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Penguins Swimming in Sea



A biological community of interacting organisms and their physical environment.

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  • The term ecosystem was coined by Sir A.G. Tansley (1935).

  • An ecosystem is the basic functional ecological unit in which living organisms interact among themselves and with their surrounding physical environment.

  • The entire biosphere is referred to as global ecosystem, which consists of several local ecosystems of earth.

  • The size of the ecosystem varies from small pond to a large forest or sea.

  • Since the global system is too big and complex to studied at one time, it is convenient to divide it into two basic categories, namely the terrestrial and the aquatic ecosystem.


Ecosystem - Structure and Function

  • The ecosystem consists of biotic and abiotic components and their interaction with each other resulting in a physical structure.

  • The flow of energy takes place within these components of the ecosystem.

  • The identification and enumeration of plant and animal species in an ecosystem gives it species composition.

  • Vertical distribution of different species occupying different levels is called stratification, e.g., in a forest ecosystem, trees occupy top vertical strata or layer, shrubs the second, and herbs and grasses occupy the bottom layers.

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The major functions of an ecosystem include

  1. Productivity

  2. Decomposition

  3. Energy flow

  4. Nutrient cycling


Pond Ecosystem

  • The solar input, the cycle of temperature, day-length and other climatic conditions regulate the rate of function of the entire pond.

  • The inorganic and organic materials are conversed with the help of the radiant energy of sun by the autotrophs.

  • The autotrophic components include the phytoplankton, some algae and the floating submerged and marginal plants.

  • Heterotrophs consume autotrophs.

  • Decomposers decompose the dead organic matter to release them back for reuse by the autotrophs.

  • The matter and minerals are recycled between biotic and abiotic components.

  • The energy flow is unidirectional.

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  1. The rate of biomass production per unit area, over a time period, by plant during photosynthesis is called productivity.

  2. It is expressed in (kcal m-2 ) yr-I or g/m2/yr or g m-2 y-1.

  3. The amount of biomass or organic matter produced per unit area over a time period in plants during photosynthesis is called primary production. It is expressed in terms of weight (g m-2 ) or energy (kcal m-2).

  4. The rate of formation of new organic matter by consumers is called secondary productivity.

  5. Productivity in an ecosystem is divided into gross primary productivity (GPP) and net primary productivity (NPP).

  6. Gross primary productivity - is the rate of production of organic matter during photosynthesis in an ecosystem.

  7. Some GPP is utilized by plants for respiration (R).

  8. Net primary productivity – is the available biomass for the consumption of heterotrophs. GPP-R=NPP

  9. The annual NPP of the whole biosphere is approximately 170 billion tons of organic matter. Productivity of the oceans are only 55 billion tons.

  10. Secondary productivity is defined as the rate of formation of new organic matter by consumers.


Factors Affecting Primary Productivity

  • Plant species inhabiting a particular area.

  • Environmental factors.

  1. Sunlight: The sunlight directly regulates the primary productivity because the plants perform photosynthesis with the help of sunlight. As tropical region receives maximum sunlight, so it exhibits higher productivity.

  2. Temperature: Temperature regulates the activity of enzyme. So, optimum temperature is required for proper functioning of enzyme.

  3. Moisture: Rain (humidity) is required for higher primary productivity. Deserts have the lowest primary productivity as the soil is deficient in moisture.

  • Availability of nutrients: Greater nutrients ensure greater primary productivity.

  • Photosynthetic efficiency: Some plants have more efficiency to trap sunlight (sugarcane), so they accumulate more primary productivity.

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            The process of breaking down complex organic matter into inorganic substances like C02, water and nutrient is called decomposition.

The raw materials which undergoes decomposition including dead plant and animal remains and their faecal matter are called detritus.


Steps in Decomposition

  1. Fragmentation: The process of breaking down of detritus into smaller particles is called fragmentation, e.g., as done by earthworm.

  2. Leaching: The process by which water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts is called leaching.

  3. Catabolism: The enzymatic process by which degraded detritus is converted into simpler inorganic substances is called catabolism.

  4. Humification: The process of accumulation of a dark coloured amorphous substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate is called humification.

  5. Mineralisation: The process by which humus is further degraded by some microbes to release inorganic nutrients is called mineralisation.


