The pollution of environment and over-exploitation of natural resources have become so extensive as to threaten the existence of entire life on this planet. Changes brought about by the unscrupulous action of man can be summed up in the following two major categories:
1. Steady impoverishment of biological systems
(a) Reduction in ecosystem complexity and diversity
(b) Reduction in genetic diversity
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2. Changes in global bio-geo-chemical cycles.
(1) Steady Impoverishment of Biological Systems :
An important consequence of the rapid rise in man’s population has been an ever rising demand for space. Human habitations are fast expanding and encroaching upon natural systems. Forests are cleared, grass-lands invaded, hill tops leveled, marshes drained and even land under water is reclaimed to provide space for human establishments.
The soil is bared, flora and fauna exterminated, natural ecosystems are destroyed and are replaced by such artificial system as agriculture, horticulture, animal farms etc. or else a jungle of steel and concrete structures comes up in place of lush green vegetation. These activities tend to reduce biological diversity of the system.
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(a) Reduction in Ecosystem Complexity and Diversity :
Man has been oversimplifying the structural complexities and diversity which occur in a natural undisturbed ecosystem. Agriculture for example is just an applied management of trophic chain. The complicated, many tiered trophic structure of a natural system is reduced to two links; primary producer and man. Man sets up an artificial agricultural system, protects the biomass which develops in his fields from pathogens, insects, birds and other herbivores.
All this increases the efficiency of production as the energy needed for maintenance of a simple structure is much lower than the complicated ones. However, simple trophic structures are more vulnerable to catastrophic changes.
An unforeseen host-specific pathogen could reduce or eliminate the single species which happens to constitute the entire population of producers. In a complicated ecosystem, with each trophic level composed of a number of species, several alternatives are available for the energy to flow, for materials to circulate in the system which is thus maintained in operative state.
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Man also tends to oversimplify microbial community present in soil or water bodies. Use of chemical fertilizers and intensive agriculture lead to depletion of organic material on which microbial populations depend for nutrition while insecticides washed down to the soil kill susceptible organisms directly. Sewage, waste-waters from industries and surface runoffs from agricultural fields pollute aquatic systems in which susceptible organisms are suppressed. The resistant ones survive and build up large populations.
The diversity of animal communities is also adversely affected by human activity. The complex ungulate fauna of Africa and India, which consisted of many species earlier, has been drastically affected. Man has taken a few species, domesticated and protected them to form huge populations. Those species which are left out are finding it increasingly difficult to cope up with the stresses imposed by human activity around them.
The overall effect of this is the replacement of diverse community with a community consisting of few species only which man chose to domesticate. This has lead to a severely damaging effect on grass-lands, forests and has caused outbreaks of animal diseases which have often acquired dimensions of an epidemic.
(b) Reduction in Genetic Diversity :
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The process of reduction in genetic diversity has also been taking place along with oversimplification of natural ecosystems. A number of plant and animal species have become extinct or are facing extinction. Evolution of new taxa and extinction of ill-adopted ones have been taking place in balanced way ever since life came into existence. However, the strain placed by human activity has speeded up the process of elimination of weaker species. This is a sad state of affairs. Each extinct species takes away with it a combination of traits or gene-pool which took millions of years to evolve. The loss is irreparable. (Mayer, 1979).
To date about 150 species of plants are cultivated on large scale. Of these 150 species only 29% provide more than 90% of the total food stock available to the mankind. Man has slowly and consciously been reducing the number of plants and animals which he breeds. Selection of varieties for better output, resistance to environmental conditions, better taste and flavours etc. has led to a drastic reduction in genetic diversity of plants and animals.
The varieties or species in which he is no longer interested are vanishing one after the other. In 1970, only two varieties provided 70% of American wheat harvest. Only four varieties provided 72% of the entire potato harvest in United States in 1980. About 73% of the total apple production in France consisted of North American varieties in 1980, of which 71% was the famous Golden variety alone (Sasson, 1987).
The reduction in genetic diversity of plants cultivated on large scale naturally places restrictions on the possibilities of creating new varieties through breeding programmes. A more immediate consequence of growing genetically uniform plants or livestock is their increased vulnerability to unkind environment or unforeseen pathogens.
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The entire crop or livestock population could be reduced by unkind weather or a pathogen, as being genetically uniform, the entire population tends to behave in the same way. For example, the Bezoskaja winter variety of wheat which was grown over an area of about 15 million hectares during successively mild winters in Russia was destroyed during 1972 by a single spell of severe winter. In 1970, American farmers lost millions of dollars in maize harvest as their genetically uniform crop was attacked by a virulent fungus. For many in Russia as well as in America this brought almost a famine like condition.
