structure and function of ecosystem


Structure of Ecosystem : The structure of ecosystem depends upon following components :

(i) Species diversity (ii) Species composition

(iii) Life cycle (iv) Stratification

Component of Ecosystem

1. Abiotic components : Consists of non living substances and factors like.

(i) Climatic factors i.e. air, water, light, temperature and precipitation.

(ii) Edaphic factors like soil composition.

(iii) Topographic factors i.e. mountains, slopes.

2. Biotic components : They constitute producers or transducers, consumers and decomposers or micro-consumers or saprotrophs.

Incomplete ecosystem:

An ecosystem lacking one or more structural components e.g., deep sea, freshly formed rain water pond ecosystem.

Functions of Ecosystem :

The functional components of ecosystem are studied with following aspects like

(i) Productivity (ii) Mineral cycling

(iii) Energy flow (iv) Food chain and web

(v) Efficiency (vi) Biotic interrelationships

(vii) Homeostasis (viii) Ecoregulation

Homeostasis in ecosystem :

Ecosystem maintains functional balance or relatively stable state of equilibrium amongst its various components. It is due to

(i) Carrying capacity (ii) Nutrient cycling (iii) Self regulation (iv) Feed back


Stratification is the structure or recognizable pattern in spatial arrangement of the members of the communities.

More specifically stratification represent vertical zonation in the community.

For example in grassland communities, there is subterranean floor containing basal portions of the vegetation.

However stratification in a forest community is most complicated where, as many as five vertical subdivisions may be recognised.

These vertical subdivisions are:

(i) Subterranean (ii) Forest floor

(iii) Herbaceous vegetation (iv) Shrubs

(v) Trees

Artificial ecosystem:

These are man-made ecosystems e.g., -Modern agriculture, dams, zoological parks, plantations, aquacultures etc.

Characteristics :

(i) Do not possess self regulatory mechanism

(ii) Have little diversity

(iii) Simple food chain

(iv) High productivity

(v) Little cycling of nutrients

Boundaries of Ecosystems

An ecosystem is generally regarded as a self-sufficient unit and a separate entity.

However, it never operates in isolation.

Boundaries between one ecosystem and another are indistinct and overlapping and all ecosystem on the earth are joined together to form a single global ecosystem known as biosphere.

Some exchange of materials and energy always occurs between different ecosystems through geological, climatic or biological processes.


The structural aspects deal with the study of number, kinds and distribution of various types of biotic (living organisms) and abiotic (e.g., light, temperature, water, oxygen, carbon, nitrogen, minerals, etc.) components.

(I) Biotic components :

Living organisms, i.e., plants, animals and micro-organisms constitute biotic component of the ecosystem.

1. Producers:

They are green photosynthetic plants that entrap solar energy through chlorophyll to synthesise organic food from inorganic raw materials.

The green plants are thus termed autotrophs as they are capable of synthesizing their own food materials.

They are also termed transducers as they change radiant energy into chemical energy.

The complex organic substances are utilized for building up their bodies and for releasing energy required for various metabolic and physiological activites.

2. Consumers:

They are the animals that are not capable of synthesizing the food materials, but feed upon other organisms or their parts.

They are thus called heterotrophs.

They are also called phagotrophs as they ingest the solid food materials.

The consumers are mainly of two types i.e., herbivores and carnivores.

Herbivores are termed primary consumers as they obtain food directly from plants.

Cattle, deer, goat, rabbit, mouse, grasshopper, etc., are common herbivores in terrestrial ecosystem and crustaceans, molluscs and protozoans are common herbivores in aquatic ecosystems.

Some carnivores (e.g., frog, cat, jackal, fox, some fishes, etc.) feed upon herbivores and thus termed as secondary consumers.

Other carnivores feed upon secondary consumers, not eating the herbivores.

They are termed as tertiary consumers (e.g., wolf, peacock, etc).

Some carnivores are thus eaten by other larger and stronger carnivores.

However, some larger and stronger carnivores (e.g., tiger and lion) never become prey to any animal and act as predators only. They are called top carnivores.

3. Decomposers:

They are saprophytic micro-organisms (bacteria, actinomycetes and fungi) deriving their food material from organic matter present in dead remains of plants and animals.

They secrete digestive enzymes which convert complex organic substances into simpler ones.

A part of the digested organic matter is assimilated by the micro-organisms and the rest is broken down into simpler inorganic compounds for recycling.

They bring about cyclic exchange of materials between biotic community and the environment. They are thus very essential components of an ecosystem.

They are also called reducers as they are capable of degrading the dead organisms.

Some workers differentiated few other categories of living beings amongst the biotic components of an ecosystem.

They are scavengers, detrivores and parasites.

Parasites belong to diverse groups, e.g., bacteria, fungi, protozoans, worms, etc.

Every type of living being can be attacked by parasites.

Detrivores are animals which feed on detritus e.g., termites, earthworm etc.

They are helpful in quick disposal of the dead bodies.

Scavengers are animals that feed on dead or injured animals and they clean the earth of organic garbages e.g., carrion, Marabou storks, Crow, Vultures (Full time scar vengers)

(II) Abiotic Components:

Non-living factors such as temperature, water, light, etc., constitute abiotic components of the ecosystem.

They are mainly of three types, i.e., climatic, topographic and edaphic.

Different abiotic factors in an ecosystem are described below.

1. Temperature: Every organism has specific range to which it is adapted to live. There are some exceptions like prokaryotes and encysted protozoan which can withstand extremes of temperature.

2. Light: It plays, a crucial role in ecosystem as it is sunlight which is the direct or indirect source of energy for all types of living organisms. It is the driving force of an ecosystem.

