The "Ecosystem" term was first coined by A.G. Tansley in 1935. E.P. Odum is the Father of the ecosystem. An Ecosystem a complete ecological unit in the ecological hierarchy that includes both biotic and abiotic compound of a particular area. Simply it is a functional unit where input and output of substance or matter occur between living and non-living component in that area. Both biotic and abiotic components have interacted through the nutrient cycles and energy flow between them and the living organisms also interact with each other.
The ecosystem is the smallest structural and functional unit of nature or environment. This term is derived from two words, namely eco and system. Eco refers to environment and system refers to a complex coordinated unit. It is an interacting system where the biotic and abiotic factors interact to produce an exchange of materials between the living and non–living factors by which they influence the property of each other. The pond is a suitable example for the ecosystem. Other examples of the ecosystem are the river, lake, estuary, ocean, marine, desert, forest, grassland, town, city etc. Even a drop of water is an Ecosystem.
4.1.    Characteristics of Ecosystem :
(1)     In any Ecosystem, the biotic communities interact with their abiotic component that affects the biotic properties of individual maintaining their flow of energy.
(2)     The ecosystem is normally an open system because of both energy and materials flow in and out of the system.
(3)     The size of an ecosystem may be micro (such as a drop of water) or macro (sea).
(4)     An ecosystem may be temporary as a fresh water pond and agriculture field or permanent like a forest, desert, grassland, ocean and river. 
(5)     Sun is an ultimate source of energy for an ecosystem.
 (6)     The ecosystem is an autoregulatory and auto sustaining unit. Each trophic level controls the other trophic level. 
(7)     The branch of science that deals with auto control of an ecosystem are called cybernetics. 
(8)    Homeostasis is the process of ecosystem equilibrium in case of changes in one trophic level of the ecosystem, the other trophic level of the same ecosystem may react according to it. So the ecosystem always remains in equilibrium.    
4.2.    Type of ecosystem
4.2.1.    Natural Ecosystem -
These are self–regulating systems without much direct human interference and manipulations.     Terrestrial Ecosystem - eg. forest, grassland, tree, desert ecosystems.     Aquatic Ecosystem - Aquatic ecosystem is again of two types.
(2a)     Lentic ecosystem - A lentic ecosystem has still waters. Examples: ponds, marshes, ditches, reservoirs and lakes.
(2b)     Lotic ecosystem - A lotic ecosystem has flowing waters. Examples: creeks, streams, rivers, springs and channels.

4.2.2.     Artificial Ecosystem
These are man-made ecosystems. e.g. agriculture land, city, town, cropland, gardens etc. 
Type of ecosystem on the basis of size:
(i)     Mega ecosystems: The mega ecosystem is further divided into subunit called macro ecosystems. e.g., Forests. The terrestrial macro ecosystem is formed of many forest ecosystems. 
(ii)     Macro ecosystem: The macro ecosystem is further divided into the meso ecosystem. For example, the forest ecosystem is formed of many meso ecosystems like the deciduous forest, coniferous forests, etc.
(iii)     Meso ecosystem: The meso ecosystem is further divided into micro-ecosystems, e.g., A low land in a forest, a mountain in a forest, pond etc. 
(iv)     Nanoecosystem - Drop of water.
4.3.    Components and Structure of Ecosystem
The ecosystem has two components
(A)     Abiotic component             
(B)     Biotic component
4.3.1.     Abiotic component: The abiotic factors of an ecosystem include the non–living substances of the environment. eg. water, soil, air, light, temperature, minerals, climate, pressure etc. The biotic factors of the ecosystem depend upon the abiotic factors for their survival. 
4.3.2.     Biotic component: The biotic factor includes the living organisms of the environment. e.g. plants, animals, bacteria, viruses etc. The biotic factors of an ecosystem are classified into three main groups, namely :
(a)     Producers             
(b)     Consumers             
(c)     Reducers or Decomposers.    Producers: These are autotrophs or organisms that produce biomass on their own basis. They carry out photosynthesis and are provided with chlorophyll. In the process of photosynthesis, producers absorb sun light and convert it into chemical energy so producers are also called as transducers or converters. The sun is the ultimate source of energy in the ecosystem.Thus the autotrophs depend on the abiotic factors of the ecosystem for producing energy. A portion of the energy synthesized, is used by the producers for their growth and survival and the remaining energy is stored for future use, and this stored energy is the ultimate source for the heterotrophs.
According to their mode of production, the producer can be divided into various terms:-
(1)    Photoautotrophs - use sunlight as an energy source. e.g. phytoplankton
(2)    Chemoautotrophs - use chemical energy in form of chemical bonds as the energy source. e.g. iron bacteria, sulphur bacteria, nitrifying bacteria.     Consumers/Heterotrophs: Consumers are organisms which eat other organisms. and heterotrophs are organisms that consume biomass alive or dead. They are not able to produce their own food. They are dependent on the producer for energy.  
The two kinds of heterotrophs are:-
(1)    Biophages (biotrophs):
the source of food is Living organisms. e.g. animals, fungi, and some bacteria and carnivores like predators and parasites.
(2)    Saprophages (saprotrophs): the source of food is dead organisms. 