(2) Changes in Bio-Geo-Chemical Cycles :
Ruthless exploitation and pollution of environment has disturbed the operations of all important bio-geo-chemical cycles. The magnitude of waste materials has been growing persistently. These wastes or their decomposition products are regularly added to various components of the environment in significant quantities to disturb even the global cycles.
Man has been extracting substantial quantities of materials which represent biological output of past ages, at a rate much faster than the rate of their formation. So are our deposits of various mineral elements and metals, good sources of which are being depleted at a very fast rate. These resources belong to our children and grand-children as much as they do to us. The policy of reckless exploitation of natural resources is one which our future generation will rightly deplore.
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In global ecosystem carbon occurs as carbon dioxide in the atmosphere, as organic compounds in plants and animal bodies, in coal and petroleum deposits and as inorganic carbonates in water, rocks, shells and testes etc. Photosynthesis brings carbon of the atmosphere into the biotic pool while respiration and decomposition of organic matter adds most of this element to the atmosphere.
A part of carbon of biotic reservoir may be fixed in coal, petroleum and carbonate deposits which constitute the biological output of carbon cycle. Some carbon dioxide may dissolve into water from the atmosphere and enter the hydrosphere while inorganic carbonates in aquatic system maybe precipitated and turned into limestone. It depicts a highly simplified carbon cycle.
Human activity has led to an enhanced rate of input of carbon into the atmosphere which has caused a measurable rise in the concentration of atmospheric carbon dioxide. This is due to increased use of organic matter, coal, petroleum and natural gas as fuel and the combustion of carbonate rocks for manufacture of cement and lime.
Rapidly growing human population is also modifying natural ecosystems over an increasingly larger area of earth’s surface by extensive deforestation, faulty agricultural practices, intensive grazing etc. while the universally distributed poisons like Lead and DDT tend to depress the photosynthetic efficiency of green plants on global scale.
So while there is more input of carbon dioxide in the atmosphere its output is somewhat reduced. No wonder between the years 1880 and 1960 the total mass of carbon in earth’s atmosphere rose by 12- 14% of which about 150 x 1012 kg has been introduced as a result of burning of fossil fuels alone.
A small rise in concentration of carbon dioxide in the atmosphere shall have no effect on plants and animals. In fact it could be beneficial to plants as photosynthesis should go up. However, carbon dioxide is a greenhouse gas and absorbs infra-red and heat waves effectively.
A high concentration of carbon dioxide in the atmosphere acts like big blanket around the globe which obstructs loss of heat from earth’s surface. It will cause an effect like that of a greenhouse in which the glass- enclosed space gets heated up due to its insulation from the outside environment.
This effect is referred to as Glass-house effect or simply as Global warming. The heat trapped in earth’s atmosphere could melt many deposits of ice on our planet causing a rise in mean sea level which in turn could change the entire map of the world.
Of more immediate concern is the high output rate of carbon locked in deposits of coal, petroleum and natural gas. The estimated rate of formation of these deposits is about 13 gm per sq. metre per year, while we are consuming them at a rate about 450 times faster than the rate of their formation naturally. This means that in near future we shall have to go without coal, natural gas and petroleum at all. We shall have to bring down the rate of consumption of fossil fuels to the level of its natural rate of production, which is an impossible task or else we shall have to look for alternative sources of energy.
2. The Oxygen Cycle
Oxygen constitutes about 45% of earth’s crust by weight and its cycle is a complicated one because of the innumerable chemical combinations in which it occurs on our planet. Atmospheric oxygen is a major reserve of this gas, in combination of hydrogen it occurs in water, with carbon it occurs in carbon dioxide, carbonates and bicarbonates etc. It forms an important constituent of organic matter. During photosynthesis water molecules dissociate to form oxygen which is fed into the atmosphere from where it is drawn in aerobic respiratory activity of living organisms to yield water molecules again. A number of other oxidation reactions require oxygen. A highly simplified diagram of Oxygen cycle.
Apart from being an essential requirement for all aerobic organisms, oxygen has played a very important part in evolution of life on this planet. It has given rise to ozone umbrella which protects terrestrial habitats from harmful solar radiations. Early life occurred in water only as there was no effective ozone concentration in the atmosphere which could prevent biocidal ultraviolet radiations from reaching earth’s surface.
Since the level of oxygen in atmosphere determines the extent of ozone concentration, it was only when enough oxygen could accumulate in the atmosphere so as to form an effective ozone shield that life could emerge from water and colonizes the land. This happened about 440 million years ago, sometime in the Silurian era when the oxygen concentration in the atmosphere rose to about 1/10 of the present-day oxygen level.
