Environmental Studies

Module 3. Ecosystem

Lesson 10
ENERGY FLOW IN THE ECOSYSTEM AND ECOLOGICAL SUCCESSION

10.1 Ecosystem Functioning

To understand clearly the nature of the ecosystem, its function must be thoroughly investigated. The function of the ecosystem is to allow flow of energy and cycling of materials which ensures stability of the system and continuity of life. These two ecological processes including interaction between the abiotic environment and the communities may be considered as the ‘heart’ of the ecosystem functioning. For the sake of convenience, the ecosystem dynamics may be analysed in terms of the following: (i) food chains, (ii) food pyramids, (iii) energy flow, (iv) nutrient cycles, (v) development and evolution of ecosystem, and (vi) homeostasis and stability of ecosystem.

10.2 Ecological Energetics

In ecological energetics one is mainly interested in the (i) quantity of solar energy reaching an ecosystem, (ii) quantity of energy used by green plants in the process of photosynthesis and (iii) the quantity and path of energy flow from producers to consumers. (Fig. 10.1)

In the earth’s atmosphere about 15 X 10⁸ calories m^-2 yr^-1 of solar energy is received (Phillipson, 1966). The fate of solar radiations upon its incidence on earth’s surface is shown in Fig. 10.2. About 34% of the solar radiations reaching the earth’s atmosphere is reflected back into space by clouds and the suspended dust particles in the atmosphere; 9% is further held by ozone, water vapour and other atmospheric gases. Remaining 47% reaches the earth’s surface. In fact, only 1 to 5% of the energy reaching the ground is converted by green plants to chemical energy, and 42 to 46% is absorbed as heat by ground, vegetation or water. Water budget showed that 45% of the incoming radiation was dissipated by transpiration of 370 t ha^-1 of water from the crop. The quantity of solar radiation received at any place not only depends upon the clarity of the atmosphere, but also on the latitude of the area. The equatorial region receives maximum solar radiation followed by other regions of the tropics. The quantity of energy goes on decreasing with increase in latitude both in the northern and southern hemispheres (Fig. 10.1).

10.2.1 Energy flow in the ecosystem

The behaviour of energy in ecosystem can be conveniently termed as energy flow because of unidirectional energy transformations. Total energy flow that constitutes the energy environment has already been dealt in detail, and now we take up the study of that portion of the total energy flow that passes through the biotic components of the ecosystem. Entrance of energy, its retention within the ecosystem and dissipation into space, are governed by two laws of thermodynamics. According to the first law, the law of conservation of energy, in a closed system, no energy comes in or escapes out and not created or destroyed but may be altered from one form to another. The second law of thermodynamics, the law of entropy, states that there is always a tendency for increase in entropy or degradation from a concentrated (non-random) to a dispersed (random) form leading to dissipation of heat. All the energy entering the earth’s surface can be accounted for. Some energy is used in photosynthesis; the rest is used in converting the water into vapours or heating the soil and air. Ultimately the energy reflected back to outer space as heat. The light energy fixed by green plants in the process of photosynthesis may be represented by the following equation:

Out of the amount of energy so fixed by green plants, some is released again in respiration. The fixed energy, in the form of food, then passes from plant source through herbivores to carnivores. At each stage of food transfer, potential energy is released, resulting in further loss of a large part of energy. The energy flow, thus follows the second law of thermodynamics.

10.3 Biogeochemical Cycles

The absorption and utilization of elements by organisms is compensated by their recycling and regeneration back into the environment by the breakdown of these organic compounds again. The more or less cyclic paths of these elements in the biosphere from environment to organisms and into the environment back are called biogeochemical cycles (Bio - living organisms, Geo - rock, soil, air, water).

Many elements enter living organisms in the gaseous state from the atmosphere or as water soluble salts from the soil. As the flux of these elements through an ecosystem gives some measure of its continuity and productivity, the analysis of exchange of various components of the biosphere is essential. Furthermore, society depends upon this life-support system of the earth for sustained and increased production of food, fodder, fibre and fuel.

These biogeochemical cycles may be categorized into three global types:

1. The hydrological cycle, involving the movement of water.
2. The gaseous cycle of carbon, oxygen and nitrogen
3. The sedimentary (non-gaseous) cycle of remaining nutrient elements e.g. phosphorus, calcium and magnesium. Sulphur is to extent intermediate, since H₂S or SO₂, formed under some circumstances, adds a gaseous component to its normally sedimentary cycle. These elements normally do not cycle through the atmosphere in the absence of a gaseous phase. The elements concerned in the sedimentary cycle are earthbound and follow a basic pattern of flow through erosion, sedimentation, mountain building, volcanic activity and biological transport (e.g. through the excreta of marine birds). Sedimentary cycles are much less perfect than gaseous in that some of the element may get stuck in certain phase of the cycle.

