• Ecosystem is the smallest structural and functional unit of nature or environment. It is a self regulatory and self sustaining unit.
  • Ecosystem may be large or small. A single drop of water may be an ecosystem.
  • Ecosystem may be temporary or permanent.


  • An ecosystem may be natural or artificial.
  • Natural ecosystem is divided into two basic categories –
    • Terrestrial ecosystem : E.g., forest, grassland, tree, desert ecosystem.
    • Aquatic ecosystem : Aquatic ecosystem is again of two types:
      • Lentic ecosystem : E.g., Stagnant fresh water, lake, pond, swamp.
      • Lotic ecosystem : Running freshwater ecosystem. E.g., river.
  • Artificial ecosystem : These are man made ecosystem. E.g., cropland, gardens etc.
  • On the basis of size, types of ecosystem are :-
    • Mega ecosystem Ocean/Sea
    • Macroecosystem Forest
    • Microecosystem Pond
    • Nanoecosystem Drop of water


An ecosystem is composed of two components –
  • Biotic component
  • Abiotic component


  • Biotic component is made up of many different inter
    dependent population, e.g., plants, animals, microbes, etc.
  • Types of biotic component are - producer, consumer & decomposer 

  • All the autotrophs of an ecosystem are called producers. The green plants are the main producers. Producers are generally chlorophyll bearing organisms which produce their own food by the process of photosynthesis, in which producers absorb solar energy and convert it into chemical energy. So producers are also called transducers or converters. E.g., yellow green algae, algal protist etc.
  • Energy enters into the ecosystem through the producers. 
The solar energy is the only ultimate source of energy in the ecosystem. This energy is available for the remaining living organisms.
  • Other examples of producers are –
    • Chemoautotrophs : Iron bacteria, sulphur bacteria, nitrifying bacteria.
    • Phytoplankton : In an aquatic ecosystem, rooting plants called phytoplankton are the major autotrophs.
Phytoplanktons may produce as much food as produced by the larger shrubs and trees in unit area.

  • All the heterotrophs of the ecosystem are known as consumers. It includes micro-organisms. These directly
    (on herbivores) or indirectly (on carnivores) depend on the producers for food.
  • Types of consumer are : macroconsumers and micro-consumers.
  • Macroconsumers / Phagotrophs or holozoic digest their food inside the body of organism i.e. first ingestion then digestion.
  • Macro-consumers are of following type :
    • Primary consumer : Such living organisms which obtain food directly from producers or plants are known as primary consumers.
E.g., Herbivores of ecosystem, cow, grazing cattle, rabbit.
They are also known as secondary producers as they synthesize complex materials in the cells by the digestion of food which is obtained from the plant.
    • Secondary consumers or primary carnivores: 
Those animals which feed upon primary consumers and obtain food. Those carnivores which kill and eat the herbivores are called predators. E.g., dog, cat, snake. In aquatic system, whale is a secondary consumer. It is an example of a filter feeder because it feeds on plankton.

  • The organisms which completely depend on dead animals are not examples of predators but they all are scavengers or detritivores. E.g., Vulture, crow, fox.
  • Vulture is a scavenger not predator because it never kills any animal. Vulture is also a decomposer. In vulture, the break down of the food material takes place inside the body and then released into the soil in the form of waste material and minerals.
  • All predators are carnivores but all carnivores are not predators.
  • Plant parasites are known as primary consumers while animals parasites (E. coli bacteria, Entamoeba histolytica, liver fluke, tapeworm) are known as secondary consumers.
  • All the insectivorous plants play the double role i.e. producer as well secondary consumer because they synthesize their own food through photosynthesis and they eat insects simultaneously.
  • Man and peacock are omnivores.
  • Organisms which use milk or curd are known as secondary consumers.
    • Top (tertiary) consumers : Those animals which kill other animals and eat them, but are not killed & eaten by other animals in nature are called tertiary consumers. E.g., Lion, man, peacock.
  • Micro-consumers/Decomposers or Scavengers/Osmotrophs are those living organisms which decompose the dead body of producers and consumers. 
  • The main decomposers in ecosystem are bacteria and fungi.
  • Decomposers play a significant role in mineral cycle.