Type of consumers according to their food and environment:-
(i)     Macro consumers         
(ii)     Micro consumers
(i)     Macro consumers (Phagotrophs or holozoic): They digest their food inside the body. ie., first ingestion of food then digestion occurs by the body. Macro consumers are of following types.
(a)    Primary consumers: They feed the autotrophs like plants, algae and bacteria. The primary consumers are also called herbivores. Elton referred the herbivores as key industry animals. Rabbit, deer, etc., are primary consumers in a terrestrial ecosystem. Plant parasites are also primary consumers.
(b)    Secondary consumers: Animals which feed upon primary consumers and obtain food and energy. Animals parasites (E.coli bacteria, Entamoeba histolitica, liver fluke, tapeworm) are also secondary consumers. Secondary consumers feed upon herbivores so they are also called as primary carnivores. Exception-insectivorous plants are not only producers but also secondary consumers.
(c)    Tertiary (Top consumers): They kill and eat the secondary consumers. They are also called secondary carnivores. Those animals which kill other animals and eat them, but they are not killed and eaten by other animals normally in nature, are called as top consumers. eg. lion, man, hawk, peacock.

(ii)    Micro Consumers: Micro consumers involve reducers or decomposers.
(a)    Reducers or Decomposers are those living organisms which decompose the dead body of producers and consumers. The decomposer is also known as reducers or transformers, saprotrophs or osmotrophs. The decomposers secrete enzymes. The enzymes digest the macromolecules of dead organisms into the monomer. These monomers are absorbed by reducers. The monomer is further disintegrated by decomposer into their chemical component, hence they play a significant role in mineral cycles. All the insectivorous plants can synthesis their own food by photosynthesis and also depends upon insects for nitrogen thus play the double role i.e., producer as well as the secondary consumer. The Inorganic material (CO2, H2O, Light), autotrophs (Producers) and decomposers are essential in the ecosystem but, macro consumers are non-essential. Detrivores are important to food webs and food chains because the majority of biomass produced on Earth is not consumed until it is dead. In food chains that do not include primary producers, the basal. Source heterotrophs are the producers. eg. vulture, crow and fox.
4.4.    Decomposition : 
Decomposition is a process of break down of complex organic matter into inorganic substance. Decomposition provides carbon dioxide, water and nutrients to the environment and recycles the nutrients in the soil. Dead plant remains such as leaves, bark, flower and animals, the dead body and their faecal matter, constitute detritus. The detritus is considered as the raw material for decomposition. The process of  decomposition has the following steps  
1.     Fragmentation,     2.     Leaching,     
3.     Catabolism,          4.     Humification   and  
5.        Mineralisation.
4.4.1.   Fragmentation: Detritivores break down detritus into smaller particles. This process is called fragmentation. (eg.-earthworm)
4.4.2.     Leaching: By the process of leaching, water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
4.4.3.     Catabolism: Bacterial and fungal enzymes degrade detritus into simple inorganic substance. This process is called catabolism.
4.4.4.     Humification: Humification leads to accumulation of a dark coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature it serves as a reservoir of nutrients.
Types of Humus -
(i)     Mor (Coarse textured humus)- It is raw hummus and is formed in acidic soil (pH - 3.8, – 4.0) in which decomposition of litter is slow because it has less number of decomposer organism.
(ii)     Mull - This is completely decomposed litter. i.e., humus because the rate of decomposition is fast due to the high PH of the soil. (Best pH of the soil 5.5 to 6.5)
4.4.5.     Mineralisation: The hummus is further degraded by some microbes and the process of release of inorganic nutrients is known as mineralisation. 
Factors affecting decomposition:-
(i)    Temperature:- Low temperature retard the growth of plants and high temperature may permit plant growth and humus accumulation in summer days.
(ii)    Soil moisture:- Extremes of both arid and anaerobic conditions retard the plant growth and microbial decomposition. Wetter conditions of soil are most favourable for both processes.
(iii)    Nutrients:- Lack of nutrients in the soil, is not favourable for decomposition.
(iv)    Soil pH:- Best pH for most of the microbes is 6–8. Lower or higher than this pH, microbes do not grow and do not favour decomposition.
(v)    Soil texture:- Soil in clays or higher altitudes or latitudes, retain more humus and favours a good and fast decomposition.
(vi)    Oxygen and other factors:- It is largely an oxygen-requiring process. The rate of decomposition is controlled by the chemical composition of detritus and climatic factors. In a particular climatic condition, decomposition rate is slower if detritus is rich in lignin and chitin and quicker if detritus is rich in nitrogen and water-soluble substances like sugars. Beside it, toxic levels of some elements like al, Mn, B, Se, cl etc. also effect decomposition.

4.5.    Structure of the Ecosystem :
Biotic and abiotic components together make the structure of the ecosystem. Another way to represent the structure of the ecosystem is through some terms related to the food relationship of producers and consumers constituting the food chain and food web as follows.
Standing Crop: An ecosystem includes many living and nonliving components. Standing crop refers to only the living things. So, the total amount or number of one or more living things in a particular area or ecosystem at any given time is called the standing crop.
Standing State: In soil, many inorganic nutrients present in every season. They vary according to season or climate and the environment. But some amount of these inorganic nutrients is fixed. This amount is termed as standing state.
4.6.    Dynamics of the ecosystem : 
The various components of the ecosystem constitute an interacting system. They are connected by energy, nutrients and minerals. The nutrients and minerals circulate and re-circulate between the abiotic and biotic factors of the ecosystem again and again. The continuous survival of the ecosystem depends on the flow of energy and the circulation of nutrients and minerals in the ecosystem. Thus the dynamics (functions) of the ecosystem includes the following :
(i)     Energy     (ii)     Primary production        
(iii)     Secondary production     (iv)     Food chain 
(v)     Food web     (vi)     Trophic levels        
(vii)     Energy flow             (viii)     Ecological pyramids.
(ix)    Nutrient Cycles
4.6.1.     Energy: Energy is the ability to do work. The flow of energy is unidirectional in the ecosystem. The main source of energy for an ecosystem is the light energy derived from the sun. The amount of solar radiation reaching the surface of the earth is 2 Cals/ It is more or less constant and is called solar constant or solar flux. About 95 to 99% of the energy is lost by reflection. Plants utilize only 0.02% of the energy reaching earth. The light energy is converted into chemical energy in the form of sugar by photosynthesis. 