10.3.1 Hydrologic (Water) cycle

The important cycle among all the materials is that of water. Water is by far the most important substance necessary for life. It is very important ecological factor that determines the structure and function of the ecosystem, and regulates the plant environment to a large extent. The cycling of all other elements is also dependent upon water as it provides the solvent medium for their uptake. It provides H⁺ for reduction of CO₂ in photosynthesis. It has moderating effect on the temperature of the surrounding area by virtue of its heat absorbing ability. Protoplasm the very basis of life is made up of 85 to 95% of water. The content varies in different tissues of the organism and in different plants and animals. Human blood is 90% water. Water cycle involves an exchange of water between the earth’s surface and the atmosphere via precipitation and evapo-transpiration. Water covers about 75% of the earth’s surface, occurring in lakes, rivers, seas, oceans, etc. The ocean occupies 70% of the surface and contains 97% of all the water on earth. Much of the remainder is frozen in the ice caps and glaciers. The water in rivers and lake is comparatively small. Less than 1% is in the form of ice-free fresh waters in rivers, lakes and aquifers. Yet this relatively negligible portion of the planet’s water is crucially important to all forms of terrestrial and aquatic life. There is also a large underground supply of water. Soils near the surface also serve as reservoirs for enormous quantities of water. Based on the data from Hutchinson (1957) (Table 10.1), prepared a diagram of hydrologic cycle (Fig. 10.2).

Every year 4.46 G of water comes in the form of rainfall of which 3.47 G precipitates over the ocean’s surface. About 1 G rainfall occurs over land mass of which 0.2 G runs away and 0.6 G evaporates again, and only a small quantity (0.2 G) is stored as underground water. 0.13 G water moves in the form of water vapour and clouds from ice caps present on South and North poles and on the top of high mountains. Only about 0.004% (~10 G) of the total water is all the time moving in the cycle as much of earth’s water is in cold storage. Glaciers and the ice caps cover 11% of the world’s land area; permanent frozen ground holds another 10% area in its grip, while 30 to 50% of the land is covered with snow at any given time. Icebergs and pack ice occupy 25% of the ocean area. Therefore of all fresh water is locked up as ice, mostly in Antarctica and Greenland.

Table 10.1 Data on water cycle from Hutchinson, 1957

10.3.2 Carbon cycle

Carbon is present in atmosphere, mainly in the form of carbon dioxide, and thus it cycles in this gaseous phase. Though it is a minor constituent of the atmosphere (0.032% v/v), as compared to oxygen (~21% v/v) and nitrogen (~79% v/v), yet without carbon dioxide no life could exist, for it is vital to the production of carbohydrates through photosynthesis in plants, the basic building blocks for other organic compounds needed in metabolic synthesis and incorporation of the carbon with the protoplasm. Fig. 10.3 illustrates the global carbon cycle. Carbon from atmospheric pool moves to green plants (producers), then to animals (consumers), and finally from these to bacteria, fungi and other microorganisms (decomposers) that return it to the atmosphere, through decomposition of dead organic matter. Some of this is also returned to the atmosphere through respiration at various levels in the food chain. It is estimated that half of the carbon fixed is subsequently returned to the soil in the form of decomposing organic matter. Fig. 10.3 illustrates the global cycle of carbon indicating the quantities involved at various levels. The atmospheric pool (711 X 10⁹ tons) is very small as compared to that of carbon in ocean (39,000 X 10⁹ tons) and in fossil fuels (12,000 X 10⁹ tons). Before the onset of industrial revolution flows among atmosphere, continents and oceans were balanced, but with industrialization and urban development this equilibrium appears to be disturbed. Fossil fuel burning, forest fire, deforestation and agriculture are some of the important sources of new input. On the contrary, forests are important carbon “sinks” as forest biomass is estimated to contain 1.5 times and forest humus 4 times the amount of carbon in the atmosphere.

There are two main sources of carbon in the abiotic world:

1. The rocks containing carbonates such as lime stone in the earth’s crust.
2. The carbon dioxide of the air and that dissolved in water.

In addition, there is present large amounts of carbon in fossil fuel (coal, petroleum, natural gas, etc.) but this is not available to the plants until and unless it is burned to produce carbon dioxide.(Fig. 10.3).

Carbon dioxide is released from carbonate rocks by acids resulting from geological action and also by acids formed during fermentation and by bacteria that produce nitric acid and sulphuric acid. An insignificant amount of carbon dioxide is also produced by activity by bacterium Carboxydismonas oligocarbophila which oxidizes carbon monoxide to carbon dioxide. Carbon monoxide (a poisonous gas for aerobic organisms including man) is not of common occurrence in nature but may be produced due to partial combustion of fossil fuel. When carbon dioxide dissolves in water, some of it reacts to form carbonic acid (H₂CO₃) which immediately produces carbonate (CO^2-₃) and bicarbonate (HCO-₃) ions.