  • Abiotic components include inorganic substances or minerals; organic substance and atmospheric factor.
  • Inorganic substances are P, S, N, H, Mg, K, CO2, NO2 etc. These are raw materials for plants.
  • Organic substances are proteins, carbohydrates, lipids in dead organic substances.
  • Atmospheric factors are light, temperature, humidity, rain, water, gas etc.
  • There are two basic functions of ecosystem :
    • Biogeochemical cycle or mineral cycle (described later)
    • Energy flow 


  • Decomposers are responsible for converting complex organic material of dead minerals or plants into simpler organic matter through the process of decomposition and release mineral substances into the soil where these are reused by the producers, so that soil is considered as the best resource of minerals.
  • The upper layer of the soil is the main site for decomposition process in the ecosystem.
  • The process of decomposition involves several processes. These processes can be categorised as:
    • fragmentation of detritus
    • leaching 
    • catabolism
    • humification
    • mineralization 
  • Detritivores (e.g., earthworm) breakdown detritus into smaller particles (called fragmentation).
  • By the process of leaching, water soluble inorganic nutrients' go down into the soil horizon and get precipitated as unavailable salts.
  • In bacteria and fungi, process of decomposition completely takes place outside the body. They  carry out catabolism and release extracellular enzymes from their body on dead remains and decompose it into simpler organic substances and then absorb it. So these are called as osmotrophs (absorptive).
  • In the process of decomposition, some nutrients get tied up with the biomass of microbes and become temporarily unavailable to other organisms. Such incorporation of nutrient in living microbes (bacteria & fungi) is called nutrient immobilisation.
  • Humification and mineralization occur during decomposition in the soil. Humification leads to formation of humus which is highly resistant to microbial action & undergoes extremely slow decomposition. Humus serves as a reservoir of nutrients.
  • Mineralization results in the release of inorganic substances (e.g., CO2, H2O) and variable nutrients (NH4+, Ca++, Mg++, K+ etc.) in the soil. 
  • Decomposition is largely an oxygen requiring process.
  • Factors affecting decomposition –
    • Temperature and soil moisture are important and regulate decomposition through their effects on the activates of soil microbes.
    • Warm and moist environment favour decomposition whereas low temperature and anaerobiosis inhibit decomposition.
Fig. : Diagrammatic representation of decomposition cycle in a terrestrial ecosystem


  • Energy is neither created nor destroyed but only transformed from one state to another state.
  • 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. That is, it flows from the producer level to the consumer level and never in the reverse direction. Hence, 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. 
Hence, 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 in a food chain. And only about 10% 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.
Fig. : How Ecosystem work
  • At each step in a food chain, a portion of the energy captured by producer is lost as heat given off during the chemical breakdown of food by respiration (metabolic heat). Thus, energy flows  through the ecosystem in a single direction and is not recycled. In contrast, nutrients cycle between organisms and the physical environment.
  • The transfer of food energy from one to another organism leads to loss of energy as heat due to metabolic activity.
  • The amount of solar radiation reaching the surface of the earth is 2 cals/sq.cm/min. 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 energy trapped by the producers (primary production) is utilized by the consumers. The producers are directly consumed by herbivores that are eaten by the primary carnivores that in turn are consumed by the secondary carnivores. The consumers store some amount of energy in their tissues. This energy, stored by the consumers, is called secondary production. Only about 10 to 20% of the primary production is converted into secondary production. The remaining 80 to 90% is lost by the consumers in the form of faeces.


  • In ecosystem, every organism depends on other organisms for food material and all organisms are (herbivores to carnivores) arranged in a series in which food energy is transferred through a repeated series of eating and being eaten. It is called food chain. 
  • In the food chain, energy flow is in the form of food.
  • In a food chain, food material or food energy transfer from one trophic level to the next trophic level.
  • Four trophic levels are present in the ecosystem because level of energy decreases during the flow of energy from one trophic to another trophic level.
    • First trophic level [T1] = Producers
    • Second trophic level [T2] = Primary consumers 
    • Third trophic level [T3] = Secondary consumers 
    • Fourth trophic level [T4] = Top consumers
  • In nature, three types of food chains are present : Grazing food chain, parasitic food chain and detritus food chain.