 6H2O + 6CO2 + light  →   C6H12O6 + 6CO2

The synthesized sugar is utilized for many purposes it can be converted into starch and stored. It combines with other sugars to form cellulose. It combines with inorganic substances (N2, P, S) to form amino acids, protein, nucleic acids, pigments, hormones. Some amount of sugar is oxidized during respiration and the energy is released to do various functions.

C6H12O6 + 6O2  →   6CO2 + 6H2O + Energy.

4.6.2.     Primary production -
The rate of biomass production is called productivity. Primary production is the synthesis of organic compounds from the atmospheric or any other inorganic raw materials i.e. the amount of biomass or organic matter produced per unit area over a period of time by plants through photosynthesis and chemosynthesis. It is expressed in term of weight (g2) or energy ( m–2). The organism carrying the production of biomass is called as primary producers or autotrophs.    Gross primary production (GPP):- 
Gross primary production is the amount of chemical energy formed by primary producers (autotrophs) in the form of biomass in a given time. It is the total amount of fixed energy (organic food) in an ecosystem (in producers) in unit time. Gross primary production includes the organic matter used up in respiration. It is also known as total (Gross) photosynthesis. The rate of accumulation of biomass is measured by gross primary production.    Net primary productivity (N.P.P.) - 
Net primary production is the rate at which all the plants in an ecosystem produce net useful chemical energy or organic matter. It is equal to the difference between the rate at which the plants in an ecosystem produce useful chemical energy (GPP) and the rate at which they use some of that energy during respiration. Some net primary production transferred to the next trophic level. Rest of it is used for growth and reproduction by primary producers. Net primary production (NPP) is the amount of energy remaining after the cellular and maintenance respiration out of total energy produced in a given time period. NPP is the available biomass for the consumption of Heterotroph.

NPP = GPP - respiration [by autotrophs]

After the respiration it is the amount of stored organic matter in plant tissues.
The unit of measurement of primary production is mass per unit area per unit time interval but in terrestrial ecosystems, the mass of carbon per unit area per year (g C m−2 yr−1) is most often used as the unit. The significant limiting factors of primary production are carbon dioxide, light, temperature, water and oxygen.    Measurement of primary production : 
Primary production refers to the amount and the rate of energy produced by autotrophs. There are many methods to measure primary production. They are the following :
(a)     Harvest method: In this method, the plants are grown on a particular area is harvested at ground level and their weight is taken. They are dried and again weighed. This is done at regular intervals. The primary production is expressed in terms of biomass or mass per unit area per unit time.
(b)     Carbon-dioxide assimilation method: Plants utilize carbon dioxide for photosynthesis. So the rate of photosynthesis can be calculated by calculating the amount of carbon dioxide utilized by plants per unit time. The incorporation of carbon dioxide in photosynthesis can be determined by using the infrared gas analyzer. With the help of this analyzer, it is possible to measure the amount of carbon dioxide entering or leaving an airtight chamber of known volume.
(c)     Oxygen production method: This method is used to measure primary production in aquatic ecosystems. In this method, the amount of oxygen produced per unit time is taken as an index to measure the rate of photosynthesis. For this, light and dark bottle technique is used. Samples of water containing the autotrophs are collected in a light bottle (transparent bottle) and in a dark bottle. The light bottle allows light to enter in the same depth from which the sample is collected. After a certain period of time, the amount of oxygen present in the two bottles is calculated by titration using sodium thiosulphate by a method. This method is known as Winkler's method.
Light bottle: In the light bottle there is photosynthesis and respiration both takes place. So  NPP = (GPP - R) 
Dark bottle: In the dark bottle there is no photosynthesis and only respiration.
eg. :- The rate of photosynthesis is calculated by calculating the amount of oxygen present in the two bottles.
Initial bottle = 12 mg O2 /L ;  Light bottle = 15 mg O2 /L ; Dark bottle = 4 mg O2 /L 
The oxygen increased in the light bottle compared to the initial due to photosynthesis, and the oxygen decreased in the dark bottle due to respiration. With this information we can calculate the Respiration, Net Primary Productivity (NPP) and Gross primary productivity (GPP) for our system: 
(Light - Initial) = (15 - 12) = 3 mg/L/hr = (GPP - R) = NPP,  
(Initial - Dark) = (12 - 4) = 8 mg/L/hr = Respiration, 
(Light-Dark) = (15 - 4) = 11 mg/L/hr = (NPP + R) = GPP 
Thus we have a measure of the net and gross primary production as well as the respiration of our system. 
(d)     Radioisotope Method: This method is similar to the oxygen producing method. In this method, a known quantity of C14 is introduced into the light and dark bottles along with the sample and the bottles are suspended for six hours. During this period the C14 is incorporated into the protoplasm of the autotrophs. The autotrophs are filtered and dried. After drying the radioactivity is measured. The amount of radioactivity is proportionate to the amount of carbohydrate produced.
4.6.3.     Secondary Production - 
Secondary productivity is the rate of formation of new organic matter by consumers. Secondary productivity by herbivores is invariably less than the plants on which they feed. The missing energy at this level is lost in the form of respiration and heat because the total mass of producers is not consumed then assimilated into consumer biomass.
(a)     The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of the organic matter, the productivity of the ocean is only 55 billion tons.
(b)     In per unit area, maximum productivity is found in the tropical rain forest.
(c)     In the aquatic ecosystem, least productivity is found in very deep lakes while coral reefs productivity is maximum.
(d)     Nitrogen is the limiting factor in ocean and phosphorus is the limiting factor in lake productivity.
(e)     Inland, highest productivity noticed in Tropical rain forest (5 kg/msg/year). While Lowest productivity is noticed in Desert and Tundra.
(f)     Most productive Agro-ecosystem is sugarcane and rice ecosystem (3-4 kg./msq/year).
(g)    Although ocean cover about two-thirds of the Earth's surface, they account for less than half of its production.
(h)    Tropical rainforest tends to be much more productive than other terrestrial ecosystems.
(i)    Temperate forests, tropical savannah, cropland and boreal forests all are exhibiting middle levels of primary productivity.    Consumption Efficiency (C.E.): Consumption Efficiency is the efficiency with which the energy of a trophic level is consumed and transferred to the next trophic level. The energy consumed through the photosynthesis and respiration in the producers and consumers respectively.
        CE    =    In   x  100
                         Pn  -   1 
Where In is ingestion and Pn is production. 
Consumption efficiency of herbivores is very low, reflecting either the difficulty of utilization of plant material or the low herbivores densities.    Assimilation Efficiency  (A.E.): Assimilation efficiency is the efficiency by which organisms take the energy by their food for their development. For herbivores and detritivores, assimilation efficiencies are low, while carnivore has high assimilation efficiency. Herbivore shows higher assimilation efficiency in the aquatic ecosystem than in terrestrial ecosystem.    Production Efficiency (P.E.): Production efficiency is the efficiency for the maximum output production at the lowest cost of input.
Generally, invertebrates have high Production efficiency, ectoderm has intermediate values for Production efficiency, while endotherm has high energy maintaining a constant temperature. Herbivores have higher Production efficiency but lower Assimilation efficiency than carnivores.    Ecological Efficiency (E.E.)- The percentage of energy transferred from one trophic level to the next is called ecological efficiency of food chain efficiency.    Net Production Efficiency :    Photosynthetic Efficiency -

