Kaushik Book: Ecosystem (Unit 2) PDF

Unit 3 Ecosystems n CONCEPT OF ECOSYSTEM Various kinds of life supporting systems like the forests, grasslands, oceans, lakes, rivers, mountains, deserts and estuaries show wide variations in their structural composition and functions. However, they all are alike in the...

Unit 3 Ecosystems n CONCEPT OF ECOSYSTEM Various kinds of life supporting systems like the forests, grasslands, oceans, lakes, rivers, mountains, deserts and estuaries show wide variations in their structural composition and functions. However, they all are alike in the fact that they consist of living entities interacting with their surroundings exchanging matter and energy. How do these different units like a hot desert, a dense evergreen forest, the Antarctic Sea or a shallow pond differ in the type of their flora and fauna, how do they derive their energy and nutrients to live together, how do they influence each other and regulate their stability are the questions that are answered by Ecology. The term Ecology was coined by Earnst Haeckel in 1869. It is derived from the Greek words Oikos- home + logos- study. So ecology deals with the study of organisms in their natural home interacting with their surroundings. The surroundings or environment consists of other living organisms (biotic) and physical (abiotic) components. Modern ecologists believe that an adequate definition of ecology must specify some unit of study and one such basic unit described by Tansley (1935) was ecosystem. An ecosystem is a group of biotic communities of species interacting with one another and with their non-living environment exchanging energy and matter. Now ecology is often defined as “the study of ecosystems”. An ecosystem is an integrated unit consisting of interacting plants, animals and microorganisms whose survival depends upon the maintenance and regulation of their biotic and abiotic structures and functions. The ecosystem is thus, a unit or a system which is composed of a number of subunits, that are all directly or indirectly linked with each other. They may be freely exchanging energy and matter from outside—an open ecosystem or may be isolated from outside—a closed ecosystem. 65 66 Environmental Science and Engineering n ECOSYSTEM CHARACTERISTICS Ecosystems show large variations in their size, structure, composition etc. However, all the ecosystems are characterized by certain basic struc- tural and functional features which are common. n STRUCTURAL FEATURES Composition and organization of biological communities and abiotic components constitute the structure of an ecosystem. I. Biotic Structure The plants, animals and microorganisms present in an ecosystem form the biotic component. These organisms have different nutritional be- haviour and status in the ecosystems and are accordingly known as Producers or Consumers, based on how do they get their food. (a) Producers: They are mainly the green plants, which can synthesize their food themselves by making use of carbondioxide present in the air and water in the presence of sunlight by involving chlorophyll, the green pigment present in the leaves, through the process of photosynthesis. They are also known as photo autotrophs (auto=self; troph=food, photo=light). There are some microorganisms also which can produce organic matter to some extent through oxidation of certain chemicals in the absence of sunlight. They are known as chemosynthetic organisms or chemo-autotrophs. For instance in the ocean depths, where there is no sunlight, chemoautotrophic sulphur bacteria make use of the heat generated by the decay of radioactive elements present in the earth’s core and released in ocean’s depths. They use this heat to convert dissolved hydrogen sulphide (H2S) and carbon dioxide (CO2) into organic compounds. (b) Consumers: All organisms which get their organic food by feeding upon other organisms are called consumers, which are of the following types: (i) Herbivores (plant eaters): They feed directly on producers and hence also known as primary consumers. e.g. rabbit, insect, man. (ii) Carnivores (meat eaters): They feed on other consumers. If they feed on herbivores they are called secondary consumers (e.g. frog) and if they feed on other carnivores (snake, big fish etc.) they are known as tertiary carnivores/consumers.  Ecosystems 67 (iii) Omnivores: They feed on both plants and animals. e.g. humans, rat, fox, many birds. (iv) Detritivores (Detritus feeders or Saprotrophs): They feed on the parts of dead organisms, wastes of living organisms, their cast- offs and partially decomposed matter e.g. beetles, termites, ants, crabs, earthworms etc. (c) Decomposers: They derive their nutrition by breaking down the complex organic molecules to simpler organic compounds and ul- timately into inorganic nutrients. Various bacteria and fungi are decomposers. In all the ecosystems, this biotic structure prevails. However, in some, it is the primary producers which predominate (e.g. in forests, agroecosystems) while in others the decomposers predominate (e.g. deep ocean). II. Abiotic Structure The physical and chemical components of an ecosystem constitute its abiotic structure. It includes climatic factors, edaphic (soil) factors, geographical factors, energy, nutrients and toxic substances. (a) Physical factors: The sunlight and shade, intensity of solar flux, duration of sun hours, average temperature, maximum-minimum temperature, annual rainfall, wind, latitude and altitude, soil type, water availability, water currents etc. are some of the important physical features which have a strong influence on the ecosystem. We can clearly see the striking differences in solar flux, temperature and precipitation (rainfall, snow etc.) pattern in a desert ecosystem, in a tropical rainforest and in tundra ecosystem. (b) Chemical factors: Availability of major essential nutrients like carbon, nitrogen, phosphorus, potassium, hydrogen, oxygen and sulphur, level of toxic substances, salts causing salinity and various organic substances present in the soil or water largely influence the functioning of the ecosystem. All the biotic components of an ecosystem are influenced by the abiotic components and vice versa, and they are linked together through energy flow and matter cycling as shown diagrammatically in Fig. 3.1. 