than the heterotrophic forms, even though the primitive living organisms were undoubtedly heterotrophic.

The aerobic environment encouraged the growth and multiplication of non-photosynthetic aerobic autotrophs like the nitrifying bacteria, the emergence of which made the operation of nitrogen cycle in nature possible and which together with carbon cycle are essential for continuity of life on earth.

Producers and Consumers:

The photosynthetic organisms are the primary producers and the neterotrophs are the consumers. In the lichens, which is an association between an alga and a fungus, the alga is the primary producer and the fungus the consumer; the algal cells however are not consumed by the fungi. The fungi grow parasitically or saprophytically and thus, are always consumers, although sometimes they may produce growth factors or hormones which may be beneficial for the autotrophic hosts.

When Anabaena lives symbiotically in the leaf cavities of Azolla, it does not photosynthesize and utilizes the organic compounds of the host plant for N₂ fixation. It is thus, a consumer; however, it returns much of the organic compounds in the form of nitrogenous substances to the host plant and so far as nitrogen nutrition is concerned it may be considered as a primary-producer.

In water-logged rice fields where Azolla multiplies rapidly, the products’ of decomposition of Azolla are utilized by the rice plant, which although, itself a primary producer, should be considered as a consumer when it receives the nitrogenous substances from Azolla, though indirectly.

In ponds and other aquatic environments, the blue-green and the green algae and the aquatic plants are the primary producers. The phytoplanktons are consumed by the zooplanktons, the zooplanktons by other small animals, which are utilized by fish and the fish may be consumed by man, birds or some other animals.

The goats feed on the grasses which are the primary-producers. Man becomes a consumer when he takes the mutton. When a man is killed by a tiger, the tiger is the consumer.

The larger is the number of organisms which intervene between the primary-producer and the ultimate consumer, the longer is the food chain. When a man eats rice or potato, the chain is shortest, when he eats fish or some animal product, the chain is longer. In the case of heterotrophic consumers also the food chain may be long.

Thus, protozoa may be preyed upon by insects, insects by bacteria and bacteria by Bdellovibrios and viruses. The viruses in this food chain are the ultimate consumers. In the food chain there may be interlocking i.e., cross utilization at several points, resulting in what is known as a food web!

In every step of the food chain, however, there is a loss of energy, which may be as much as 90%. With every passing step, there is a decrease in size and volume and increase in protein content.

Decomposers:

When organisms—-autotrophs or heterotrophs—die, microorganisms start decom­posing the corpse or detritus, as also the excreta in various ways. Plants are also known to leach or exude organic and inorganic matter.

Insects and other small animals help in splitting the larger particles into smaller ones. The degradation continues depending on the enzyme-makeup of the decom­posers. Many such decomposers produce hydrolases, which degrade cellulose, pectins, proteins, and lipids to simple monomers like sugars, amino acids, and organic acids etc., which are then metabolized rapidly by other microorganisms. In the case of plant remains, in many cases the fungi are the first attackers because many of them are active producers of enzymes which degrade cellulose, pectin and lignin.

However, the degra­dation is scarcely complete because aromatic substances resulting from degradation of lignin and other large molecular weight aromatic substances cannot be degraded and metabolized by most microorganisms. These undefined aromatic substances along with polysaccharides etc. constitute “humus”, which is so important for soil formation and its physical structure.

A good amount of nitrogen is also locked up in humus. Animal remains are degraded much rapidly. In the case of invertebrates it is quite rapid; in the case of vertebrates the bones are much difficult to degrade and they may be recognised in soil even after hundreds of years. The decomposers utilize the products of decomposition and are thus consumers. The CO₂ evolved and the heat generated provides a measure of decomposition.

Biogeochemical Cycles:

The ultimate products of degradation are usually small molecules, gases, oxides and salts. The CO₂ liberated in respiration or decomposition of organic matter returns to the food chain via photosynthesis, the primary producers and the consumers. The ammonia produced is usually oxidised to nitrate by the nitrifiers and denitrified by denitrifying bacteria to N₂ or N₂O. N₂ comes back to the biosphere through N₂ fixation by bacteria, freely or symbiotically with other plants. H₂S is oxidized to S° or SO”₄ which being solids are retained in the biosphere. The phosphates present in plants or animal bones are slowly solubilized by phosphate-solubilizing bacteria and recycled through plants, animals and microorganisms. Similar cycles also exist for Mn, Fe, and other minerals.

The turnover time depends on the nature of the compound being recycled. In the case of gases it may be of the order of a million years or more.

The Consequences of Interference with the Ecological Balance:

Human activity is interfering seriously with the operation of these—cyclic processes endangering the life of man in the distant future. Because the rate of photosynthesis is higher than the rate of respiration, there has been an accumulation of photo-synthetically evolved O₂ in our atmosphere.

Its utilization in respiration is preventing the attainment of high oxidation levels. The atmosphere has changed from a reducing to a moderately oxidizing one to which the primitive and more evolved forms of life have adopted themselves. Industrial processes are consuming considerable O₂ and producing large amounts of CO₂. By 1959 CO₂ concentration in our atmosphere in­creased by 13% and by the year 2000 the increase may be as much as 25%, which in turn will increase the temperature of many places affecting the life of various orga­nisms.

Alteration of CO₂ concentrations had profoundly influenced the climate in the geological past. Application of heavy doses of nitrogenous fertilizers and the use of jet aircrafts are enhancing the concentration of nitrogen oxides in our atmosphere, which in turn is depleting the ozone shield of our atmosphere.

The excessive appli­cation of fungicides and insecticides etc. has now come to a point that several species have totally disappeared from certain places and the resistant forms, which have gradually become selected, are now becoming predominant. Every man now carries about 21 mg of DDT in his adipose tissues; even in the breast milk of mothers, the concentration is not far from the value lethal for the babies.

Malarial parasite which was controlled by DDT is now reappearing with vengeance, being resistant to many insecticides. Excessive use of antibiotics has selected the drug resistant forms to an extent that unless new and more powerful drugs are discovered continually, disastrous epidemics may occur in the near future.

Extensive deforestation has been suggested to be the cause of decrease in rainfall in several places; attempts to convert arid areas into green belts by irrigation etc. have also been suspected to change the location of low pressure areas resulting in diversion of clouds to other regions. The prudence of the introduction of crops and other plants to distant places has also been questioned.

Some of the high-yielding varieties of crops developed in Philippines or Mexico have ousted the local low-yielding disease resistant cultivars and the consequences may well be disastrous, since the introduced varieties are not always resistant to the indigenous pathogens. Along with the food grains imported from foreign countries, we have also “imported” weeds like Parthenium and several pathogens which have already started causing serious problems in many places.

Parthenium is spreading at a fast rate changing the composition of the weed flora and causing considerable health hazards. Man has to pay heavily for upsetting the ecological balance. He still has a long way to go if he has to understand the ways of nature and what she has up her sleeves, if she is disturbed.