4.6.4.     Food chain:
It is simply a linear representation of the pattern of eating or being eaten, in an ecosystem. Here the sun is the ultimate source of energy for all ecosystems found on the earth and the trophic levels. The biotic factors of the ecosystem are linked together by food. For example, the producers are connected to the herbivores and the herbivores to the carnivores for food. The sequence of the eaters and being eaten to another trophic level is called the food chain. 

Producers  →  Herbivores  →  Carnivores

The various steps in a food chain are called trophic levels. Owing to repeated eating being eaten, the energy is transferred from one trophic level to another trophic level. Some examples of food chains are –
1.    Phytoplankton  →  Zooplankton →  Fishes →  Snakes 
2.    Plants   →   Mouse →  Snake  →   Hawk   =  Grassland
3.    Plants     →   Goat     →    Man  →     Lion    =  Forest
This transfer of energy from one trophic level to another is called energy flow. A typical food chain can be seen in a pond ecosystem. The algae and phytoplankton are eaten by the zooplankton. The zooplankton is eaten by fishes which are eaten by snakes.

 Types of food chains: The food chains are of two types, namely : 
(a)     Grazing food chain 
(b)     Detritus food chain     Grazing food chain: This food chain starts from plants, goes through herbivores and ends in carnivores.

Plants → Herbivores → Primary Carnivores → Secondary Carnivores

This type of food chain depends on the autotrophs which capture the energy from solar radiation. A few chains are given below :
1.    Grass     →      Mouse    →     Snake   →    Vulture
2.    Phytoplankton   →  Zooplankton  →  Fish   →   Whalefish
3.    Grass  →  Grasshopper  →   Lizard   →   Eagle
The grazing food chain is further divided into two types, namely : 
*    Predator chain: In the predator food chain, one animal is captured and another animal is being eaten. The animal which is eaten is called prey and the animal which eats other animals is called a predator. The predator food chain is formed of plants, herbivores, primary carnivores, secondary carnivores and so on.
*    Parasitic chain: The plants and animals of the grazing food chain are infected by parasites. The parasites derive their energy from their hosts. Thus, the parasitic chain within the grazing food chain is formed.    Detritus food chain: It starts from dead organic matter and ends with inorganic compounds. There are certain groups of organisms which feed exclusively on the dead bodies of animals and plants. These organisms are called detritivores. The detritivores include algae, bacteria, fungi, protozoans, insects, millipedes, centipedes, crustaceans, mussels, clams, annelid, nematodes, ducks, etc. These organisms ingest and digest dead organic materials. Some amount of energy is trapped and the remainder is excreted in the form of simple organic compounds. These are again used by another set of detritivores until the organic compounds are converted into CO2 and water.

Dead organic materials  →  Detritivores →  CO2 + H2O

Linking of grazing and detritus food chains – The two main food chains cannot operate independently. They are interconnected at various levels. According to Wilson and Bossert (1971), the stability of the ecosystem is directly proportional to the number of links between these various food chains. The detritus feeders obtain energy from the dead bodies of animals and plants which are components of the grazing food chain. Again some of the detritus feeders are eaten by the consumers of the grazing food chain. For example, in a pond ecosystem earthworms belonging to the detritus food chain are eaten by fishes belonging to the grazing food chain. 

4.6.5.     Food web: In an ecosystem, the various food chains are interconnected with each other to form a network called food web i.e. the interlocking of many food chains is called food web. Simple food chains are very rare in nature. This is because each organism may obtain food from more than one trophic level. In other words, one organism forms food for more than one organisms of the higher trophic level.
Examples: In a grassland ecosystem, the grass is eaten by grasshopper, rabbit and mouse. Grasshopper is eaten by lizard which is eaten by the hawk. Rabbit is eaten by the hawk. The mouse is eaten by the snake which is eaten by the hawk. In addition hawk also directly eats grasshopper and mouse. Thus there are five linear food chains which are interconnected to form a food web.