68 Environmental Science and Engineering Nutrient cycling Abiotic component Energy Biotic flow compo- nent Nutrient flow Bacteria Plants Animals Abiotic component Nutrie nt Cycling Fig. 3.1. Nutrient cycling and energy flow mediated through food- chain. The flow of energy is unidirectional while the nutrients move in a cyclic manner from the abiotic to biotic (food chain) to abiotic and so on. n FUNCTIONAL ATTRIBUTES Every ecosystem performs under natural conditions in a systematic way. It receives energy from the sun and passes it on through various biotic components and in fact, all life depends upon this flow of energy. Besides energy, various nutrients and water are also required for life processes which are exchanged by the biotic components within themselves and with their abiotic components within or outside the ecosystem. The biotic components also regulate themselves in a very systematic manner and show mechanisms to encounter some degree of environmental stress. The major functional attributes of an ecosystems are as follows: (i) Food chain, food webs and trophic structure (ii) Energy flow (iii) Cycling of nutrients (Biogeochemical cycles) (iv) Primary and Secondary production (v) Ecosystem development and regulation  Ecosystems 69 n TROPHIC STRUCTURE The structure and functions of ecosystems are very closely related and influence each other so intimately that they need to be studied together. The flow of energy is mediated through a series of feeding relation- ships in a definite sequence or pattern which is known as food chain. Nutrients too move along the food chain. The producers and consum- ers are arranged in the ecosystem in a definite manner and their inter- action along with population size are expressed together as trophic structure. Each food level is known as trophic level and the amount of living matter at each trophic level at a given time is known as standing crop or standing biomass. Before we study about energy flow or nutrient cycling, we must learn about the food-chains, that provide the path through which the flow of energy and matter take place in ecosystem. n FOOD CHAINS The sequence of eating and being eaten in an ecosystem is known as food chain. All organisms, living or dead, are potential food for some other organism and thus, there is essentially no waste in the functioning of a natural ecosystem. A caterpillar eats a plant leaf, a sparrow eats the caterpillar, a cat or a hawk eats the sparrow and when they all die, they are all consumed by microorganisms like bacteria or fungi (decomposers) which break down the organic matter and convert it into simple inorganic substances that can again be used by the plants- the primary producers. Some common examples of simple food chains are: l Grass → grasshopper → Frog → Snake → Hawk (Grassland ecosystem) l Phytoplanktons → water fleas → small fish → Tuna (Pond ecosystem) l Lichens → reindeer → Man (Arctic tundra) Each organism in the ecosystem is assigned a feeding level or trophic level depending on its nutritional status. Thus, in the grassland food chain, grasshopper occupies the Ist trophic level, frog the IInd and snake and hawk occupy the IIIrd and the IVth trophic levels, re- spectively. The decomposers consume the dead matter of all these trophic levels. In nature, we come across two major types of food chains: 70 Environmental Science and Engineering I. Grazing food chain: It starts with green plants (primary pro- ducers) and culminates in carnivores. All the examples cited above show this type of food chain. Another example could be Grass → Rabbit → Fox Small fish Phytoplanktons Zooplanktons (Algae, diatoms) Cornivorous fish Fig. 3.2. A grazing food chain in a pond ecosystem. II. Detritus food chain: It starts with dead organic matter which the detritivores and decomposers consume. Partially decomposed dead organic matter and even the decomposers are consumed by detritivores and their predators. An example of the detritus food chain is seen in a Mangrove (estuary). Dead mangrove tree leaves Detritus feeders Phytoplanktons Carnivores Decomposers (Bacteria, fungi) Fig. 3.3. A detritus food chain in an estuary based on dead leaves of mangrove trees.  Ecosystems 71 Here, a large quantity of leaf material falls in the form of litter into the water. The leaf fragments are eaten by saprotrophs. (Saprotrophs are those organisms which feed on dead organic matter). These fallen leaves are colonized by small algae, which are also consumed by the saprotrophs or detritivores consisting of crabs, mollusks, shrimps, insect larvae, nematodes and fishes. The detritivores are eaten by small carnivorous fishes, which is turn are eaten by large carnivorous fishes. Leaf litter → algae → crabs → small carnivorous fish → large carnivorous fish (Mangrove ecosystem) Dead organic matter → fungi → bacteria (Forest ecosystem) Thus the grazing food chain derives its energy basically from plant energy while in the detritus food chain it is obtained primarily from plant biomass, secondarily from microbial biomass and tertiarily from carnivores. Both the food chains occur together in natural ecosystems, but grazing food chain usually predominates. n FOOD WEB Food chains in ecosystems are rarely found to operate as isolated linear sequences. Rather, they are found to be interconnected and usually form a complex network with several linkages and are known as food webs. Thus, food web is a network of food chains where different types of organisms are connected at different trophic levels, so that there are a number of options of eating and being eaten at each trophic level. Fig. 3.4 illustrates an example of a food-web in the unique Antarctic Ecosystem. This is representing the total ecosystem including the Antarctic sea and the continental land. The land does not show any higher life forms of plants. The only species are that of some algae, lichens and mosses. The animals include penguins and snow petrel which depend upon the aquatic chain for their food energy. In a tropical region, on the other hand, the ecosystems are much more complex. They have a rich species diversity and therefore, the food webs are much more complex. Why nature has evolved food webs in ecosystems instead of sim- ple linear food chains? This is because food webs give greater stability to the ecosystem. In a linear food chain, if one species becomes extinct or one species suffers then the species in the subsequent trophic levels are also affected. In a food web, on the other hand, there are a number of options available at each trophic level. So if one species is affected, it does not affect other trophic levels so seriously. 72 Environmental Science and Engineering Humans Blue Whale Sperm Whale Elephant Seal Killer Whale Leopard Seal Squid Penguin Emperor Penguin Snow Petrel Fish Carnivorous Herbivorous Krill Zooplankton Plankton Phytoplankton Fig. 3.4. A simplified food web in Antarctic ecosystem. Just consider the simple food chains of arctic tundra ecosystem: Cladonia → Reindeer → Man Grass → Caribou → Wolf If due to some stress, the population of reindeer or Caribou falls, it will leave little option for man or wolf to eat from the ecosystem. Had there been more biodiversity, it would have led to complex food web giving the ecosystem more stability. Significance of food chains and food webs l Food chains and food webs play a very significant role in the ecosystem because the two most important functions of en- ergy flow and nutrient cycling take place through them.  