Though photochemical dissociation of water in the stratosphere has been estimated to produce about 1.5 Gms of oxygen per metre sq. per year, the main process responsible for production of oxygen is photosynthesis which has been estimated to produce about 260 gm of oxygen per sq. metre per year.
Almost an equal amount of oxygen is consumed in respiration of animals and other organisms. Oxygen is also consumed in reactions involving oxidation of a large number of substances such as CO, Fe, H2, CH4 etc. but the precise quantitative significance of these reactions is not well understood.
The quantity of oxygen required for burning coal, petroleum and natural gas has been estimated to be 10-11 gm per sq. metre per year and this has no detectable effect on the global concentration of this gas. Without any replacement it should take about 230,000 years to use up all the atmospheric oxygen at the present rate of its consumption.
Of a more immediate concern is the concentration of ozone in the atmosphere. In lower atmosphere, ozone is harmful to both plants and animals and causes photochemical or oxidizing type of air pollution. But high up in the stratosphere its presence is essential as it absorbs harmful ultra-violet radiations and provides a protecting umbrella to the biosphere below.
Ultra-violet rays split up oxygen molecules to produce oxygen atoms which combine with molecular oxygen to form ozone. Photolytic dissociation of ozone also occurs naturally and its formation and dissociation balance each other. However, we have been introducing a variety of such materials in the upper atmosphere which accelerate the dissociation of ozone catalytically. This has the undesirable consequence of thinning out the protective ozone shield which could in the long run turn the geologic clock back to the period when earth wore very thin mantle of this gas and harmful ultra-violet radiations of the sun could reach right up to earth’s surface. Terrestrial life shall be drastically affected.
Nitrogen, an essential element for all living organisms has a more complicated cycle. The main reservoirs of nitrogen are rock deposits, atmospheric air and the living organisms. Biological nitrogen fixation carried on by some bacteria and blue green algae brings the nitrogen of the atmosphere into soil or aquatic bodies from where it is taken up in the biosphere.
Abiological nitrogen fixation also provides some nitrogen to the living system. Decay and decomposition of plant and animal wastes and dead organic matter yield ammonia which is converted to nitrite by bacteria, Nitrosomonas and then nitrates by the activity of bacteria like Nitrobacter. The process of denitrification, brought about by the activity of bacteria such as Pseudomonas, Thiobacillus, Micrococcus denitrificans etc., converts nitrates to elemental nitrogen which is finally added to the atmospheric pool.
It should be obvious that the cyclic flow of nitrogen in an ecosystem involves a precise balance of activity of a few species of bacteria so that adequate level of nutrients is maintained without excessive accumulation of inorganic and organic compounds of nitrogen which are, beyond a certain concentration, harmful to a living system. Life on this planet could be drastically affected if only a few dozen species involved in nitrogen cycle are eliminated.
Of particular interest are the oxides of nitrogen which are responsible for causing environmental degradation. Nitrous oxide is produced microbially by oxidation of organic nitrogen and by the reduction of nitrates both in the soil and in waters. This gas is readily absorbed by soil. However, some of it may be turned into nitric oxide by the action of ultra-violet radiations.
Nitric oxide is more reactive gas which is also produced by microbial oxidation of ammonia, combustion of coal, oil and organic matter and by lightning in the atmosphere. Though this gas is not very harmful to plants and animals, it is a source of several more harmful pollutants such as Peroxyacetyl nitrate (PAN) and nitrogen dioxide which forms nitrous and nitric acids in the atmosphere and thus contributes to acid rains. Nitrogen dioxide is a constituent of oxidizing type of air pollution and is at least partially involved in ozone depletion in the stratosphere.
Phosphorus plays an essential role in almost every step of organic synthesis. This element is morfe abundant in living organisms than in abiotic systems. The main reservoirs of phosphorus on earth are living organisms and relatively insoluble calcium phosphate deposits in rocks and sediments. Phosphorus is solubilized from its deposits under conditions of low pH and is taken up by plants which pass it on to the food chain.
Plant and animal secretions and decomposition of dead organic material bring it back to the surrounding medium. A part of this phosphorus is leached away to oceans through rivers where it forms marine deposits being available to fishes and marine birds which convert it to phosphate rocks, guano-deposits and bone deposits as relatively insoluble tricalcium phosphate.
Therefore, only a little amount of phosphorus returns to land from ocean through fishes and guano birds while much of it is lost to relatively deep sea deposits. Fig. 2.5 depicts a highly simplified phosphorus cycle.