The richest source of stored carbon today is in the ocean, and in the form of these ions. The oceans contain about 50 times more carbon dioxide than in the atmosphere. This regulates atmospheric carbon dioxide than in the atmosphere. This regulates atmospheric carbon dioxide content level to 0.03% despite photosynthetic uptake. Thus, there is a continuous exchange of carbon dioxide between the atmosphere and organisms on the one hand and between the atmosphere and sea on the other hand. However, the majority of ocean-dissolved CO₂ (HCO-₃) is below the thermocline and inaccessible for rapid exchange with the atmosphere. The immediate source of CO₂ for exchange is thus restricted to relatively small quantity of epilimnic CO₂. The sea water being rich in calcium and being alkaline (NaOH) helps in accelerating the process of carbonate decomposition. About 48 ml l^-1 CO₂ occurs as carbonate in sea water. Such deposits in the form of coral reefs and calcium carbonate rocks are common in the tropical regions of the oceans. In warm climates, high temperatures and greater salinity and alkalinity favour the process of carbonate decomposition, and it is also reflected in thicker, shells of moluscs. (Fig. 10.4).

The carbon dioxide has the unique property of absorbing infra-red radiations. While the small quantities of carbon dioxide are helpful in keeping the earth warm, the enhanced atmospheric carbon dioxide results in rise in the temperature of the atmosphere much in the same way as glass houses do (i.e. they permit the radiations to pass through and strike the earth, but once converted into heat and reflected upwards, the heat waves are absorbed by carbon dioxide rich atmosphere and cause rise in temperature) and in turn, causes rise in ocean level. Fig. 10.4 shows the carbon cycle in an ecosystem.

10.3.3 Oxygen cycle

Oxygen which is in abundance (20.9476% v/v) in the atmosphere is another indispensable material for life. According to Broecker (1970), each square metre of the earth’s surface is covered by 60,000 moles (about a ton) of oxygen gas. Terrestrial, aquatic and marine plants, during photosynthesis release about 8 moles of oxygen annually for each square metre of the earth’s surface. Nearly all of this gaseous oxygen is utilized in the process of respiration by plants, animals and bacteria with the result that the amount of oxygen consumed is almost equal to that of released in the atmosphere. However, there is a small net addition of oxygen to the atmosphere (about 1 part in 15 million parts of the oxygen present), which probably does not bring about any change in the oxygen content, as much of this is utilized in the oxidation of carbon, iron, sulphur and other minerals during the normal process of weathering.

Oxygen in bound state, occurs as oxides of carbonates in rocks, and in water. Oxygen dissolved in water is the main source of oxygen for aquatic plants, which may act as one of the limiting factors in their growth and development. Another important phase of oxygen is the ozone layer (oxygen acted on by short-wave radiation to produce ozone), of the outer atmosphere, which by shielding out the deadly ionizing short-wave ultraviolet radiations, protects the life. Oxygen is thus present in atmosphere in sufficiently large quantities and there is no possibility of oxygen deficiency on global scale even if all the earth’s organic matter including the fossil fuel is burnt.

10.3.4 Nitrogen cycle

Gaseous nitrogen is the most abundant element of the atmosphere (78.084% v/v), and seems to have a highly complex nutrient cycle in the terrestrial and aquatic ecosystems. This substance is very important for plants and animals as an essential, constituent component of chlorophyll and proteins. Despite its immense value and indispensable nature it is never taken directly from the atmosphere by animals or higher plants. Atmospheric nitrogen is rather inert and does not readily participate in any reaction. A generalized nitrogen cycle is shown in .

The chief sources of nitrogen for plants are nitrates in the soil. The atmospheric nitrogen is fixed symbiotically as well as asymbiotically by a variety of microorganisms. The chief nitrogen fixers are bacteria belonging to the genus Rhizobium found in root nodules of legumes. Asymbiotic nitrogen fixers are some blue green algae, like Anabaena and Nostoc, aerobic bacteria like Azotobacter, and anaerobic bacteria like Clostridium. Certain photosynthetic bacteria like Rhodospirillum are also nitrogen fixers. Some proportion of atmospheric nitrogen is fixed during lightening also. The fixed atmospheric nitrogen reaches the soil as nitrates, which are taken up by plants for manufacture of complex nitrogenous compounds which in turn, are eaten by animals. The dead organic matter formed due to death of plants and animals is decomposed by various types of bacteria, actinomycetes and fungi occurring in soil and water. This releases nitrogen either in free stage or as ammonia gas in the atmosphere. Ammonia gas may reach the soil as nitrates through the activity of nitrifying microbes, Nitrosomonas and Nitrobacter. Some nitrates of soil due to activity of denitrifying microbes, Pseudomonas, may also be converted to free nitrogen gas returning to the atmosphere. This inorganic nitrogen is again recycled into the organic system upon absorption by higher plants. It is presumed that the fixation of nitrogen by microorganisms is generally in equilibrium with denitrification. (Fig. 10.5).