Ecosystems 73 l The food chains also help in maintaining and regulating the population size of different animals and thus, help maintain the ecological balance. l Food chains show a unique property of biological magnifica- tion of some chemicals. There are several pesticides, heavy metals and other chemicals which are non-biodegradable in nature. Such chemicals are not decomposed by microorgan- isms and they keep on passing from one trophic level to an- other. And, at each successive trophic level, they keep on in- creasing in concentration. This phenomenon is known as biomagnification or biological magnification. CASE STUDY A build-up of DDT concentration : A striking case of biomagnification of DDT (a broad range insecticide) was observed when some birds like Osprey were found to suffer a sharp decline in their population. The young ones of these birds were found to hatch out in premature condition leading to their death. This was later found to be due to bio-magnification of DDT through the food chain. DDT sprayed for pest control was in very low concentration, but its concentration increased along the food chain through phytoplanktons to zooplanktons and then to fish which was eaten by the birds. The concentration of DDT was magnified several thousand times in the birds which caused thinning of shells in the birds’ eggs, causing death of the young ones. It becomes very clear from the above instance that the animals occupying the higher trophic levels are at a greater risk of biomagnification of toxic chemicals. Human beings consuming milk, eggs and meat are at a higher trophic level. So, we have to stop indiscriminate use of pesticides and heavy metals if we wish to save ourselves from their biologically magnified toxic levels. n ECOLOGICAL PYRAMIDS Graphic representation of trophic structure and function of an ecosys- tem, starting with producers at the base and successive trophic levels forming the apex is knows as an ecological pyramid. Ecological pyra- mids are of three types: Pyramid of numbers: It represents the number of individual organisms at each trophic level. We may have upright or inverted pyramid 74 Environmental Science and Engineering of numbers, depending upon the type of ecosystem and food chain as shown in Fig. 3.5. A grassland ecosystem (Fig. 3.5a) and a pond ecosystem show an upright pyramid of numbers. The producers in the grasslands are grasses and that in a pond are phytoplanktons (algae etc.), which are small in size and very large in number. So the producers form a broad base. The herbivores in a grassland are insects while tertiary carnivores are hawks or other birds which are gradually less and less in number and hence the pyramid apex becomes gradually narrower forming an upright pyramid. Similar is the case with the herbivores, carnivores and top carnivores in pond which decrease in number at higher trophic levels. Top carnivores Top carnivores Hawks, Lion, Tiger other birds Carnivores Frogs, birds Snakes, foxes, Carnivores lizards Herbivores Insects Herbivores Insects, birds Producers Grasses Producers Trees (a) (b) Hyper parasites Fleas, microbes Parasites Lice, bugs Herbivores Birds Producers Trees (c) Fig. 3.5. Pyramid of numbers (a) grassland (b) forest (c) Parasitic food chain. In a forest ecosystem, big trees are the producers, which are less in number and hence form a narrow base. A larger number of herbivores including birds, insects and several species of animals feed upon the trees (on leaves, fruits, flowers, bark etc.) and form a much broader middle level. The secondary consumers like fox, snakes, lizards etc. are less in number than herbivores while top carnivores like lion, tiger etc. are still smaller in number. So the pyramid is narrow on both sides and broader in the middle (Fig. 3.5 b). Parasitic food chain shows an inverted pyramid of number. The producers like a few big trees harbour fruit eating birds acting like  Ecosystems 75 herbivores which are larger in number. A much higher number of lice, bugs etc. grow as parasites on these birds while a still greater number of hyperparasites like bugs, fleas and microbes feed upon them, thus making an inverted pyramid (Fig. 3.5 c). Pyramid of biomass: It is based upon the total biomass (dry matter) at each trophic level in a food chain. The pyramid of biomass can also be upright or inverted. Fig. 3.6 (a, b) show pyramids of biomass in a forest and an aquatic ecosystem. The pyramid of biomass in a forest is upright in contrast to its pyramid of numbers. This is because the producers (trees) accumulate a huge biomass while the consumers’ total biomass feeding on them declines at higher trophic levels, resulting in broad base and narrowing top. Tertiary Carnivores Big fish Carnivores Carnivores Small fish Snakes, frog, birds Herbivores Insects Squirrel, rabbit, Producers Herbivores insects Phytoplanktons Producers Grasses, herbs (a) (b) Fig. 3.6. Pyramid of biomass (a) Grassland (b) Pond. The pond ecosystem shows an inverted pyramid of biomass (Fig. 3.6 b). The total biomass of producers (phytoplanktons) is much less as compared to herbivores (zooplanktons, insects), Carnivores (Small fish) and tertiary carnivores (big fish). Thus the pyramid takes an inverted shape with narrow base and broad apex. Pyramid of Energy: The amount of energy present at each trophic level is considered for this type of pyramid. Pyramid of energy gives the best representation of the trophic relationships and it is always upright. At every successive trophic level, there is a huge loss of energy (about 90%) in the form of heat, respiration etc. Thus, at each next higher level only 10% of the energy passes on. Hence, there is a sharp decline in energy level of each successive trophic level as we move from producers to top carnivores. Therefore, the pyramid of energy is always upright as shown in Fig. 3.7. 76 Environmental Science and Engineering Top carnivores Carnivores Herbivores Producers Fig. 3.7. Pyramid of energy. n ENERGY FLOW IN AN ECOSYSTEM Flow of energy in an ecosystem takes place through the food chain and it is this energy flow which keeps the ecosystem going. The most important feature of this energy flow is that it is unidirectional or one- way flow. Unlike the nutrients (like carbon, nitrogen, phosphorus etc.) which move in a cyclic manner and are reused by the producers after flowing through the food chain, energy is not reused in the food chain. Also, the flow of energy follows the two laws of Thermodynamics: Ist law of Thermodynamics states that energy can neither be created nor be destroyed but it can be transformed from one form to another. The solar energy captured by the green plants (producers) gets converted into biochemical energy of plants and later into that of consumers. IInd law of Thermodynamics states that energy dissipates as it is used or in other words, its gets converted from a more concentrated to dispersed form. As energy flows through the food chain, there occurs dissipation of energy at every trophic level. The loss of energy takes place through respiration, loss of energy in locomotion, running, hunt- ing and other activities. At every level there is about 90% loss of energy and the energy transferred from one trophic level to the other is only about 10%. Energy flow models: The flow of energy through various trophic levels in an ecosystem can be explained with the help of various energy flow models. (a) Universal energy flow model: Energy flow through an ecosystem was explained by E.P. Odum as the universal energy flow model (Fig. 3.8). As the flow of energy takes place, there is a gradual loss of energy at every level, thereby resulting in less energy available at next trophic level as indicated by narrower pipes (energy flow) and smaller boxes (stored energy in biomass). The loss of energy is mainly the energy not utilized (NU). This is the energy lost in locomotion,  Ecosystems 77 excretion