Our understanding of the rate of transfer of phosphorus between various components of the cycle is very poor. It is likely that only up to 1900 A.D. phosphorus cycle was in a balanced state in which the rate of its solubilization was almost equal to the rate of its transfer through rivers to oceans. However, rapid soil erosion has greatly enhanced the rate of loss of phosphorus from soils to the sea. Hence, phosphorus has to be added to agricultural fields to sustain productivity.
The industrial production of phosphorus overtook its natural rate of cycling around 1910 A.D. and about 70- 80% of the industrially produced phosphorus is ultimately applied to agricultural fields. The remaining phosphorus is usually used in water softening or in detergents and soaps which are discharged directly into fresh water systems or oceans wherein they cause nutrient enrichment, excessive algal growth etc., e.g., they accelerate eutrophication.
The practice of burning dead plant material and cow-dung cakes instead of using them as natural manures cuts short the natural cycling and releases the primary nutrient rather in one stroke without providing the much needed infrastructure – the humus – to the soil which is helpful in retention of mineral nutrients.
The released phosphorus is quickly leached away by running waters. Poor yields and subsequent degradation of agricultural land are the major consequences of the practice which has been prevalent in many Asian countries including India.
Major reserves of sulphur on earth’s crust are deposits of sulphide and sulphates while as organic sulphur it occurs in bodies of all living beings. Plant and animal secretions and decomposition of dead organic matter yields hydrogen sulphide gas which is converted to sulphates by bacterial action. Plants take up sulphur as sulphates.
Bacterial action may turn some sulphates to hydrogen sulphide which is changed to sulphates again. A simplified sulphur cycle is presented in Fig. 2.6. Sulphur in coal and petroleum yields sulphur dioxide gas upon combustion.
A number of metals occur as sulphide or sulphate deposits which during their mining and processing operations give rise mostly to sulphur dioxide gas. Sulphur dioxide is changed to corresponding acid which finally yields sulphates which are ultimately added to the sulphate content of environment. Some sulphur is also added to the soil or to aquatic systems as a result of volcanic activity.
There are two major factors contributed by human beings which have disturbed the natural sulphur cycle. Combustion of fossil fuels yields an enormous amount of sulphur dioxide. It is the injection of sulphur into the atmosphere which has been causing great concern. A lot of sulphur is added to the atmosphere as a result of mining and processing operations during extraction of metals.
Volcanic emissions which are the only known natural source of sulphur dioxide, have been estimated to contribute 2 x 109 kg to 5 x 109 kg of sulphur per year, while the annual amount of industrial sulphur injected into the atmosphere is thought to be about 83 x 109 kg. Sulphur dioxide is a serious local pollutant with a wide range of harmful effects. This gas is mainly responsible for causing acid rains.
Rapid exploitation of sulphur deposits is another feature which causes anxiety. These deposits are believed to have been produced by the activity of anaerobic bacteria (Baggiota sp.) from hydrogen sulphide gas. We are consuming these deposits faster than they are formed naturally. A day may come when there will be no good deposits of sulphur left for human use.
6. Bio-Geo-Chemical Cycles of Trace Elements :
In addition to major elements which compose the bulk of living organisms, there are a number of trace elements which also circulate in an ecosystem. Some of these are essential ones, some may enter a living being as a substitute for essential ones, while there are some elements which are simply drawn in and circulated in the biosphere along with the general flow of materials.
They are released in more soluble state in the soil as a consequence of weathering of the parent rocks from where they move to aquatic bodies as well. Primary producers take them up and pass them on to higher trophic levels in the food chain. Exudates of living organisms and products of decomposition of dead plant and animal bodies bring them back to the soil.
A part of these elements are leached away with rapidly flowing waters to rivers and then to see where they are finally laid down in sedimentary deposits. Some of these elements may also combine with organic molecules to form volatile organ metallic compounds and contaminate the atmosphere as well.
It is mainly the human activity which has disturbed the natural cycles of these elements. Important ones which are causing environmental problems these days are lead, mercury, cadmium, chromium, arsenic, zinc and nickel etc.
Industrial production of most of these metals has already exceeded their natural rate of circulation in global systems. Coal and petroleum crude is a complex mixture of a number of such elements. Combustion of coal, petroleum and their mishandling during processing, transportation and storage etc., release these elements in a much higher concentration than those attained during their natural cycling.
We are exploiting the deposits of these elements at a much faster rate than they are being laid down. Obviously there may come a time when there are no good quality, economically viable deposits left for our use. Unlike the consumption of world’s stock of fossil fuels, metals are not irreversibly consumed – instead they tend to be scattered and broadcast in diluted state by human activity. In the process these elements may get accumulated at places in sufficient concentrations to be harmful to a biological system as has been the case of mercury, cadmium and lead.