Factors Affecting Decomposition

  • Chemical composition of detritus

  1. The decomposition rate is slow if detritus is rich in lignin and chitin.

  2. The decomposition rate is higher when detritus is rich in nitrogen and water-soluble substances like sugars.

  • Climatic factors

  1. Warm and moist environment favours decomposition.

  2. Low temperature and anaerobiosis inhibit decomposition.

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Energy Flow

  • The sun is the only source of energy for all ecosystems on earth.

  • Out of the total incident solar radiation, only 50 per cent of it is photosynthetically active radiation (PAR).

  • Plants capture only 2-10 per cent of the PAR and this small amount of energy sustains the entire living world. So, there is unidirectional flow of energy from the sun to producers and then to consumers.

  • The energy is transferred in an ecosystem in the form of food which is degraded and lose major part of food energy as heat during metabolic activities and only a very small fraction becomes stored as biomass.

  • This is correlated to second law of thermodynamics.

  • The green plants in the ecosystem which can trap solar energy to convert it into chemical bond energy are called producers.

  • All the animals that depend for food on plants are called consumers.

  • Consumers are divided into the following categories:

  1. Primary consumers: Animals which feed directly on plants, i.e., herbivores.

  2. Secondary consumers: Consumers that feed on primary consumers, i.e., carnivores.

  3. Tertiary consumers: Consumers that feed on secondary consumers.

  • Lindeman's 10 per cent law: At each step of food chain, when food energy is transferred from one trophic level to the next higher trophic level, only about 10 per cent of energy is passed on to the next trophic level rest of it lost in the form of heat. This is known as Lindeman's 10 per cent law given by Lindeman in 1942.

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Types of Food Chains

Two types of food chains can be observed in the ecosystem:

  • Grazing Food Chain (GFC):

  1. It begins with green plants called producers as trophic level.

  2. A much less fraction of energy flows through this type of food chain.

  3. Energy for food chain comes from sun.

e.g., Grass-----Rabbit------Lion


  • Detritus Food Chain (DFC):

  1. It begins with dead organic matter and decomposers called saprophytes as first trophic level.

  2. A much large fraction of energy flows through this type of food chain.

  3. Energy for the food chain comes from organic remain or detritus.

e.g., Dead leaves------Woodlouse-------Blackbird


Trophic Level

  1. A trophic level is the group of living organisms which help in transfer of energy in a food chain within an ecosystem.

  2. Each trophic level has a certain mass of living material at a particular time called the standing crop.

  3. The standing crop is measured as the biomass of living organisms (biomass), or the number in a unit area.

Trophic levels.png
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Ecological Pyramid

  • The relation between producers and consumers in an ecosystem can be graphically represented in the form of a pyramid called ecological pyramid.

  • Structure: The base always represents the producers or the first trophic level and the apex represents top level consumer or the last trophic level.

  • Ecological pyramids are of three types:

  1. Pyramid of number

  2. Pyramid of biomass

  3. Pyramid of energy

  • In most of the ecosystem all type of pyramids are upright.


1. Pyramid of number: A pyramid of number is graphical representation that shows the number of organisms at each trophic level. The pyramid of number was advanced by Charles Elton in 1927.

Pyramid of number is always can be upright except in tree ecosystem.


2. Pyramid of biomass: Pyramid of biomass A diagrammatic representation of the amount of organic material (see biomass), measured in grams of dry mass per square metre (g m–2), found in a particular habitat at ascending trophic levels of a food chain.

Pyramid of biomass in sea is generally inverted because the biomass of fishes far exceeds that of phytoplankton.


3. Pyramid of energy: An energy pyramid is a graphical model of energy flow in a community. Pyramid of energy is always upright, can never be inverted.


Limitations of ecological pyramids:

  1. It never takes into account the same species belonging to two or more trophic levels.

  2. It assumes a simple food chain, which never exists in nature.

  3. In spite of the vital role played by saprophytes/decomposers, they are not given any position in ecological pyramids.

Ideal Pyramid of Energy.png
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Ecological Succession

  • The sequential, gradual and predictable changes in the species composition in an area are called succession or ecological succession.

  • The entire sequence of communities that successively changes in a given area are called sere(s).

  • The individual transitional communities are termed as seral stages or seral communities.