etc. or it is the energy lost in respiration (R) which is for maintenance. The rest of the energy is used for production (P). NU Energy storage Input I P Output A energy energy Standing Biomass Respiration Fig. 3.8. Universal energy flow model applicable to all living components (I = Energy input; A : assimilated energy ; P = Production ; NU = Energy not used. (b) Single channel energy flow model: The flow of energy takes place in a unidirectional manner through a single channel of green plants or producers to herbivores and carnivores. Fig. 3.9 depicts such a model and illustrated the gradual decline in energy level due to loss of energy at each successive trophic level in a grazing food chain. NU NA NU NA Sunlight I GPP NPP Carnivores Herbivores Producers R R R Heat loss Fig. 3.9. One-way energy flow model showing unidirectional flow through primary producers, herbivores and carnivores. At each successive trophic level there is huge loss of energy (I = Solar energy input ; GPP = Gross primary production ; NPP = Net primary production ; NU = Energy not used ; NA = Energy not assimilated e.g. excretion ; R = Respiratory loss). 78 Environmental Science and Engineering (c) Double channel or Y-shaped energy flow model: In nature, both grazing food chain and detritus food chain operate in the same ecosystem. However, sometimes it is the grazing food chain which predominates. It happens in marine ecosystem where primary production in the open sea is limited and a major portion of it is eaten by herbivorous marine animals. Therefore, very little primary production is left to be passed on to the dead or detritus compartment. On the other hand, in a forest ecosystem the huge quantity of biomass produced cannot be all consumed by herbivores. Rather, a large proportion of the live biomass enters into detritus (dead) compartment in the form of litter. Hence the detritus food chain is more important there. The two channel or Y-shaped model of energy flow shows the passage of energy through these two chains, which are separated in time and space (Fig 3.10). Respiration R R RR R s Carnivores ivore Herb Tree Grazing food chain canopy ( in forest canopy) Ground R level Litter, roots etc. D D Storage Producers D (Trees) Detritus food chain (in soil) Detritivores Decomposers Fig. 3.10. Y-shaped or 2-channel energy flow model showing energy flow through the grazing food chain and the detritus food chain (R = Respiration, D = Detritus or dead matter). n NUTRIENT CYCLING Besides energy flow, the other important functional attribute of an ecosystem is nutrient cycling. Nutrients like carbon, nitrogen, sulphur, oxygen, hydrogen, phosphorus etc. move in circular paths through biotic and abiotic components and are therefore known as biogeochemical cycles. Water also moves in a cycle, known as hydrological cycle. The nutrients too move through the food chain and ultimately reach the  Ecosystems 79 detritus compartment (containing dead organic matter) where various micro-organisms carry out decomposition. Various organically bound nutrients of dead plants and animals are converted into inorganic substances by microbial decomposition that are readily used up by plants (primary producers) and the cycle starts afresh. Nitrogen cycle Cycling of one such important nutrient nitrogen is shown in Fig. 3.11. Nitrogen is present in the atmosphere as N2 in large amount (78%) and it is fixed either by the physical process of lightening or biologically by some bacteria and/or cyanobacteria (blue green algae). The nitrogen is taken up by plants and used in metabolism for biosynthesis of amino acids, proteins, vitamins etc. and passes through the food chain. After death of the plants and animals, the organic nitrogen in dead tissues is decomposed by several groups of ammonifying and nitrifying bacteria which convert them into ammonia, nitrites and nitrates, which are again used by plants. Some bacteria convert nitrates, into molecular nitrogen or N2 which is released back into the atmosphere and the cycle goes on. Atmosphere Nitrogen Volcanic NOX Acid rain eruptions N2 Biological Electrification N2 fixation Acid rain Fertilizer Runoff Hydro- (Eutrophication) sphere Litho- Animal Animals protoplasm Shallow marine sphere Nitrates sediments (soil) Industrial Plant activities protoplasm Nitrites Loss to deep Nitrification sediments Death and Deacy Ammonia Ammonification Denitrification Excretion Organic (Urea, uric Nitrogen acid) (Proteins, amino acids) Fig. 3.11. Nitrogen cycle—a gaseous cycle with major reserve as N2 (78%) in the atmosphere. Circulation of N- between living components and soil/atmosphere is mediated by a group of micro-organisms which convert one form of N into another. 80 Environmental Science and Engineering Carbon Cycle Sometimes human interferences disturb the normal cycling of such nutrients and create imbalances. For example, nature has a very bal- anced carbon cycle (Fig. 3.12). Carbon, in the form of carbon dioxide is taken up by green plants as a raw material for photosynthesis, through which a variety of carbohydrates and other organic substances are pro- duced. Through the food chain it moves and ultimately organic carbon present in the dead matter is returned to the atmosphere as carbon dioxide by microorganisms. Respiration by all organisms produces carbon dioxide, while the latter is used up by plants. In the recent years carbon dioxide levels have increased in the atmosphere due to burning of fossil fuels etc. which has caused an imbalance in the natural cycle and the world today is facing the serious problem of global warming due to enhanced carbon dioxide emissions. n Atmospheric atio s pir re Carbondioxide al t n m (CO2) a n io P l i rat i An y pl rest on b sp re ti ts l an ria te ixa Fossil fuel f Decomposition 2 CO burning r Direct CO2 fixation absorption (by aquatic plants) Carbonates, CO2 Dead organic matter (Organic carbon) Microbial action Fig. 3.12. Carbon cycle. Phosphorus cycle Phosphorous cycle is another important nutrient cycle-which is shown in Fig. 3.13. The reservoir of phosphorus lies in the rocks, fossils etc. which is excavated by man for using it as a fertilizer. Farmers use the phosphate fertilizers indiscriminately and as a result excess phosphates are lost as run-off, which causes the problem of eutrophication or overnourishment of lakes leading to algal blooms as already discussed  Ecosystems 81 P-reserves Death and Excreta Guano(P-rich) decay deposits Sea birds Phosphate Bones and Animal rocks teeth protoplasm Eutro- Runoff phication fossil, bones, teeth Plant Marine protoplasm fish etc. Fertilizers Mining (PO4) ia er Erosion ct Loss to deep g ba marine sediments izin Plant Phosphat Dissolved phosphate uptake Fig. 3.13. Phosphorus cycle—a sedimentary cycle with major reserves of phosphorus in the sediments. in unit 2. A good proportion of phosphates moving with surface run- off reaches the oceans and are lost into the deep sediments. Our limited supply of phosphorus lying in the phosphate rocks of this earth are thus over-exploited by man and a large part is taken out of the normal cycle due to loss into oceans. So human beings are making the phosphorous cycle acyclic. Sea birds, on the other hand, are playing an important role in