  • There are five trophic levels found in highly complex ecosystem in which tertiary consumer is present in between  the secondary consumers and top consumers. Then the fifth trophic level (T5) is formed by the top consumer.
  • Shorter food chains will provide greater energy.
  • Generally the decomposers (bacteria and fungi) are not included in the food chain but when included then there are included as the last tropic level.


  • Most of the food chain in nature are of this type. 
This food chain begins with producers (plants) and in successive order it goes from small organism to large organism.

Aquatic ecosystem

Grassland ecosystem


  • This food chain also starts from producers but in successive order it goes from big organism to the smaller organism.

Tree ecosystem
Both grazing and parasitic food chains are directly dependent on solar radiation (as a primary source of energy) and have rapid energy flow.


  • This food chain begins with decomposition of dead organic matter by decomposers so it is also known as saprophytic food chain.
  • In this food chain, primary consumers are bacteria and fungi.
  • In mangrove vegetation, this food chain goes up to big organism.
Dead mangroves leaves → Bacteria & fungi → Amphipods, molluscs, crabs, nematodes → Small fishes → Fish eating birds.
  • In detritus food chain, energy flow is rather very slow yet magnitude of energy is great because vast number of decomposers are involved.
  • Detritus food chain does not depend on light.


  • Food web, refers to the group of interconnecting food chains in a particular community.
  • In a food web, transfer of food energy is unidirectional but from many different alternative pathways.
  • In a food web, members of a particular trophic level obtain their food according to their choice and taste. It means they have more than one option or alternative for getting food.
  • As much as food web in complex ecosystem is more permanent or stable, such type of ecosystem is not destroyed naturally and continues for a long time. This ecosystem is not affected by loss of any organism of any particular trophic level. Those ecosystems which have simple food web are not very stable it means that they can be finished at any time, if there is a change in any particular trophic level.


Ecosystem is a dynamic system because continuous interaction is going on in between abiotic or biotic components, so ecosystem is present in an equilibrium position. Ecosystem is also self maintainable and self regulatory system. It means an ecosystem maintains a balance in between different trophic levels. Each trophic level controls the other trophic level and if any change takes place in any trophic level of ecosystem, the other trophic levels of  this ecosystem may react according to it. So, ecosystem always remain in equilibrium. This feature of the system is known as homeostasis.


  • Graphical representation of ecological parameters at different trophic levels in the ecosystem is called pyramids. These parameters are number, biomass and energy. Pyramid was formed by Charls Elton; so called as Eltonian pyramids.
  • In pyramids, basal, mid and top tiers show the parameter values for producers, herbivores and carnivores in the ecosystem.
  • There are three types of pyramids
    • Pyramids of number
    • Pyramids of energy
    • Pyramids of biomass
  • In most of the ecosystems, the number and biomass of producers are more and those of consumers are less. This type of ecosystem has a pyramid where the apex is pointed upwards. This type of pyramid is called upright pyramid. In some ecosystems, the number and the biomass of the producers are less and those of consumers are more. This type of ecosystem produces a pyramid where the apex is directed downwards. This type of pyramid is called the inverted pyramid.
  • When the ecosystem contains lesser number of producers and more number of consumers, the pyramid will be inverted in shape. Inverted pyramid occurs in a tree ecosystem. A single tree (producer) contains many fruit eating birds (primary consumers). The birds contain numerous parasites (secondary consumers).
  • When the biomass of producers is less and that of consumers is more, the pyramid will have inverted shape. It occurs in a pond or lake ecosystem. Here, the biomass of diatoms and phytoplanktons are negligible as compared to that of crustaceans and small fishes.