  • The community that is in near equilibrium with the environment is called a climax community.

  • The species that invade a bare area are called pioneer species.

  • The changes that occur in successive seral stages to reach a climax community are:

  1. Changes in the diversity of species of organisms.

  2. Increase in the total biomass.

  3. Increase in the number of species and organisms.


Ecological succession is of two types:

  1. Primary succession: It begins in areas where no living organisms ever existed. Therefore, the establishment of a biotic community is very slow. e.g., newly cooled lava, bare rock, newly created pond or reservoir.

  2. Secondary succession: It begins in areas where natural biotic communities have been destroyed. e.g., abandoned farm lands, buried or cut forests. Since soil is available, it is a faster process.

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Succession of Plants

The plant succession is of two types:

  1. Hydrarch succession: The plant succession which takes place in wet area or water, leading to a successional series progress from hydric to the mesic conditions.

  2. Xerarch succession: The plant succession which takes place in dry area, leading to a successional series from xeric to mesic conditions.


1. Primary succession in water

  • The pioneer species are phytoplanktons.

  • The phytoplanktons are replaced by free-floating angiosperms.

  • Then, rooted angiosperms invade sedges, grasses and finally the trees.

  • At last, a stable climax forest is formed.

  • An aquatic habitat is converted into mesic habitat.


2. Primary succession on rocks

  • Lichens are the pioneer species on a bare area.

  • The lichen secretes some acids to dissolve rock and help in weathering and soil formation.

  • Later, some small bryophytes invade and hold the small amount of soil.

  • The bryophytes are succeeded by herbs, shrubs and ultimately big trees.

  • At last, a stable climax forest is formed.

  • The xerophytic habitat gets converted into a mesophytic one.

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Nutrient Cycle

  • The movement of nutrient elements through various components (abiotic and biotic) of an ecosystem is called nutrient cycling or biogeochemical cycle.

  • The total amount of nutrients like carbon, phosphorus, calcium, etc., present in soil at any time is called standing state.

  • Standing state varies with the kind of ecosystem and season.

  • The nutrient reservoir meets the deficit arising due to imbalance in the rate of influx and efflux.

  • The nutrient cycles are of two types:

  1. Gaseous cycle

  2. Sedimentary cycle.

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Carbon cycle

  • Carbon constitutes 49 per cent of dry weight of an organism.

  • 71 per cent of the carbon is found dissolved in oceans which is responsible for its regulation in atmosphere.

  • The carbon cycle occurs through atmosphere, oceans and through living and dead organisms.

  • It is estimated that 4 x 1013 kg of carbon is fixed in the biosphere through photosynthesis annually.

  • Carbon is returned to atmosphere as C02 by animals and plants through respiration and the activities of decomposers.

  • Some amount of fixed carbon is lost as sediments and removed from circulation.

  • Burning of wood, forest fire, volcanic activity and combustion of organic matter and fossil fuels are some essential sources for releasing C02 in the atmosphere.

  • Human activities like deforestation and vehicular burning of fossil fuels has caused an increase in the amount of C02 in atmosphere.

Carbon cycle.png
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Phosphorus Cycle

  • Importance of phosphorus:

  1. It is a major constituent of biological membranes, nucleic acids and cellular energy transfer systems.

  2. It is required for making shells, bones and teeth.

  • Rocks are the natural reservoirs of phosphorus.

  • During weathering of rocks, minute amounts of these phosphates dissolve in soil solution and are absorbed by plant through roots.

  • Herbivores and carnivores obtain this element from plants directly or indirectly.

  • The waste products and the dead organisms are decomposed by phosphate-solubilising bacteria and in turn release phosphorus.

Phosphorous Cycle.png

Ecosystem Services

  • The products of ecosystem processes are called ecosystem services.

  • Forests are the major source of ecosystem services. They

  1. purify air and water,

  2. cycle nutrients,

  3. provide wildlife habitat,

  4. pollinate crops,

  5. mitigate droughts and flood,

  6. generate fertile soils,

  7. maintain biodiversity,

  8. provide storage site for carbon,

  9. provide aesthetic, cultural and spiritual values.


Robert Constanza and his colleagues tried to put price tags on nature's life-support services which

came up to US$ 33 trillion a year.

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