phosphorus cycling. They eat sea-fishes which are phosphorus rich and the droppings or excreta of the birds return the phosphorus on the land. The Guano deposits on the coasts of Peru are very rich sources of phosphorus. n PRIMARY PRODUCTION Primary productivity of an ecosystem is defined as the rate at which radiant energy is converted into organic substances by photosyn- thesis or chemo-synthesis by the primary producers. When organic matter is produced by the primary producers (mainly green plants and some microorganisms), some of it is oxidized or burnt inside their body and converted into carbon-dioxide which is released during respiration and is accompanied by loss of energy. Respiratory loss of energy is a must, because it is required for the maintenance of the organism. Now, the producers are left with a little less organic matter than what was actually produced by them. This is known as the net primary production (NPP) and the respiratory loss (R) added to it gives the gross primary production (GPP). Thus, NPP = GPP – R. 82 Environmental Science and Engineering Primary production of an ecosystem depends upon the solar radiations, availability of water and nutrients and upon the type of the plants and their chlorophyll content. Table 3.1 shows the average gross primary productivity of some major ecosystems. Table 3.1. Annual average of gross primary production of some major ecosystems Ecosystem Gross Primary Productivity (K Cal/m2/yr) Deserts and Tundra 200 Open Oceans 1,000 Grasslands 2,500 Moist Temperate Forests 8,000 Agro-ecosystems 12,000 Wet Tropical Forests 20,000 Estuaries 20,000 Productivity of tropical forests and estuaries are the highest. This is because tropical forests have abundant rainfall, warm temperature congenial for growth, abundant sunlight and a rich diversity of species. Estuaries get natural energy subsidies in the form of wave currents that bring along with them nutrients required for production. Deserts on the other hand, have limitations of adequate water supply while Tundra have very low temperature as limiting factor and hence show low primary production. Agro-ecosystems get lots of energy subsidies in the form of irrigation water, good quality seeds, fertilizers and pesticides and show a high productivity of 12,000 K Cal/m2/yr. Still, it is noteworthy that their productivity is less than that of tropical forests which are not receiving any artificial energy subsidies. Nature itself has designed its species composition, structure, energy capture and flow, and a closed nutrient cycling system that ensures a high primary production of 20,000 K Cal/m2/yr. Also, the qualitative variety of the primary production is enormous in the tropical forests. This makes it all the more important to conserve our tropical forests.  Ecosystems 83 Secondary Production The food synthesized by green plants through photosynthesis is the primary production which is eaten by herbivores. The plant energy is used up for producing organic matter of the herbivores which, in turn, is used up by the carnivores. The amount of organic matter stored by the herbivores or carnivores (in excess of respiratory loss) is known as secondary production. The energy stored at consumer level for use by the next trophic level is thus defined as secondary production. n ECOSYSTEM REGULATION All ecosystems regulate themselves and maintain themselves under a set of environmental conditions. Any environmental stress tries to disturb the normal ecosystem functions. However, the ecosystem, by itself, tries to resist the change and maintain itself in equilibrium with the environment due to a property known as homeostasis. Homeostasis is the inherent property of all living systems to resist change. However, the system can show this tolerance or resistance only within a maximum and a minimum range, which is its range of tolerance known as homeostatic plateau. Within this range, if any stress tries to cause a deviation, then the system has its own mechanisms to counteract these deviations which are known as negative feedback mechanisms. So negative feedback mechanisms are deviation counteracting mechanisms which try to bring the system back to its ideal conditions. But, if the stress is too high and beyond the range of homeostatic plateau, then another type of mechanisms known as positive feedback mechanisms start operating. These are the deviation accelerating mechanisms. So the positive feedback mechanisms add to the stress conditions and tend to take the system away from the optimal conditions. Fig. 3.14 depicts the ecosystem regulation mechanisms. Human beings should try to keep the ecosystems within the homeostatic plateau. They should not contribute to positive feedbacks otherwise the ecosystems will collapse. 84 Environmental Science and Engineering Death or + ve collapse Feedback System function – ve – ve Homeostatic Feedback Feedback plateau + ve Feedback Death or collapse (–) O (+) Stress conditions Fig. 3.14. Ecosystem regulation by homeostasis. On application of a stress, the negative feedback mechanisms start operating, trying to counter the stress to regulate the system. But beyond the homeostatic plateau, positive feedback starts which further accelerate the stress effects causing death or collapse of the organism/system. n ECOLOGICAL SUCCESSION An ecosystem is not static in nature. It is dynamic and changes its structure as well as function with time and quite interestingly, these changes are very orderly and can be predicted. It is observed that one type of a community is totally replaced by another type of community over a period of time and simultaneously several changes also occur. This process is known as ecological succession. Ecological succession is defined as an orderly process of changes in the community structure and function with time mediated through modifications in the physical environment and ultimately culminating in a stabilized ecosystem known as climax. The whole sequence of communities which are transitory are known as Seral stages or seres whereas the community establishing first of all in the area is called a pioneer community. Ecological successions starting on different types of areas or substrata are named differently as follows: (i) Hydrarch or Hydrosere: Starting in watery area like pond, swamp, bog  Ecosystems 85 (ii) Mesarch: starting in an area of adequate moisture. (iii) Xerarch or Xerosere: Starting in a dry area with little moisture. They can be of the following types: Lithosere : starting on a bare rock Psammosere : starting on sand Halosere : starting on saline soil Process of Succession The process of succession takes place in a systematic order of sequential steps as follows: (i) Nudation: It is the development of a bare area without any life form. The bare area may be caused due to landslides, volcanic eruption etc. (topographic factor), or due to drought, glaciers, frost etc. (Climatic factor), or due to overgrazing, disease outbreak, agricultural/ industrial activities (biotic factors). (ii) Invasion: It is the successful establishment of one or more species on a bare area through dispersal or migration, followed by ecesis or establishment. Dispersal of the seeds, spores etc. is brought about by wind, water, insects