  • Pyramid of numbers is a graphical representation of numerical strength of various populations in different trophic levels per unit area of an ecosystem with producers forming the base, intermediate levels forming intermediate tiers and apex formed by top carnivores.
  • These pyramids are mostly upright because number of producers [T1] is maximum and number of herbivores and carnivores decrease towards apex or at successive trophic levels, such as grassland ecosystem and aquatic ecosystem.
  • In aquatic ecosystem, producers are maximum. Number of individuals of any trophic level depends on the availability of those individuals which are used in the form of food for the lower trophic level I. Therefore, the availability of food is an important component. 
  • But in a tree ecosystem, the pyramid of numbers is inverted. This is called parasitic ecosystem because birds (herbivores) depend on the tree (producer) and parasites (consumer) depend on birds, therefore, with increase in the no. of trophic levels, the number of organisms increase sequentially.

  • Pyramid of number shows biotic potential of an ecosystem. The number of members of any particular species in favourable conditions is called their biotic potential. When the numbers of the members of any species increases then it is called population explosion. Because of this, the existence of the species comes in danger. Human population is approaching this condition at present.
  • In a cropland ecosystem, the crops are more in numbers. The grasshoppers feeding on crop plants are lesser in number. The frogs feeding on grasshoppers are still lesser in number. The snakes feeding on frogs are fewer in number.
Crop →  Grasshopper → Frogs → Snakes → Hawks
  • In a grassland ecosystem, the grasses are present  in large numbers. The consumers decrease in the following order:
Grass → Grasshopper → Lizard → Hawk 
Grass → Rabbit → Fox → Lion
  • In a pond ecosystem, the number decreases in the following order – 
Phytoplankton  → Zooplankton → Fishes → Snakes


  • Pyramid of biomass is a graphical representation of the amount of biomass per unit area sequence wise in rising tropic levels with producers at the base and top carnivores at the apex.
  • Mostly these pyramids are also upright (erect). E.g., Tree ecosystem, forest ecosystem.
  • Pyramids of biomass in aquatic ecosystem is inverted because in it producers are microorganisms and their biomass is very less.
  • In a grassland, the biomass of grasses is the maximum, and it gradually decreases towards the consumer level in the following order: 
Grass → Mouse → Snake → Hawk
Grass → Grasshopper → Lizard → Hawk
  • In a forest, the biomass of trees is maximum and the biomass of the top consumer is the minimum. The decrease in weight occurs in the following order :
Plants → Deer → Fox → Tiger
Plants → Rabbit → Fox → Lion
  • Some special points related to biomass:
    • Standing crop : Total amount of living organic matter present in particular area in particular time in an ecosystem is known as standing crop. It may be expressed in terms of number or weight per unit area.
Biomass is the standing crop expressed in terms of weight (i.e. organism mass). Biomass is measured by bomb calorimeter.
    • Standing quality or Standing state : Total amount of inorganic substances such as P, S, N, H present in particular area at a particular time in an ecosystem is known as standing state.

The pyramids of biomass show the standing crop.


  • It represents the amount of energy at different trophic levels.  Energy pyramids are always upright or erect because there is a gradual decrease in energy at successive trophic levels. 
  • According to the 10% law of Lindeman, 90% part of obtained energy of each organism is utilized in their various metabolic activities and heat and only 10% of energy is transferred to the next trophic level. So, 90% energy is lost at each trophic level, therefore, top consumers like lion etc. are ecologically weakest but physically they are strong.
  • In a grassland, green plants trap the maximum light energy. The energy gradually decreases towards the top consumer level–
Fig. : Pyramid of numbers in a grassland ecosystem. 
Only three top-carnivores are supported in an ecosystem 
based on production of nearly 6 millions plants.
Fig. : Pyramid of  biomass shows a sharp
decrease in biomass at higher trophic levels

Fig.  : Inverted pyramid of biomass-small standing crop 
of phytoplankton supports large standing crop of zooplankton.
Fig. : An ideal pyramid of energy. Observe that primary producers convert only 1% of the energy in the sunlight available to them into NPP

Grass → Grasshopper → Lizard → Hawk
Grass → Rabbit → Fox → Lion
Grass → Mouse → Snake → Hawk
  • In a pond, maximum energy is trapped by the phytoplankton. Then the amount of energy decreases towards the top-consumer level. 
Phytoplankton → Zooplankton → Fish → Snake
Phytoplankton → Zooplankton → Small fish → Large fish

Pyramids of energy represent the productivity of the ecosystem as well as transfer of production in ecosystem.