But in recent years there has been high quantity of atmospheric nitrogen fixation by Industrial process (Haber’s process). Nitrogen so fixed is not readily and fully denitrified so as to cause accumulation of nitrates or ammonia in water and soil. The accumulation of nitrates in water causes eutrophication. NO₂ from the incomplete combustion of fossil fuel in automobiles further pollute the environment. It appears that through photochemical and electrical fixation 2.5 x 107 ty^-1 and through biological fixation 5-(6)x 10⁹ ty^-1 of nitrate is formed. Industrial nitrogen fixation including oxides of nitrogen formed during fossil fuel combustion is 8 x 10⁷ ty^-1. Nitrogen fixed by microorganisms is 1-(2) x 10⁸ ty^-1, which is presumed almost equal to that of denitrification. A tiny fraction of annual N-fixation is lost to fossilization in sediments because the anaerobic sedimentary environment is favourable to denitrifying bacteria.

10.3.5 Sulphur cycle

Sulphur is a component of sedimentary cycle. It is found in the gaseous forms (H2S, SO2, etc.) in the atmosphere, and as sulphates, sulphides and organic-sulphur in the soil. SO2 gas present in the atmosphere is produced volcanically, by burning of vegetation, and now in copious quantities by oxidation of sulphides and organo-S in fossil fuels. H₂S and dimethyl sulphide are commonly formed by the activity of anaerobic bacteria. The elemental and organic sulphur, and SO₄^2- are formed through oxidation of H2S. SO₂ and H₂S from the atmosphere are returned to the soil through precipitation. Sulphur in the form of sulphates (SO₄^2-) is the principal available form that is reduced and incorporated into proteins by autotrophs. Sulphur is an essential constituent of certain amino acids (cysteine, cystine, and methionine), the peptide glutathione and certain vitamins or enzyme cofactors (thiamine, biotine, and thiotic acid). It is the mercaptan, containing the thiol (-SH, or sulphydryl) group, and as the corresponding oxidized disulfide form that sulphur is most reactive in the plant.

The sulphur cycle links air, water and soil, where microbes play a key role. The sulphur is incorporated in the tissues of autotrophs as -SH in the proteins. It passes through the grazing food chain and excess of it is released through the faeces of animals. Within the detritus food chain the decomposition of proteins releases sulphur. Under aerobic conditions Aspergillus and Neurospora and under anaerobic conditions the bacteria like Escherichia and Proteus are largely responsible for the decomposition. In anaerobic soils and sediments H2S is formed by sulphate reducing bacteria like Desulphonovibrio desulfuricans which utilize the oxygen in the sulphate molecule to obtain energy and in turn reduce the sulphate in deep sediments to H2S gas:

In iron-rich materials, much of this H2S is scavenged by ferrous iron to produce the very insoluble, black FeS. Many photosynthetic and chemosynthetic bacteria play an important role in sulphur metabolism. Chemoautotrophic colourless bacteria like Beggiatoa, Thiothrix and Thiobacillus occurring in H2S containing water oxidizes H₂S to S or S to SO₄^2- when the H₂S supply is exhausted.

Thiobacillus thiooxidans under highly acidic conditions (up to pH 0.6) may convert sulphur to sulphuric acid of 10% concentration and thus strongly acidify the soil. There are also green sulphur (e.g. Chlorobium) and purple-sulphur (e.g. Chromatium) photosynthetic bacteria that use the H2S as the source of hydrogen in reducing CO₂.
Light

Green bacteria are able to oxidize H₂S only to elemental sulphur, whereas the purple one can carry oxidation to sulphate stage.

Sulphur cycle plays a key role in the metabolism of other nutrients like iron, copper, cadmium, zinc, cobalt etc. For example, when iron is precipitated as sulphide, phosphorus is converted from insoluble to soluble form and thus becomes available to organisms.

10.3.6 Phosphorus cycle

Like sulphur, phosphorus is also a component of sedimentary cycle. It is an essential component as in the form of ATP it acts as an energy carrier. It is comparatively less abundant in natural ecosystems, particularly in terrestrial ecosystems and occurs in meager amounts in aquatic ecosystems too. The phosphorus is made available to the plants form the phosphatic rocks by slow weathering process. The phosphatic (inorganic phosphates typically orthophosphate ions) are metabolised in the plant body and pass through the food chain to animals, and then to decomposers (as food as well as through death and decay) in the form of organic phosphate, which is subsequently made available in the soil for reutilization through mineralisation and decomposition. However, a major proportion of phosphorus becomes lost to this central cycle through run off to the deep sediments of the oceans and in biological processes, such as formation of teeth and bones. On the contrary some quantities of phosphates are returned back to the earth in the form of bird guana (excreta) and fishes. In recent years the excessive use of phosphate fertilizers and the detergents is a problem of global concern as it has been considered responsible for accelerated eutrophication of water bodies.