or birds. Then the seeds germinate and grow on the land. As growth and reproduction start, these pioneer species increase in number and form groups or aggregations. (iii) Competition and coaction: As the number of individuals grows there is competition, both inter-specific (between different species) and intra-specific (within the same species), for space, water and nutrition. They influence each other in a number of ways, known as coaction. (iv) Reaction: The living organisms grow, use water and nutrients from the substratum, and in turn, they have a strong influence on the environment which is modified to a large extent and this is known as reaction. The modifications are very often such that they become unsuitable for the existing species and favour some new species, which replace them. Thus, reaction leads to several seral communities. (v) Stabilization: The succession ultimately culminates in a more or less stable community called climax which is in equilibrium with the environment. The climax community is characterized by maximum biomass and symbiotic (mutually beneficial) linkages between organisms and are maintained quite efficiently per unit of available energy. Let us consider very briefly two types of succession. 86 Environmental Science and Engineering A. Hydrosere (Hydrarch): This type of succession starts in a water body like pond. A number of intermediate stages come and ultimately it culminates in a climax community which is a forest. The pioneer community consists of phytoplanktons, which are free floating algae, diatoms etc. Gradually these are replaced by rooted- submerged plants followed by rooted-floating plants. Growth of these plants keep on adding organic matter to the substratum by death and Free floating stage (a) Open water body (lake), sediment brought in by river. Rooted floating stage Sediment (b) Sediment accumulation continues, organic debris from plants too add to soil formation and shrinking of water body occurs. Marshy vegetation Soil with standing water (c) A mat of vegetation covers the water which is mostly a marshy habitat now, with a small part as aquatic system.  Ecosystems 87 Woodland Stage Soil (d) Eventually the former lake is covered by climax woodland community, representating a terrestrial ecosystem. Fig. 3.15. Ecological succession: A hydrach—from lake to woodland community. decay and thus a layer of soil builds up and shallowing of water takes place. Then Reed swamp (marshy) stage follows in which the plants are partly in water and partly on land. This is followed by a sedge- meadow stage of grasses then by a woodland consisting of shrubs and trees and finally by a forest acting as climax. (Fig. 3.15) B. Xerosere (Xerarch): This type of succession originates on a bare rock, which lacks water and organic matter. Interestingly, here also the climax community is a forest, although the intermediate stages are very different. The pioneer community here consists of crustose and foliose lichens. These lichens produce some weak acids and help in disintegrating the rock, a process known as weathering. Their growth helps in building up gradually some organic matter, humus and soil. Then comes the community of mosses, followed by herbs, shrubs and finally the forest trees. Throughout this gradual process there is a slow build up of organic matter and water in the substratum. Thus, succession tends to move towards mesic conditions (moderate condition), irrespective of the fact, whether it started from a dry (Xeric) condition or a moist (hydric) condition and it culminates in a stable climax community, which is usually a forest. MAJOR ECOSYSTEM TYPES Let us consider types, characteristic features, structure and functions of some major ecosystems. 88 Environmental Science and Engineering n FOREST ECOSYSTEM These are the ecosystems having a predominance of trees that are interspersed with a large number of species of herbs, shrubs, climbers, lichens, algae and a wide variety of wild animals and birds. As discussed above forests are found in undisturbed areas receiving moderate to high rainfall and usually occur as stable climax communities. Depending upon the prevailing climatic conditions forests can be of various types: (a) Tropical Rain Forests: They are evergreen broadleaf forests found near the equator. They are characterized by high temperature, high humidity and high rainfall, all of which favour the growth of trees. All through the year the climate remains more or less uniform. They are the richest in biodiversity. Different forms of life occupy specialized areas (niches) within different layers and spaces of the ecosystem depending upon their needs for food, sunlight, water, nutrient etc. We come across different types and layers of plants and animals in the tropical rain forests. e.g. the emergent layer is the topmost layer of the tallest broad-leaf evergreen trees, below which lies the canopy where top branches of shorter trees form an umbrella like cover. Below this is present the understory of still smaller trees. On the tree trunks some woody climbers are found to grow which are known as Lianas. There are some other plants like Orchids which are epiphytes i.e. they are attached to the trunks or branches of big trees and they take up water and nutrients falling from above. The orchids have special type of leaves to capture and hold the water. Some large epiphytes can hold as much as 4 litres of water, equivalent to a small bucket! Thus, these epiphytes almost act like mini-ponds suspended up in the air, in the forest crown. That is the reason why a large variety of birds, insects and animals like monkeys have made their natural homes (habitats) in these forests (Plate II). The understorey trees usually receive very dim sunlight. They usually develop dark green leaves with high chlorophyll content so that they can use the diffused sunlight for photosynthesis. The shrub layer receives even less sunlight and the ground layer commonly known as forest floor receives almost no sunlight and is a dark layer. Most of the animals like bats, birds, insects etc. occupy the bright canopy layer while monkeys, toads, snakes, chameleons etc. keep on moving up and down in sunny and darker layers. Termites, fungi, mushrooms etc. grow on the ground layer. Warm temperature and high availability of moisture facilitate rapid breakdown (decomposition) of the dropped leaves, twigs etc. releasing the nutrients rapidly. These nutrients are immediately taken up by the mycorrhizal roots of the trees.  Ecosystems 89 Plate II. Tropical rain forest. Interestingly, the flowers of forest trees are very large, colourful, fragrant and attractive which helps in pollination by insects, birds, bats etc. Rafflesia arnoldi, the biggest flower (7 kg weight) is known to smell like rotten meat and attracts flies and beetles which help in its pollination (Plate III). Plate III. Rafflesia—the biggest flower. 