1.    Grass     →     Grasshopper        →     Lizard         →     Hawk
2.    Grass     →     Mouse         →    Hawk
3.    Grass         →    Rabbit         →     Lion
4.    Grass     →    Mouse    →    Snake    →    Vulture
5.    Grass     →     Insect     →    Frog

This is a very simple food web. But in any ecosystem, the food web is more complex. For example, in the grassland itself, in addition to hawk, there are many other carnivores such as vulture, crow, fox, man, etc. 
The significance of food web: Food webs are very important in maintaining the stability of an ecosystem. For example, the overgrowth of grasses is controlled by the herbivores. When one type of herbivores increase in number and control the vegetation. Similarly, when the herbivorous animal becomes extinct, the carnivore predating on this type may eat another type of herbivore and herbivorous animals number is controlled or balanced by carnivores. Thus food web is essential for the equilibrium of all trophic levels in the ecosystem.4.6.6.    Trophic levels: Each food chain contains many steps like producers, herbivores, primary carnivores and so on. Each step of the food chain is called trophic level. The number of trophic levels in a food chain is restricted to five or six.  Green plants make first trophic level. 
4.6.7.     Energy flow: The transfer of energy from one trophic level to another trophic level is called energy flow. The flow of energy in an ecosystem is unidirectional. This energy can be used only once in the ecosystem. But the minerals circulate and recirculate many times in the ecosystem. A large amount of energy is lost at each trophic level. It is estimated that 90% of the energy is lost when it is transferred from one trophic level to another. Thus the amount of energy available decreases from step to step. Only about 10% of the biomass is transferred from one trophic level to the next one is a food chain. And only about 10% of chemical energy is retained at each trophic level. 

This is called 10% law of Lindeman (1942). When the food chain is short, the final consumers may get a large amount of energy. But when the food chain is long, the final consumer may get a lesser amount of energy. As shown in the figure about 4000 Kcal of light falls on the green plants. Of this, approximately 50% (2000 Kcal) is absorbed. Of the 50%, only 1% (20 Kcal) is converted at the first trophic level. Thus the net primary production is merely 15K cal. Secondary productivity (P2 and P3) is 10% (2.0 Kcal & 1.5 Kcal) at the herbivores and carnivores level.
(i)    Energy flow is the key function of the ecosystem. The storage and expenditure of energy in the ecosystem is based on the two basic laws of thermodynamics.
(a)     1st law of thermodynamics - Energy is neither created nor destroyed but only transformed from one form to another form.
(b)     The law of entropy - The transfer of food energy from one to another organism leads to loss of energy as heat due to metabolic activity.
(c)     Thermodynamics is useful to examine the relationship between incident.
(ii)     Energy in food is in concentrated form, heat energy is highly dispersed. It must be understood that all changes in energy forms can be accounted for energy flow in any system.
In the ecosystem energy used only once but the mineral circulation and recirculation many times. According to 10% law of lindeman-90% of the energy is lost when it is transferred from one trophic level to another and remaining 10% of biomass is transferred from one trophic level to next trophic level and only 10% chemical energy is retained at each trophic level. In a short food chain the top consumer gets a large amount of energy and in a long food chain, the top consumer gets a lesser amount of energy. 

(iii)     It is useful to examine the relationship between incident radiant energy and the energy captured by the producers. Only the visible light, or the photosynthetically active radiation (PAR), which carries about 50 percent of the energy of total incident solar radiation, is available to producers for absorption. 
4.6.8.     Ecological pyramids: The Graphical representation of ecological parameters like biomass, number and energy of organism at different successive trophic levels and trophic structure in the ecosystem is called pyramids. When an ecosystem has a pyramid where the apex is pointed upwards called upright pyramid. Whereas in some ecosystems the number and the biomass of the producers are less and those of consumers are more. This type of ecosystem characterized by a pyramid where the apex is directed downwards and is called an inverted pyramid.
The pyramid was formed by Charis Elton; So we called it Eltonian pyramids. The three general classes of such pyramids are:-  Pyramids of number- The number of the individual organism in a various trophic level is shown according to decreasing or increasing order in this type of pyramid. These pyramids can be upright or reverse but mostly upright, because the number of producers [T1] is maximum and the number of herbivores and carnivores decrease up to higher trophic levels, such as grassland ecosystem and aquatic ecosystem. In a food chain the number of individual decrease from producer to the consumer level. As in an ecosystem, the number of producers is far high. The number of herbivores is lesser in number than the producers. Similarly, the number of carnivores is lesser than herbivores.
(a)    Cropland ecosystem: In croplands, the crops are more in numbers. The herbivore i.e. grasshoppers feeding on crop plants are lesser in number. The primary carnivore frogs feeding on grasshopper are still lesser in number. The snakes feeding on frogs are fewer in number and hence

Crop     →     Grasshopper    →     Frogs         →    Lion

(b)    Grassland ecosystem: In a grassland, the grasses are there in large numbers. The consumers decrease in the following order.
(i)    Grass  →  Grasshopper  →  Lizard  →  Snake
(ii)    Grass   →  Rabbit   →  Fox  →   Vulture
(c)    Pond ecosystem: In a pond, the producers (Phytoplanktons ) are also in a great number and then Zooplanktons feed on it are lesser and all organisms decrease towards the top consumer.
Phytoplankton   →    Zooplankton      →   Fishes   →  Snakes  Pyramids of biomass: These pyramids show the decrease of biomass from base to apex.  Since the total biomass of producers ingested by the herbivores is greater than the herbivores, and the carnivores of the succeeding trophic level are also having low biomass then the herbivores eaten by them. Pyramids of biomass represent the total amount of biomass at each trophic level of the ecosystem, mostly these pyramids are also upright (erect) eg., tree ecosystem.
(a)    In a Grassland: In a grassland, the biomass of grasses is the maximum, and at every trophic level, it gradually decreases towards the top consumer level in the following order.