10.3.7 Calcium cycle

It is important element needed by plants for building their cell walls and by animals for bone formation. It is being regularly added to the soil pool through the weathering of rocks and through atmosphere. A large proportion of this is kept in a state of cycling by uptake from soil into the biotic pool of plants and animals and their return through litter fall, death and decay via detritus food chain. Only a small portion is lost out of the ecosystem through stream flow and this is replenished by weathering and precipitation.

10.3.8 Cycle of toxic elements

Several non-essential elements like mercury, lead, cadmium, arsenic and fluorine, despite their substantial toxicity are freely cycled through biological systems in well regulated and balanced manner. Growing industrial use, mining operations and other man’s activities tended to perturb this equilibrium and upset the balance towards greater accumulation and lesser dispersion of toxic elements. A very significant role in the mobility and dispersion of these elements in the biosphere is played by microorganisms.

10.3.8.1 Mercury

It is one of the most important toxic elements which is now increasingly (about four-fold) discharged in soils and water as an unwanted by-product of certain industrial and agricultural activities. Mercury cycle is better known and the potential rate determining the role of biomethylation of mercury in an ecosystem involving lakes, rivers, coastal environment, soil, etc., is now well established. The natural level of mercury in soils is as high as 0.04 ppm, and in water 0.06 ppm. The amount of mercury found in the air depends on conditions of the environment. The element is poisonous in the metallic state, as inorganic salts of mercury or in the form of organic mercury compounds. It does not have to be ingested being poisonous. Metallic mercury gives off vapours at room temperature; some of the metal even vaporizes at the freezing point of water and this being highly volatile gets dispersed into biosphere. Elemental mercury can exist in three alternative states, viz., Hg2²⁺, Hg²⁺ and HgO and certain microorganisms are capable of interconverting the three forms. Naturally occurring methyl-vitamin B₁₂ compounds can aid the synthesis of methyl mercury as well as dimethyl mercury in natural habitats. The bioaccumulation of mercury is greatly facilitated by the natural synthesis of stable alkylmercury compounds (Wood, 1974). About 25% of the world mercury production form chlorine plant, where mercury is used as in electrolyte electrode, escapes in fuel gases. Methyl mercury compounds formed probably in sulphide-rich sediments by the activity of Methanobacterium amelankis are also highly toxic and move in the ecosystem either in solution or as atmospheric volatiles. Methyl mercury chloride is particularly toxic to animals as it is easily passed across cell membranes. Dimethyl mercury, which is highly volatile, passes into the air and decomposes into CH₄, C₂H₆ and Hg₂O, thus causing air pollution.

The mercury cycle shows that the mercury in ecosystem passes through food chain or by inhalation of dust or ingestion of surface-contaminated food. Mercury pollution can be best assessed by measuring the concentration of total mercury in sediments and also the rate of uptake of methyl mercury by fish.

10.3.8.2 Arsenic

It also has a biological cycle in nature. It is an element that is intermediate between the metals and non-metals. It is more abundant in nature as compared to mercury. In drinking water it may occur at levels of upto 50 ppm, whereas mercury levels commonly do not exceed 1 ppm. Arsenic compounds are known as to accumulate through food chains (Summers and Silver, 1978), with the result that even small doses can be lethal. Severe poisoning of human can be caused by as little as 100 mg, and 130 mg found to be fatal. It occurs in rocks, soils and water at much higher levels than does in mercury. It is found in many vegetables and fruits. Some marine organisms, especially shellfish tend to concentrate arsenic within their bodies, which may contain more than 100 ppm. For example, 174 ppm in prawn, 42 ppm in shrimp, and 40 ppm in bass. In moist soils, it is present upto 500 ppm. It has also been detected at concentration of 10 to 70 ppm in several commonly marketed house hold detergents. It may often stimulate plant growth in very low concentrations, but is injurious in excessive quantities. Destruction of chlorophyll appears to be the main effect. As little as 1 ppm of arsenic trioxides in the water has caused injury into plants. U.S. Public Health Service in 1942 set a safe limit of 0.05 ppm, and in 1962 it recommended a maximum of 0.01 ppm in drinking water. There is also evidence that arsenic accumulates in the livers of mammals. Skin cancer has been found to be associated in several regions with arsenic intake in drinking water.

Arsenate is reduced to arsenite and then microbially methylated to form dimethylarsine and trimethylarsine. The conversion of arsenate through arsenite and methylarsenic acid occurs in lake sediments; di-and tri-methylarsines are released in water. These become oxidized in air to less toxic dimethylarsenic acid. The dimethylarsenic acid is thus cycled between air and sediment (Wood, 1974). Dimethylarsine is highly toxic to fish and other organisms.

10.3.8.3 Lead

The lead is prevalent in the natural environment. The earth’s crust contains an average of about 10 to 15 ppm lead, though the content in rock, soil and water is extremely variable. Lead enters the environment in enormous quantities and particularly efficiently dispersed to the atmosphere by the use of tetraethyl and tetramethyl lead as antiknock additives to petrol (gasoline), which may contain about 2 g Pb gal^-1. About 2.5 X 108 kg y^-1 Pb enters the oceans from this source and the mean sea-water concentration has increased almost seven fold during the past 50 years and is now about 0.07 µ g kg^-1 (Goldberg, 1971).