90 Environmental Science and Engineering The Silent Valley in Kerala is the only tropical rain forest lying in India which is the natural habitat for a wide variety of species. Being the store-house of biodiversity, the forests provide us with an array of commercial goods like timber, fuel wood, drugs, resins, gums etc. Unfortunately there is cutting down of these forests at an alarming rate. Within the next 30-40 years we are likely to be left with only scattered fragments of such forests, thereby losing the rich biodiversity and the ecological uses of forests, discussed earlier in unit II. (b) Tropical deciduous forests: They are found a little away from the equator and are characterized by a warm climate the year round. Rain occurs only during monsoon. A large part of the year remains dry and therefore different types of deciduous trees are found here, which lose their leaves during dry season. (c) Tropical scrub forests: They are found in areas where the dry season is even longer. Here there are small deciduous trees and shrubs. (d) Temperate rain forests: They are found in temperate areas with adequate rainfall. These are dominated by coniferous trees like pines, firs, redwoods etc. They also consist of some evergreen broad- leaf trees. (e) Temperate deciduous forests: They are found in areas with moderate temperatures. There is a marked seasonality with long sum- mers, cold but not too severe winter and abundant rainfall throughout the year. The major trees include broad leaf deciduous trees like oak, hickory, poplar etc. (f ) Evergreen coniferous forests (Boreal Forests): They are found just south of arctic tundra. Here winters are long, cold and dry. Sunlight is available for a few hours only. In summer the temperature is mild, sun-shines for long hours but the season is quite short. The major trees include pines, spruce, fir, cedar etc. which have tiny, nee- dle-shaped leaves having a waxy coating so that they can withstand severe cold and drought. The soil is found to get frozen during winter when few species can survive. The leaves, also know as needles, fall on the forest floor and cover the nutrient poor soil. These soils are acidic and prevent other plants from growing. Species diversity is rather low in these forests.  Ecosystems 91 n GRASSLAND ECOSYSTEMS Grasslands are dominated by grass species but sometimes also allow the growth of a few trees and shrubs. Rainfall is average but erratic. Limited grazing helps to improve the net primary production of the grasslands but overgrazing leads to degradation of these grasslands resulting in desertification. Three types of grasslands are found to occur in different climatic regions: (a) Tropical grasslands: They occur near the borders of tropical rain forests in regions of high average temperature and low to moderate rainfall. In Africa, these are typically known as Savannas, which have tall grasses with scattered shrubs and stunted trees. The Savannas have a wide diversity of animals including zebras, giraffes, gazelle, antelopes etc. During dry season, fires are quite common. Termite mounds are very common here. The termites gather the detritus (dead organic matter) containing a lot of cellulose and build up a mound. On the top of the mound fungi are found to grow which feed upon this dead matter including cellulose and in turn release methane, a greenhouse gas. Tropical savannas have a highly efficient system of photosynthesis. Most of the carbon assimilated by them in the form of carbohydrates is in the perennating bulbs, rhizomes, runners etc. which are present underground. Deliberate burning of these grasslands can relase huge quantities of carbon dioxide, another green house gas, responsible for global warming. (b) Temperate grasslands: They are usually found on flat, gentle sloped hills, winters are very cold but summers are hot and dry. Intense grazing and summer fires do not allow shrubs or trees to grow. In United States and Canada these grasslands are known as prairies, in South America as Pampas, in Africa as Velds and in central Europe and Asia they are known as Steppes. Winds keep blowing and evaporation rate is very high. It also favours rapid fires in summer. The soils are quite fertile and therefore, very often these grasslands are cleared for agriculture. (c) Polar grasslands (Arctic Tundra): They are found in arctic polar region where severe cold and strong, frigid winds along with ice and snow create too harsh a climate for trees to grow. In summers the sun-shines almost round the clock and hence several small annual plants grow in the summer. The animals include arctic wolf, weasel, arctic fox, reindeer etc. A thick layer of ice remains frozen under the soil surface throughout the year and is known as permafrost. In summer, the tundra shows the appearance of shallow lakes, bogs etc. where mosquitoes, different type of insects and migratory birds appear. 92 Environmental Science and Engineering n DESERT ECOSYSTEMS These ecosystems occur in regions where evaporation exceeds precipitation (rainfall, snow etc.). The precipitation is less than 25 cm per year. About 1/3rd of our world’s land area is covered by deserts. Deserts have little species diversity and consist of drought resistant or drought avoiding plants. The atmosphere is very dry and hence it is a poor insulator. That is why in deserts the soil gets cooled up quickly, making the nights cool. Deserts are of three major types, based on climatic conditions: (a) Tropical deserts like Sahara and Namib in Africa and Thar desert, Rajasthan, India are the driest of all with only a few species. Wind blown sand dunes are very common. (b) Temperate deserts like Mojave in Southern California where day time temperatures are very hot in summer but cool in winters. (c) Cold deserts like the Gobi desert in China has cold winters and warm summers. Desert plants and animals are having most typical adaptations for conservation of water. Many desert plants are found to have re- duced, scaly leaves so as to cut down loss of water due to transpiration or have succulent leaves to store water. Many a times their stems get flattened and develop chlorophyll so that they can take up the function of photosynthesis. Some plants show very deep roots to tap the groundwater. Many plants have a waxy, thick cuticle over the leaf to reduce loss of water through transpiration. Desert animals like insects and reptiles have thick outer coverings to minimize loss of water. They usually live inside burrows where humidity is better and heat is less. Desert soil is rich in nutrients but deficient in water. Due to low species diversity, shortage of water and slow growth rate, the desert plant communities, if faced with a severe stress take a long time to recover. n AQUATIC ECOSYSTEMS Aquatic ecosystems dealing with water bodies and the biotic communities present in them are either freshwater or marine. Freshwater ecosystems are further of standing type (lentic) like ponds and lakes or free-flowing type (lotic), like rivers. Let us consider some important aquatic ecosystems. (a) Pond ecosystem: It is a small freshwater aquatic ecosystem where water is stagnant. Ponds may be seasonal in nature i.e. receiving  Ecosystems 93 enough water during rainy season. Ponds are usually shallow water bodies which play a very important role in the villages where most of the activities center around ponds. They contain several types of algae, aquatic plants, insects, fishes and birds. The ponds are, however, very often exposed to tremendous anthropogenic (human-generated) pres- sures. They are used