 Grass    →     Grasshopper     →     Lizard         →     Hawk

(b)    In a forest: In a forest, the biomass or trees are the maxima in number and the biomass of the top which are consumer exist on trees is minimum. The decrease in weight occurs in the following order :
            Plants         →     Deer         →    Fox     →     Tiger
            Plants         →    Rabbit     →    Fox     →     Lion Pyramids of energy: Pyramid of energy shows the energy flow in an ecosystem from the producer level to the consumer level. At each trophic level, 80 to 90% of energy is lost. Inform of heat, the amount of energy decreases from the producer level to the consumer level and the lowest energy is transferred to the top level. It is unit rational only.  
(a)    In a grassland: In a grassland green plants trap the maximum light energy. The energy gradually decreases towards the top consumer level.
The energy flow in an ecosystem also follows the second law of thermodynamics. According to it, energy loses when it is transferred from one state to other and one level to other

Grass         →     Grasshopper     →     Lizard         →     frog
Grass         →     Mouse         →     Snake         →     Vulture

(b)    In a pond: In a pond, maximum energy is trapped by the phytoplankton and of course they are small in size but large in number. So the amount of energy decreases towards the top–consumer level.

Phytoplankton  →   Zooplankton    →   Fish  →  Snake   →   Eagle
Phytoplankton  →  Zooplankton   →  Small fish   →  Whale Fish

The energy pyramids are always upright without any exception because there is always a loss of energy at each trophic level.  Inverted pyramids : 
There are many ecosystems where the number and biomass of producers are more and the number of consumers is less, for its stability. When the type of ecosystem characterized by a pyramid where the apex is directed downwards and is called an inverted pyramid. Inverted pyramid occurs for number and biomass. The pyramid of energy is always upright. 
(A)    The inverted pyramid of numbers: Pyramid of numbers in a tree ecosystem is inverted. It is also called the parasitic ecosystem. Because birds (herbivores) live on the tree (producer) and parasites (consumer) like bugs, lice live on birds, and here with the increase in the number of trophic levels, the number of the organisms increase sequentially, unlike other ecosystems. 

(B)    The inverted pyramid of biomass: When the biomass of producers is less and consumers are more the pyramid will have inverted shape. For example, the biomass of diatoms and phytoplankton are negligible as compared to crustaceans and small fishes, in lake or pond ecosystem.

4.6.9.    Mineral/Nutrient/Biogeochemical cycles : 
The biocomponent includes the Living Organism. The geo-component includes the rock, soil, water. The chemical component includes the Material or Nutrients. The following types of nutrient cycles are found in an ecosystem
(i)     Gaseous Cycle - C, H, N, O cycle.
(ii)     Sedimentary cycle - P, S, Ca cycles.
For the cyclic movement or flow of these mineral earth crust like sediment of sea or water bodies work as a reservoir. It contains a bulk amount in the form of inactive inorganic material. The absolute amounts of nutrient moving in and out in a cyclic form is known as nutrient cycles. 
ex.    The plant uptake the nutrient from the soil through absorption and the absorbed nutrients are metabolically incorporated in plants during growth. and hence nutrients are recycled i.e., brought back to the soil through litter fall from vegetation, animal remains and faecal matter, etc. Decomposition also regenerates the nutrients in the soil.
When the uptake of nutrient exceeds the amount recycled (e.g., as in the case of a young growing forest), a fraction of the uptake is retained in the standing crop. This retention of balance nutrients in the standing crop leads to an increase in nutrients content of the ecosystem. 
(i)     Gaseous Cycles - Carbon Cycle: Rocks of carbonates are the main source of carbon in the atmosphere and in the hydrosphere. Carbon is present in lithosphere in the form of coal and petroleum. The carbon released from them is present in the atmosphere in the form of carbon dioxide (CO2). The green autotrophs can utilize CO2 from the air to synthesize food materials which are obtained by other organisms as food. Herbivores supply the organic food to carnivore and after the oxidation or respiration of these organic matters, CO2 is produced which then dissolve in air or water again and again by plants in air or water and again utilized by the plants.

(a)     The flow of Carbon into the biotic system: Carbon flows into the biotic system through photosynthesis and many other processes. In the photosynthesis process, the green plants uptake CO2 and incorporate into glucose. The glucose is converted into other types of macromolecules like other carbohydrates, proteins and lipids. These organic compounds are stored up in the plant tissues. When the primary consumer eats these producers the carbon flows into the body of herbivorous animals through the food chain or food web. When these herbivores are eaten by carnivores or secondary consumer, the carbon enters in the body of carnivorous animals. By the food chain or food web, the carbon cycle is operated among populations of the biotic community.
The producer of aquatic biotic community uptake the dissolved CO2 of water. The remaining way of transfer is just like the terrestrial. Formation of the shell by marine animals like protozoans, corals, molluscs, algae, etc., are also one of the ways to fix the CO2. These organism use CO2 for the construction of the shell. These animals then convert the CO2 into calcium carbonate (CaCO3) and construct their shells as follows:-