Normally lead is not strongly absorbed from soil, by plants. The main toxicity hazard is therefore, from inhalation of dust or ingestion of surface-contaminated food. However, plants grown on heavily contaminated soil absorb several thousand µ g g^-1 compared as the normal plant content of between 1 and 15 µ g g^-1 (Johnston and Proctor, 1977).

10.3.8.4 Cadmium

Cadmium belongs to same family of elements as zinc and mercury. A major source of cadmium is zinc mining and smelting in addition to its release by other industries such as metal plating, and in making pigments, ceramics, photographic equipments, and nuclear reactors as well as those engaged in textile printing, lead mines and various chemical industries.

There is no evidence that cadmium has any role in nutrition of plants and animals. It is toxic in relatively small amounts. Being highly mobile in soil and water it is taken up freely by plants and passed on to grazing food chain (Coughtrey and Martin, 1976). In animals and humans, cadmium tends to accumulate in kidneys, pancreas and bones. In Japan the disease itai itai was caused by people’s consumption of heavy metals, primarily cadmium either by drinking water or by eating rice which had accumulated the metal from the irrigation water. The affliction is characterized by kidney malfunction, a drop in phosphate level of blood serum, loss of minerals from the bones, and a condition called osteomalacia, which is a rickets-like condition characterized by pathogenic bone fracture and intense pains.

10.3.8.5 Fluorine

Fluorine makes up about 0.1 per cent of the earth’s crust. In its elemental state it is a gas. However, in nature it is always found in various combinations. The greater proportion is in the form of the mineral fluorspar (Calcium fluorate, CaF) and in large deposits of mineral cryolite (sodium aluminium fluoride, NaAIF). Sources of atmospheric fluorine are aluminium smelting using cryolite as a flux, coal burning and the firing of clays in brick manufacture.

Fluorine is freely mobile in the atmosphere and ultimately appears in rainfall as fluoride. Plants take it from soil and water. In gaseous form, it enters open stomata, causes collapse of mesophyll cells, loss of photosynthetic activity and necrosis. Animals derive it from food, water, and minerals. The effect on tooth decay from drinking the water deficient in fluorine was noted. On the other hand, teeth impairment, called dentineri or black teeth, was observed among people.

10.4 Food Chains

The transfer of food energy from the source in plants through a series of organisms with repeated stages of eating and being eaten is known as the food chain. The green plants, in the food chain, occupy the first trophic (nutritional or energy) - the producer level, the herbivores that eat the plants the second trophic - the primary consumer level, the carnivores that eat the herbivores the third trophic - the secondary consumer level and perhaps even a fourth- the tertiary consumer level. Some organisms are omnivores that eat the plant as well as animals at their lower level in the food chain and they may occupy more than one trophic level in the food chain. Thus, in any food chain, energy flows from producers -----> primary consumers (herbivores) -----> secondary consumers (carnivores) A tertiary consumers (carnivores), and so on. At each step of food transfer, a large proportion, 80 to 90% of the potential energy is lost through dissipation of heat resulting in continuous diminution of available energy. This is the reason that rarely more than five trophic levels occur in a food chain. The efficiency of energy transfer also varies from one trophic level to another.

In nature, three types of food chains have been distinguished:

10.4.1 Grazing food chain

The consumers which utilise the living plant parts as their food or energy source constitute the grazing food chain. The food chain, thus begins from a green plant base. It is common in the terrestrial and aquatic ecosystems where most of the primary production is edible by herbivores. Some of the common examples of grazing food chain are given in Table 10.2

10.4.2 Parasitic food chain

It also begins from a green plant base and goes to herbivores, which may be the host of a huge number of lice living as ectoparasites.

10.4.3 Detritus food chain

The food chain goes from dead organic matters of decaying animal and plant bodies to the microorganisms and then to detritus feeding organisms (detrivores or saprovores) and their predators is known as “detritus food chain”. Soil organisms are thus less dependent on direct solar energy and depend chiefly on the influx of organic matter produced in another system. This is very clear from the following illustration:

Table 10.2 Some examples of grazing food chains

A good example of detritus food chain based on mangrove leaves.

Some examples of food chains are shown in (Fig. 10.6).

In the brackish zone of Southern Florida, leaves of the red mangrove (Rhizophore mangle) fall into the warm, shallow waters. The fallen leaf fragments acted on by such saprotrophs as fungi, bacteria, and protozoa, and colonised by phytoplanktonic and benthic algae are eaten and reeaten by a group of small animals. These animals include crabs, copepods, insect larvae, mysids, nematodes, grass shrimps, amphipods, etc. All these animals are called detritus consumers. These animals, in turn, are eaten by some minnows, small game fish, etc. The small carnivores, which in turn, serve as the food for large game fish, and so on. Mangrove leaves, through detritus food chain make substantial contribution to the food chain that is upto 90% of the stored energy in the dead organic material is consumed through detritus food chain. This chain is further important from the view point of mineral cycles within the ecosystem.