  • Productivity of the ecosystem refers to the rate of biomass production i.e. the amount of organic matter accumulated per unit area per unit time.
  • It is generally expressed in g–2 yr –1 or (kcal m–2) yr–1.
  • There are two types of productivity present primary and secondary.


  • Primary productivity is defined as the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.
  • Primary productivity depends on the plant species inhabiting a particular area. It also depends on a variety of environmental factors, like, availability of nutrients and photosynthetic capacity of plants. Therefore, it varies in different types of ecosystems. 
  • The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of the organic matter. 
  • Productivity of the ocean is only 55 billion tons.
  • In per unit area, maximum productivity is found in tropical rainforest.
  • In water, least productive ecosystems are very deep lakes and highly productive ecosystems are coral reefs.
  • Nitrogen is the limiting factor in ocean and phosphorus is the limiting factor in lake productivity.
  • Most productive agro-ecosystem is sugarcane and rice ecosystem - 3 - 4 kg/m sq/year.
  • Primary productivity can be divided into - gross primary productivity (GPP) and net primary productivity (NPP).
  • Gross primary productivity (G.P.P.) is the total amount of energy fixed (organic food) in an ecosystem (in producers) in unit time including the organic matter used up in respiration during the measurement period. 
It is also known as total (gross) photosynthesis. A considerable amount of GPP is utilized by plants in respiration.
  • Net primary productivity (N.P.P.) is the amount of stored organic matter in plant tissues after respiratory utilization.
NPP = GPP – R (R =   Respiration + Metabolic activities) .
            GPP = NPP + R


  • Secondary productivity is the rate of formation of new organic matter by consumers.
  • The food of consumers has been produced by the primary producers & secondary productivity reflects only the utilization of this food for the production of consumer biomass.
Net community productivity or Net productivity is the rate of storage of organic matter not used by the heterotrophs
NCP = N.P.P. – HR
(HR = Energy used by heterotrophs or consumers)


The percentage of energy transferred from one trophic level to the next is called ecological efficiency or food chain efficiency.
Ecological efficiencies may be expressed as –
  • Assimilation efficiency (AE)
    It is the proportion of consumed energy that is assimilated. 
  • Net production efficiency (NPE)
    It is the percentage of net primary productivity in relation to gross primary productivity. 
  • Photosynthetic efficiency (PE)
    It is the percentage of incident solar radiation trapped by producers to perform photosynthesis & produce gross primary productivity.

Photosynthetic efficiency is 1-5%. It can also be expressed in relation to PAR when it is 2-10%.
  • Ecological efficiency(EE)/trophic level efficiency
    (TLE) : It is the percentage of energy converted into biomass by a higher trophic level over the energy of food resources available at the lower trophic level.  


  • All the types of materials required by ecosystem in addition of energy, are available continuously to system through recycling. Thus, there is a constant exchange of materials between the living organisms and their abiotic environment through the recycling of materials. This phenomenon is called biogeochemical cycle.
  • The mineral elements taken up from the environment (soil as well as air) by the green plant - the producers, are again returned to the environment through consumers and decomposers.
  • The following types of cycle are found in an ecosystem–
    • Gaseous cycle : C, H, N, O cycles. Reservoir is in the atmosphere (air) or in hydrosphere (water). 
    • Sedimentary cycle : P, S, Ca cycles. Reservoirs are in earth's crust (lithosphere).