        CO2 + H2O → H2CO3 (Carbonic acid)
        H2CO3 → H+ + HCO3 (Bicarbonate)
        HCO3 + Ca+ → H+ + CaCO3 (Calcium carbonate)
(b)     The flow of Carbon into the abiotic system from Biotic community: There are five ways of carbon flow from biotic to the abiotic system.
(i)    Respiration: Plants and animals release CO2 by respiration. The CO2 is produced due to biological oxidation of macromolecules.
(ii)    Decomposition: When plants and animals die, the dead bodies are decomposed into CO2 by decomposers like bacteria, algae, etc.
(iii)    Shells: After the death of marine animals, CaCO3 stored in the shells is either deposited as sedimentary rocks or dissolved in water to release CO2 by the reversion of the above all reactions.
(iv)    Coal: As plants are converted into coal after combustion. Carbon from coal returns to the air in the form of CO2 through combustion and weathering.
(v)    Forest fire: In the forest fire, CO2  is produced after combustion of wood and release into the air.  Nitrogen cycle
It is a gaseous cycle. The main source of nitrogen is air which contains 79% nitrogen (N2). Plants do not use environmental nitrogen directly. They take it in the form of ammonium salts, nitrites and nitrates.
Ammonium salts, nitrites and nitrates are formed by the process of nitrogen fixation of environmental N2 by the plants. After this process, it is followed by various steps. So. N2 cycle includes major steps:-
    (1)    Nitrogen Fixation                (2)    Nitrification
    (3)    Assimilation                    (4)    Ammonification
    (5)    Denitrification
Nitrogen is present in the atmosphere (78%) as a reservoir and it is present in rocks and sediments. Nitrogen is an important component of amino acids, nucleic acids and ATP. 
(1)    Nitrogen Fixation: It is an anaerobic (without oxygen) process in which atmospheric nitrogen is reduced to NH3. Some bacteria do this job. These bacteria are known as nitrogen-fixing bacteria. eg. Legumes and symbiotic bacteria of Rhizobium species do this process. These bacteria are present in legumes root nodules. These organisms convert the nitrogen to ammonia within the soil. Which can be taken by plants. The same process occurs in the aquatic ecosystem where the cyanobacteria convert the nitrogen into ammonia. Another example of nitrogen fixation or symbiosis occurs between a number of woody plant species and the diazotrophic actinomycete Frankia. The plant gives energy materials to the diazotrophs, which in turn reduce atmospheric nitrogen (N2) to ammonia (NH4+). This ammonia is transferred from the bacteria to the plant for nutrition purpose.

(2)    Nitrification: After N2 has been fixed, some other bacteria convert it into nitrate. This process is known as nitrification. Bacteria like Nitrosomonas, Nitrococcus and Nitrobacter participate in this process.
This process occurs in two steps:-
(i)    In the first step, Nitrosomonas convert ammonia into Nitrite.
(ii)    In the second step, Nitrobacter converts nitrite into Nitrate.

Many bacteria are present in the soil, they also help in this process.
(3)    Assimilation: The nitrate is consumed by the plants. When animals eat the plants, Nitrogen is assimilated by them also.
(4)    Ammonification: Through the excretion of animals and through the decomposition of plants, NH4 (Ammonia) again excreted in the environment. This process is called as Ammonification.

(5)    Denitrification: Nitrate, which is consumed by plants, is again broken down in the form of Nitrogen (N2) by some organisms or denitrifying bacteria. Then this N2 is released in the environment or atmosphere again. So the denitrification includes the reduction of Nitrates to gaseous Nitrogen. Denitrifying bacteria performs exactly vice versa of Nitrogen-fixing bacteria. The Phosphorus /Sedimentary Cycle
The cycling of phosphorus between biotic and abiotic environment is termed as phosphorus cycle. Phosphorous is the main constituent of DNA, RNA, AMP, ADP, ATP, GTP, NADP, Phospholipids, Plasma membrane, protoplasm, bones, teeth and food etc. The main source of phosphorus is rocks or weathering of rocks. P-cycle is the slowest and non-gaseous cycle. By the weathering of rocks, phosphorus is released in seawater or in the soil. In the sea, it gets available for sea plants or seaweeds, which pass into fish and sea birds. If it passes in the soil than it passes to herbivores and then carnivores and then it is found in the faeces of these animals and birds. The excretory material of birds is rich in phosphorus and it is termed as guano. Guano or faeces is again released in the soil. In the soil, the plant absorbs the phosphate in the form of orthophosphate.  Sulphur cycle : 

Cycling of sulphur between biotic and abiotic systems is called sulphur cycle. It is also a sedimentary cycle. Sulphur is an important element for proteins, lipids and many co-factors. It is released by volcanos, sea reservoirs, rocks weathering, and sediments of the ocean in the environment. Sulphur cycle is followed by the mineralisation of organic sulphur, the oxidation of sulphides and the reduction of sulphates with the incorporation of sulphur into organic molecules. Sulphur is released by E.Coli bacteria also, it is involved in metal deposition and fossil fuel production. Incomplete combustion also causes sulphur dioxide (SO2) release. From the rock weathering or sea, it is transferred to plants. After this, it again passes to Herbivores and carnivores and then again it is released in soil or water through the excretory material.
    Bacteria oxidise H2S of air to sulphate.

H2S + O2  → SO4  + 2H+

This sulphur is used by plants.

If Fe (iron) is present on rocks, sulphur, which is excreted in the soils or sea, combines with Fe.

Fe + S →  FeS

After it, sulphur is no longer available in free form as it remains in the form of FeS only. It is available in the case of human interference or natural disasters.  Water Cycle :
Sea is the main source of water. Water is the most important component of living organisms. It is evaporated from water bodies (ponds, oceans, lakes, rivers, plants, human etc.) It reaches an atmosphere. In the atmosphere, it forms clouds. Clouds are made by the condensation of water vapour, moist, fog and ice at high altitudes. After it, rain comes again on earth and the cycle again goes on like this continuously.