10.5 Food Web

Food chain, normally do not operate in isolated but are interlocked with each other forming some sort of pattern known as food web. An organism in the ecosystem may operate at more than one trophic level, i.e. it derives its food from more than one source and in turn, may serve as a source of food for several organisms of higher trophic level. This results into linking together, but intersecting each other, of several food chains. Another reason for the formation of food web seems to be successive loss of energy at higher trophic levels till no more energy is available to support yet another link in the food chain. A food web delineated for small organisms of a stream community in South Wales. This illustrates: (i) the interlinking of food chain, (ii) three trophic levels, (iii) intermediate position of the organisms e.g. Hydropsyche, and (iv) an “open” system in which part of the basic food is “imported” from outside the stream.

The food webs are very important in maintaining the stability of an ecosystem, in nature. For example, in grazing food chain of a grassland, (Fig. 10.7) in the absence of rabbit, grass may be eaten by mouse. The mouse in turn may be eaten directly, either by hawk or snake. The snake then may be eaten by hawk.

Absence of rabbit thus would not disturb the ecosystem as the alternative (mouse) may serve for the maintenance of its stability. Moreover, a balanced ecosystem is essential for the survival of all the living organisms of the system. For example, if the primary consumers (herbivores) are not in nature than the producers would perish due to overcrowding and competition. In the same way, the survival of the primary consumers is linked with the secondary consumers (carnivores) and so on. Thus each species of an ecosystem is indeed kept under some sort of a natural check so that the system may remain stable.

A food web, unlike a food chain has therefore, several alternative pathways for flow of energy. Sudden decrease in population of one category of consumers at any trophic level does not affect much the functioning of an ecosystem, as at that trophic level, the second category of consumers multiply and build up their numbers. An ecosystem is, therefore, more stable, if it has a greater number of alternative pathways. Some examples of food webs are given in (Fig. 10.8), (Fig. 10.9) , & (Fig. 10.10)

10.6 Ecological Pyramids

The concept of ecological pyramids was developed by Charles Elton (1927), the pioneer British Ecologist. There is some sort of relationship between the number, biomass and energy content of the primary producers, consumers of the first and second orders and so on to top carnivores in the ecosystem. This relationship may be represented graphically by means of pyramids which is referred to as ecological pyramids, where the first or producer level forms the base of the pyramid and the successive levels (the tiers) making the apex. Ecological pyramids are of three general types: (i) Pyramid of numbers, showing the number of organisms at each trophic level (number m^-2), (ii) Pyramid of biomass, showing the total dry weight or any other suitable measure of the total amount of living matter (g m^-2), and (iii) Pyramid of energy, showing the amount of energy flow and/or productivity at successive trophic levels (calories m^-2 year^-1).

10.6.1 Pyramid of numbers

The relationship between the number of producers, consumers of primary, secondary and tertiary orders constitutes the pyramid of numbers. The form of the pyramid of numbers will vary widely with different communities, depending on whether producers are small (phytoplankton, grass) or large (oak trees). Sometimes, number of individuals varies so widely that it is difficult to represent the entire ecosystem on the same numerical scale. Such data could best be presented in a tabular form. The pyramids of numbers in grassland, pond, and forest ecosystem are shown in following Figures. (Fig. 10.11), (Fig. 10.12), (Fig. 10.13).In a grassland, the producers which are mainly grasses, are always maximum in number. This number then shows a successive decrease towards apex, as the primary consumers (herbivores), which are rabbits, mice, etc., are lesser in number than the grasses; the secondary consumers, the snakes and lizards are lesser in number than the rabbits and mice. Finally, the top (tertiary) consumers, the hawks and birds, are least in number. Thus, the pyramid becomes upright. Similarly, in pond ecosystem, the pyramid is upright. Here the producers, which are mainly phytoplanktons as algae, bacteria, etc. are maximum in number; the herbivores which are very small fish, rotifers, etc., are lesser in number than the producers; and the secondary consumers (carnivores), such as water beetles and small fish, etc., are lesser in number than the herbivores. Finally, the top (tertiary, consumers), the bigger fish and birds are least in number.

In a forest ecosystem (Fig. 10.12), however, the pyramid of numbers is somewhat different in shape the producers which are mainly large-sized trees are lesser in number, and form base of the pyramid. The herbivores, which are the fruit eating birds, deers, etc., are more in number than the producers. Then, there is a gradual decrease in the number of successive carnivores, thus making the pyramid again upright one.