for washing clothes, bathing, swimming, cattle bathing and drinking etc. and therefore get polluted. (b) Lake ecosystems: Lakes are usually big freshwater bodies with standing water. They have a shallow water zone called Littoral zone, an open-water zone where effective penetration of solar light takes place, called Limnetic zone and a deep bottom area where light penetration is negligible, known as profundal zone (Fig. 3.16). Littoral zone Rooted plants Euphotic zone Limnetic zone (High productivity) Compensation level Profundal zone Aphotic zone (Dark) (Little productivity) Fig. 3.16. Zonation in a lake ecosystem. The Dal Lake in Srinagar (J & K), Naini Lake in Nainital (Uttaranchal) and Loktak lake in Manipur are some of the famous lakes of our country. Organisms : The lakes have several types of organisms: (a) Planktons that float on the surface of waters e.g. phytoplanktons like algae and zooplanktons like rotifers. (b) Nektons that swim e.g. fishes. (c) Neustons that rest or swim on the surface. (d) Benthos that are attached to bottom sediments e.g. snails. (e) Periphytons that are attached or clinging to other plants or any other surface e.g. crustaceans. Stratification : The lakes show stratification or zonation based on temperature differences. During summer, the top waters become warmer than the bottom waters. Therefore, only the warm top layer 94 Environmental Science and Engineering circulates without mixing with the colder layer, thus forming a distinct zonation: Epilimnion : Warm, lighter, circulating surface layer Hypolimnion : Cold, viscous, non-circulating bottom layer. In between the two layers is thermocline, the region of sharp drop in temperature. Types of Lakes : Some important types of lakes are: (a) Oligotrophic lakes which have low nutrient concentrations. (b) Eutrophic lakes which are overnourished by nutrients like nitrogen and phosphorus, usually as a result of agricultural run-off or municipal sewage discharge. They are covered with “algal blooms” e.g. Dal Lake. (c) Dystrophic lakes that have low pH, high humic acid content and brown waters e.g. bog lakes. (d) Endemic lakes that are very ancient, deep and have endemic fauna which are restricted only to that lake e.g. the Lake Baikal in Russia; the deepest lake, which is now suffering a threat due to industrial pollution. (e) Desert salt lakes that occur in arid regions and have devel- oped high salt concentrations as a result of high evaporation. e.g. great salt lake, Utah; Sambhar lake in Rajasthan. (f ) Volcanic lakes that receive water from magma after volcanic eruptions e.g. many lakes in Japan. They have highly restricted biota. (g) Meromictic lakes that are rich in salts and are permanently stratified e.g. lake Nevada. (h) Artificial lakes or impoundments that are created due to con- struction of dams e.g. Govindsagar lake at Bhakra-Nangal. Streams These are freshwater aquatic ecosystems where water current is a major controlling factor, oxygen and nutrient in the water is more uniform and land-water exchange is more extensive. Although stream organisms have to face more extremes of temperature and action of currents as compared to pond or lake organisms, but they do not have to face oxygen deficiency under natural conditions. This is because the streams are shallow, have a large surface exposed to air and constant motion which churns the water and provides abundant oxygen. Their dissolved oxygen level is higher than that of ponds even though the green plants  Ecosystems 95 are much less in number. The stream animals usually have a narrow range of tolerance to oxygen. That is the reason why they are very susceptible to any organic pollution which depletes dissolved oxygen in the water. Thus, streams are the worst victims of industrial development. River Ecosystem: Rivers are large streams that flow downward from mountain highlands and flowing through the plains fall into the sea. So the river ecosystems show a series of different conditions. The mountain highland part has cold, clear waters rushing down as water falls with large amounts of dissolved oxygen. The plants are attached to rocks (periphytons) and fishes are cold-water, high oxygen requiring fish like trouts. In the second phase on the gentle slopes, the waters are warmer and support a luxuriant growth of plants and less oxygen requiring fishes. In the third phase, the river waters are very rich in biotic diversity. Moving down the hills, rivers shape the land. They bring with them lots of silt rich in nutrients which is deposited in the plains and in the delta before reaching the ocean. Oceans These are gigantic reservoirs of water covering more than 70% of our earth’s surface and play a key role in the survival of about 2,50,000 marine species, serving as food for humans and other organisms, give a huge variety of sea-products and drugs. Oceans provide us iron, phosphorus, magnesium, oil, natural gas, sand and gravel. Oceans are the major sinks of carbon dioxide and play an important role in regulating many biogeochemical cycles and hydrological cycle, thereby regulating the earth’s climate. The oceans have two major life zones: (Fig. 3.17) Coastal zone with relatively warm, nutrient rich shallow water. Due to high nutrients and ample sunlight this is the zone of high primary productivity. Open sea: It is the deeper part of the ocean, away from the continental shelf (The submerged part of the continent). It is vertically divided into three regions: (i) Euphotic zone which receives abundant light and shows high photosynthetic activity. 96 Environmental Science and Engineering Intertidal Neritic Oceanic Euphotic zone Aphotic zone Mid oceanic ridges Bathyal zone Trench Continental shelf Continental slope Continental Abyssal plain rise Fig. 3.17. Vertical and horizontal zonation of a marine ecosystem. (ii) Bathyal zone receives dim light and is usually geologically active. (iii) Abyssal zone is the dark zone, 2000 to 5000 metres deep. The abyssal zone has no primary source of energy i.e. solar energy. It is the world’s largest ecological unit but it is an incomplete ecosystem. Estuary An estuary is a partially enclosed coastal area at the mouth of a river where fresh water and salty seawater meet. These are the transition zones which are strongly affected by tidal action. Constant mixing of water stirs up the silt which makes the nutrients available for the pri- mary producers. There are wide variations in the stream flow and tidal currents at any given location diurnally, monthly and seasonally. There- fore, the organisms present in estuaries show a wide range of tolerance to temperature and salinity. Such organisms are known as eurythermal and euryhaline. Coastal bays, and tidal marshes are examples of estu- aries. Estuaries have a rich biodiversity and many of the species are en- demic. There are many migratory species of fishes like eels and salmons in which half of the life is spent in fresh water and half in salty water. For them estuaries are ideal places for resting during migration, where they also get abundant food. Estuaries are highly productive ecosys- tems. The river flow and tidal action provide energy subsidies for the estuary thereby enhancing its productivity. Estuaries are of much use