  • The main source of carbon is atmosphere and hydrosphere is the rocks of carbonates. 
  • Carbon present in lithosphere is in the form of coal and petroleum. The carbon released from them is present in the atmosphere in the form of carbon dioxide. 
  • The green autotrophs utilize CO2 from the air to synthesize food materials which is obtained by other organisms as food. Carnivores obtain their carbonic food from the herbivores. These carbonic matter produce CO2 through the oxidation or respiration which dissolve in air or water and are again utilized by the plants.
Fig. : Carbon cycle


  • The cycling of phosphorus between biotic and abiotic components of the environment represents phosphorus cycle.
  • In this, organic phosphates (or H2PO4) are absorbed by plants from the soil and bodies of water and eventually pass into animals through food chains.
  • Phosphorus is the main constituent of protoplasm, plasma membrane, bones and teeth. 
  • Main source of phosphorus is rocks. It comes from the weathering of phosphorus containing rock in the soil. Plants absorb this phosphorus from the soil and transfer this phosphate to animals and after the death of animals, it is released again into the lithosphere by the action of decomposers.
  • Sometimes some of the elements like phosphorus and calcium reach into the sea through water, from where they transform into rocks. They separate from the cycle for a long time so it is also known as sedimentary cycle.
But when these rocks break after sometime, then this phosphorus is again made available to the sea plant or sea weeds, which pass into fish and sea birds. The excretory materials of birds on the rocks of sea shore is called Guano and it is a source of phosphorus.

Plants absorbs phosphate from the soil in the form of orthophosphate (PO43–).
Fig. : Phosphorus cycle


  • Development of plant community on barren area is called ecological succession or biotic succession.
  • The replacement of existing community by new ones, in an orderly sequence in barren area with time due to change in environmental conditions is succession.
  • Biotic communities are never stable. They are changing more or less over time and space, due to the presence of different types of climatic and environmental conditions. So a continuous interaction is going on between the community and environment till state of stability.
  • Term of community in succession :
    • Pioneer community : The first community to inhabit an area.
    • Climax community : The last and stable community in an area. This is more stable. Usually mesophytes are present in climax community.
    • Seral communities or seral stage : In succession, communities or stages which comes in between pioneer community and climax community is called transitional or seral communities.
    • Sere : The entire series of communities is called sere. The name of sere depends on where the succession occurs or takes place.
Succession in fresh water Hydrosere
Succession in salty water Halosere
Succession in acidic water Oxalosere
Succession at dry region Xerosere
Succession on rocks Lithosere
Succession on sand Psammosere
Succession at moistened region Mesosere
Succession of microbes Serula
Fig. : Succession : When strong winds cause moving sand dunes to form at the southern end of Lake Michigan, succession often leads to a community dominated by black oak.  Succession on such dunes occurs in three stages and forms black oak communities that have lasted upto 12,000 years. Under different local environmental conditions, succession in Michigan sand dunes can lead to the establishment of stable communities as different as grasslands, swamps, and sugar maple forests.

  • Ecological succession shows certain characteristics :
    • Gradual replacement from short lived to long lived plant.
    • Continuous change in communities towards a state of stability or climax.
    • Increased species diversity, biomass, niche specialization, humus content.
    • Decreased net community productivity or annual yield.
    • Future seral communities can be predicted as it is a directional process.


  • Biotic factors : The action of each seral community (interaction with its environment) makes the area less favourable for itself and more favourable for next seral community in the succession.
  • Physiographic factors : These include climatic and other physical factors like soil erosion, soil deposition, landslides, volcanic lava. These all factors make an area barren.


  • Ecological succession are of two types – primary and secondary.
  • Primary succession occurs in the barren area where there was no type of living matter (previously). E.g., volcanic lava, estuarine,  mud bank, igneous rock, sand dunes.
  • Secondary succession occurs  where vegetation was present previously but vegetation was destroyed due to natural or artificial causes i.e. fire, flood, sudden changes in climate, landslide.
  • This succession is comparatively more rapid. It requires 50-100 years for grassland and 100-200 years for forest.


  • On the basis of replacement
    • Autogenic succession : During the succession, the community reacts with the environment and changes it. This community is replaced by new community. This is known as autogenic succession.
    • Allogenic succession : Community is replaced due to external conditions or forces, not by existing vegetation itself. This kind of succession is known as allogenic succession.  E.g., Fire, flood.
  • On the basis of changes in nutritional and energy contents
    • Autotrophic succession : This is the succession of plants communities. It begins in a predominantly inorganic environment & the energy flow is maintained indefinitely. There is a gradual increase in the organic matter content supported by energy flow. 
    • Heterotrophic succession : This is the succession of animal communities. It begins in a predominantly organic environment and there is a progressive decline in the energy content.