However, in a parasitic food chain (Fig. 10.14), the pyramids are always inverted. This is due to the fact that a single plant may support the growth of many herbivore birds and each one of these, in turn, may provide nutrition to several hyperparasites like bugs and lice. Thus from the producers towards consumers, the number of organisms successively shows an increase, making the pyramid inverted one. In crop ecosystem, the pyramid is upright one where primary consumers, viz., grasshoppers are lesser in number than the crops; frogs, snakes, and eagle- the primary, the secondary and the top consumers respectively are present in decreasing number.

10.6.2 Pyramids of biomass

In this type of pyramid, the relationship between different trophic levels is presented in terms of weight of organisms (biomass). The pyramids of biomass in different ecosystems are shown in (Fig.10.15). In grassland and forest, there is generally a gradual decrease in mass of organisms at successive levels from the producers to the top consumers. Thus, pyramids are upright. In an aquatic ecosystem (like pond), however, the biomass of producers is least. This value gradually shows an increase towards the apex of the pyramid, thus making the pyramid inverted one. In this case the biomass of diatoms and phytoplanktons (primary consumers) that feed on them. The biomass of large carnivore fishes (secondary consumers) which feed on smaller fishes is the highest of all the trophic levels. In English Channel the biomass of primary producers is only 4 g m^-2 whereas that of the consumers is 21 g m^-2. Infact, this is the case in most aquatic bodies . In lakes and sea, on the other hand, the phytoplanktons usually outweigh their grazers (zooplanktons) during periods of high primary productivity, as during the spring “bloom”, but at other times, as in winter the reverse may be true. This difference in biomass trend can be explained if the time is also taken into account.

10.6.3 Pyramid of energy

The pyramid of energy represents the total quantity of energy utilized by different trophic level organisms of an ecosystem per unit area over a set period of time (usually, per square metre per year). The primary producers of an ecosystem trap the radiant energy of the sun and covert it into potential chemical energy. This trapped energy flows in the food chain from the producers to the top carnivores, decreasing at successive trophic levels. If the relationship of total quantity of energy utilized in unit area over a particular period of time by different trophic levels is diagrammatically represented, an upright pyramid is invariably formed. As against the pyramid of numbers and biomass, the shape of the pyramid of energy is always upright because in this the time factor is taken into account. In a grassland the green plants (primary producers) trap the maximum light energy in a particular area over a fixed period of time. Similarly, in a pond ecosystem, the phytoplanktons, in a particular area, trap and accumulate much more energy than the herbivore fishes in the course of year because of their large numbers and quicker rate of multiplication. Comparatively, the amount of energy utilized in a year by the top carnivores is much less than that of herbivore fishes.

Of the three types of pyramids as discussed above, the energy pyramid gives by far the best overall picture of the functional role of communities in an ecosystem. This is because of the fact that energy pyramid is a picture of rate of passage of food mass through the food chain, whereas number and biomass pyramids are pictures of standing states, i.e. organisms present at any moment. Its shape is invariably an upright one, and not affected by variation in the size and metabolic state of individuals, if all the sources of energy in the ecosystem are considered. The number and biomass pyramids on the other hand, may be upright or inverted depending upon the size and biomass of the producer organisms as compared to consumers.

10.7 Ecological Succession

Ecological succession is the phenomenon or process by which an ecological community undergoes more or less orderly and predictable changes following disturbance or initial colonization of new habitat. Succession was among the first theories advanced in ecology and the study of succession remains at the core of ecological science. Succession may be initiated either by formation of new, unoccupied habitat (e.g., a lava flow or a severe landslide) or by some form of disturbance (e.g. fire, severe wind throw, logging) of an existing community.

10.7.1 Primary succession

Succession that begins in new habitats, uninfluenced by pre-existing communities is called primary succession. In primary succession pioneer species like lichen, algae and fungus as well as other abiotic factors like wind and water start to "normalize" the habitat. This creating conditions nearer optimum for vascular plant growth; pedogenesis or the formation of soil is the most important process.

These pioneer plants are then dominated and often replaced by plants better adapted to less odd conditions, these plants include vascular plants like grasses and some shrubs that are able to live in thin soils that are often mineral based.

For example, spores of lichen or fungus, being the pioneer species, are spread onto a land of rocks. Then, the rocks are broken down into smaller pieces and organic matter gradually accumulates, favouring the growth of larger plants like grasses, ferns and herbs. These plants further improve the habitat and help the adaptation of larger vascular plants like shrubs, or even medium- or large-sized trees. More animals are then attracted to the place and finally a climax community is reached.

10.7.2 Secondary succession

Succession that follows disruption of a pre-existing community is called secondary succession. (e.g. forest fire, harvesting, hurricane) that reduces an already established ecosystem (e.g. a forest or a wheat field) to a smaller population of species, and as such secondary succession occurs on preexisting soil whereas primary succession usually occurs in a place lacking soil.

Simply put, secondary succession is the succession that occurs after the initial succession has been disrupted and some plants and animals still exist. It is usually faster than primary succession as:

1. Soil is already present, so there is no need for pioneer species;
2. Seeds, roots and underground vegetative organs of plants may still survive in the soil.