4.7.    Green House Effect :
Generally, because of the low amount in the environment, the carbon dioxide is not considered as a pollutant, but its higher concentration forms a thick layer above the earth's surface and doesn't allow the radiations of sunlight go back from the earth surface. This results in an increase in temperature of the earth's surface. That’s means just like in the greenhouse the CO2 is forming a layer around the earth surface. This effect is called the "Green House Effect". The increase of the earth temperature because of the greenhouse effect is known as Global warming.
CO2, CH4, CFC, N2O are the main greenhouse gases. These are termed as relation active gases also SO2, NO2, and O3 are not greenhouse gases.
.Effects of Green House Gases :
1.     Rise in temperature
2.    Melting of glaciers
3.    Floods in rivers
4.    The rise in sea level
5.    Changes in the cycle of rain
6.    Global Warming
7.    CO2 fixation (Increased rate of photosynthesis due to increased CO2 concentration)

Control of Global Warming :
1.      Alternative source or green and renewable source of energy like wind and solar energy those not have the emission of greenhouse gases.
2.     Large scale afforestation should be done for photosynthetic utilization of CO2.
3.     By decreasing the use of Nitrogen fertilizers in agriculture for reducing N2O emission.
4.    An eco-friendly alternative of Chloro Fluoro Carbon (CFC) should be developed.
Depletion of Ozone layer : 
A protective layer is presently made up of ozone molecules (O3) above 30 km from the surface of the earth. This layer protects the earth from ultra-violet rays that are harmful to all living organisms and sometimes causes cancer. But nowadays, this thick layer of ozone is becoming thinner and causing the radiations allowed to come on earth. The reason for the decreasing thickness of the ozone layer is the freon gas and CFC gas released by the refrigerators. The use of plastic and aerosol also causes the release of filling agents. These agents react from the O3 present in the environment or breakdown the O3. So due to this reason, O3 does not remain free to make that protective ozone layer. O3 breakdowns in O2 and Oxygen free radicals. These free radicals then start a chain and inhibit the formation of the ozone layer. If the thickness of the ozone layer decreases from a particular area, it is called an ozone hole. An ozone hole has been seen in Antarctica.

O3    →    O2   + oxygen free radical 
O3 +  O.  →   O2  +  2O 

4.8.    Acid rain : 
As the pollutants are raised in the environment. The environment has to bear the suffer from acid rain. Rain in which water has low pH is termed as acidic rain. As the population increases, the vehicles are used more in frequency or percentage as compared to the last decade. Oxygen (O2) and Nitrous oxides are released from vehicles and sulphur oxides are released from chemical factories in the environment. These nitrous oxides and O2 and sulphur oxides are changed in Nitric and Carbonic acids and sulphuric acids respectively. These acids combine with water and oxidants, which are already present in the environment and they all make an acid precipitation form in the environment when sun rays and water comes from the clouds in the form of rain. It becomes acidic before it drops to the earth.
Acid rain does not only effect on living organism but also effects on materials, building soil etc. It attacks building materials, especially like sandstone, limestone, marble, steel and nickel. In plants, it causes chlorosis. Chlorosis causes yellowing the leaves of plants, which becomes an obstacle in the chlorophyll formation. Apart from this, acid rain effects the water animals and forests also by increasing the acidity of water bodies and soil too. So, acid rain disturbs the aquatic ecosystem, as well as the forest and sand ecosystem.  
Biochemical Oxygen Demand (B.O.D.) :
Bacteria use the oxygen which is present in the water already. They use the oxygen to decompose the organic wastes present in water. The amount of oxygen which bacteria use is known as Biochemical Oxygen Demand (BOD). As the use of oxygen is increased in water, dissolved oxygen (D.O) is decreased.
Hence we can measure water pollution by the value of B.O.D. or we can say that Biochemical oxygen demand is a parameter to measure water pollution by organic wastes and decomposition.
    Chemical Oxygen Demand (C.O.D.) :
    Organic matter includes Biodegradable and Non-Biodegradable things present in the environment. The oxygen requirement by chemicals for oxidation of total organic matter in water is termed as C.O.D.

C.O.D.   >  B.O.D.     →    (Always)

4.9.    Biological magnification :
This is the process in which some nondegradable heavy metals or toxic or pesticides are involved in the food chain and taken up by the plants and then carnivores or animals and then top consumers. These toxic substances like D.D.T., Al, Fe, Hg, ABS (Alkyl benzene sulphonate) cannot be decomposed by plants so it is only accumulated by the organism and it's accumulation value always increased from lower trophic level to higher trophic level.
D.D.T. Concentration starts from 0.003 ppm in water. It is increased in the aquatic plants and then more increased in Herbivores and than carnivores and finally, it can ultimately reach on 25 ppm in fish-eating birds through the accumulation of toxins or Biomagnification. 

The higher concentration of D.D.T. disturbs calcium metabolism in birds, which causes thinning of eggshell and this leads to premature breaking and eventually effects the bird population. This affects the whole aquatic ecosystem or food chain.
4.10.    Eutrophication :
If the process of nutrient enrichment of water causes the consequent loss of species diversity in that area, is termed as eutrophication. In this process, the nutrients in a lake, are increased in drastic speed. This causes the successive growth of algal and organic loading. This causes a reduction in the oxygen content of water. Due to the decrease of oxygen, aquatic animals die and their population reaches at the lowest number and in the end, they are finished. This vast increase of algae in a lake is termed as algal boom and due to algal boom, the lake also starts to disappear and gradually a land appears in the place of that lake. This phenomenon is termed as "ageing of the lake". Ageing of the lake may take place in thousand or crore of years.

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