Sometimes succession is in retrogressive direction. E.g., Forest to grass.


  • Nudation : It involves development of barren area (i.e. removal of community) by topographic (soil erosion, landslides, volcanic eruptions), biotic (human activity) and climatic factors (fire, flood, hail). It is the early stage of soil formation.
  • Invasion :  Successful establishment of a species in a barren area. This process is completed in three steps :-
    • Migration (dispersal) : Reaching of different reproductive structures like seed, spores through water or air on barren area.
    • Ecosis (ecesis) : Successful establishment of species in new environment is called ecosis. Many juvenile plants are formed due to the germination of different spores or seeds. Out of them, some of the plant species are modified or adapted according to the new climatic conditions and established there.
    • Aggregation : After ecesis (establishment), as a result of reproduction, members of the species increase in number.
  • Competition or co-action : Due to increasing number of species at limited place, there develops competition for habitat and nutrition. 
  • Reaction : Species which have survived, will react with the environment and modify the environment (change soil, water, light, temperature). The modified environment is less favourable for the existing community so it is replaced by another community.
  • Stabilization (Climax) : Finally there occurs a stage in the process when the final terminal community becomes stabilized for a longer period of time, maintains itself with the climate of the area. This community is called climax community. It is complex, stable, no more species can replace them.


Stages of hydrosere or hydrarch succession in the newly formed pond or lake.
  • Phytoplankton stage : It is pioneer community, first coming minute autotrophic organism. These produce organic matter. E.g., soft mud diatom, cyanobacteria.
  • Rooted submerged stages : E.g., Vallisneria, Hydrilla, Utricularia etc.
  • Rooted floating stages : E.g., Nymphaea, Nelumbium, Trapa, Azolla, Lemna, Wolffia.
  • Reed swamp stage (amphibious stage) : Most part of these rooted plants remain exposed to air. E.g., Typha, Sagittaria.
  • Sedge Meadow stage or marsh meadow stage : Muddy plants. E.g., Ipomea.
  • Scrub stage : Woody shrubs, tolerates water logging. E.g., Cornus, Salix, Almus.
  • Climax forest stage : e.g., Tree

Stages of lithosere (succession on rocks) are :
  • Crustose lichens stage : It is a pionner community, tolerates desiccation, produces organic acid which causes weathering of rocks, so first minerals are released for own use. E.g., Rhizocarpon, Rinodina.
  • Foliose lichens stage : Large lichens with leafy thalli. E.g., Parmellia, Dermatocarpon.
  • Moss stage : E.g. Polytrichum, Torula.
  • Herb stage : Annual hardy grasses, E.g., Poa, Eleusin, Aristida.
  • Shrub stage : E.g., Zizyphus, Rhus.
  • Climax forest stage (Mesophytic) : E.g., Tree.


  • Information gained through biotic succession is used in having controlled growth of one or more species by preventing their superiors to invade the area, e.g., maintenance of teak forests.
  • It gives information about the techniques to be used during reforestation & afforestation.
  • Dams are protected by preventing situation & biotic succession to occur.
  • It tells us how a biotic seral stage can be maintained by net allowing the biotic succession to proceed further through interference like grazing and fire.


  • Ecological services are those humankind benefits from a multitude of resources and processes that are supplied by natural ecosystems.
  • These services include carbon fixation, pollination, oxygen release, cycle nutrients, mitigate droughts and floods etc.
  • Services can be subdivided into five categories–
    • Provisioning, such as the production of food and water
    • Regulating, such as the control by climate and disease
    • Supporting, such as nutrient cycle and crop pollination
    • Cultural, such as spiritual and recreational benefits
    • Preserving, which includes guarding against uncertainty through the maintenance of diversity

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Ecosystem | Biology Notes for NEET/AIIMS/JIPMER
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