3. Wind: Wind has more pronounced effect on plants than on animals.

(i) Wind velocity increases the rate of transpiration.

(ii) Wind brings about pollination in most of the gymnosperms and some angiosperms which is essential for seed formation and hence perpetuation of species.

(iii) It brings about dispersal of fruits and seeds, necessary to avoid overcrowding and competition.

(iv) Wind affects the plant and animal populations by causing soil erosion in dry areas.

4. Humidity: It refers to water vapour or moisture content of the atmosphere and affects the water loss from the body surface of terrestrial organisms that occurs through evaporation, perspiration and transpiration.

5. Precipitation: It occurs in different forms like rainfall, dew, hail, snow, etc. Rainfall is the most significant of these and is the main determinant of composition of biotic community.

6. Water: Availability of water in soil, ponds, rivers, lakes, etc., mainly depends upon rainfall which controls the distribution of animals through its effect on water availability.

7. Topography: Topography is the surface behaviour of the earth like slope, altitude, hills, plains, mountain chain, exposure, etc. These factors affect vegetation and consequently animal life indirectly through their effect on rainfall, light intensity, wind velocity, water content in soil, etc. Vegetation on two sides of a hill, one facing the sun and other away, differ because of the difference in environmental conditions, like humidity, light duration, light intensity, rainfall, etc., as two faces of hill receive different amount of solar radiations and wind action. Flora and fauna on the edge of pond and middle of pond and, on or underside of the rock are different for similar reason.

8. Soil: The edaphic factors, i.e., the factors relating to soil such as soil texture (sand, loam or clay), soil pH (acidic or alkaline), soil water, soil aeration, mineral contents of the soil, etc.,determine the distribution of plants and of animals too, which depends upon vegetation.


The components of the ecosystem are seen to function as a unit when following aspects are considered.

(A) Productivity

(B) Decomposition

(C) Energy flow

(D) Nutrient cycling


 Productivity of Ecosystem  

The rate of biomass production is called productivity. It is expressed in terms of (gm–2)yr–1 or (k cal m–2) yr1 to compare the productivity of different ecosystems.

(i) Coral reefs, tropical rain forests, sugarcane fields are most productive.

(ii) Deserts and deep sea ecosystems are least productive.

Ecosystem productivity is maintained by the flow of energy derived from the sun.

Energy trapped by plants varies from ecosystem to ecosystem.

Table below shows energy absorption at different levels:

Types of productivity :

(i) Primary Productivity : The rate at which radiant energy is stored by photoautotrophs and chemoautotrophs.

a. Gross Primary Productivity (GPP) : It is the rate of organic matter synthesized by producers per unit area per unit time.

b. Net Primary Productivity. It is the rate of organic matter built up or stored by producers in their bodies per unit time and area. Net productivity is equal to gross primary productivity minus loss due to respiration and other reasons. NPP is the available biomass for the consumption to heterotrophs i.e. herbivores and decomposers.

Net primary productivity = Gross primary productivity – Respiratory loss.

Primary productivity depends on the plant species inhabiting a particular area, availability of nutrients and photosynthetic capacity of plants. This is available to herbivore level.

The annual NPP of whole biosphere is approximately 170 billion tons (dry wt.) of organic matter, despite occupying about 70% of the surface, the productivity of the oceans is only 55 billion tons.ln deep marine habitats, both light and nutrients become limiting. The most limiting nutrient of marine ecosystem is nitrogen.

(ii) Secondary productivity. Rate of increase in energy containing organic matter or biomass by heterotrophs or consumers per unit time and area is known as secondary productivity. It is available to carnivore level.

(iii) Community productivity. It is the rate of net synthesis of built up of organic matter by a community per unit time and area.

(iv) Ecological efficiency/Trophic level efficiency. The percentage of energy converted into biomass by a higher trophic level over the energy of food resources available at the lower trophic level is called ecological efficiency.



It is the process of physical and chemical breakdown of complex organic remains by organisms called decomposers, so as to produce inorganic raw materials (CO2, H2O, minerals, etc.) for recycling.

The major site for decomposition is the upper layer of soil in terrestrial habitats and bottom of water bodies.

Freshly deposited organic matter constitutes raw material and is called litter.

Detritus is degrading dead organic matter and is differentiated into above ground and below ground detritus.

Above ground detritus consists of dried plant parts (leaves, twigs, bark, flowers), excreta and dead remains of animals.

Below ground detritus is also called root detritus, because it is mainly composed of dead roots.

Underground organisms and their excreta also form a part of below ground detritus.

Decomposition Processes

Three types of processes occur simultaneously during decomposition of detritus, viz. fragmentation, leaching and catabolism.

1. Fragmentation of Detritus: Small invertebrate animals called detrivores feed on detritus, e.g., Earthworms, termites. They bring about its fragmentation. A part of detritus eaten by detrivores comes out in highly pulverised state in their faeces. Due to fragmentation during eating and pulverisation in digestive tracts, detritus is changed into fine particles which have a large surface area.

2. Leaching: Part of soluble substances present in the fragmented and decomposing detritus (e.g., sugars, inorganic nutrients) get leached to upper layers of soil by percolating water.

Fig. : Diagrammatic representation of decomposition cycle in a terrestrial ecosystem

3. Catabolism: It is carried out by saprotrophic bacteria and fungi. They secrete digestive enzymes over the fragmented detritus. The enzymes change complex organic compounds into simple compounds. Inorganic substances are also released in the process.

The rate of catabolic action or breakdown of different complex substances is different.

This differential decomposition produces two substances, humus and inorganic nutrients in processes respectively called humification and mineralisation, which occurs in soil.

(i) Humification. It is the process of partial decomposition of detritus to form humus. Humus is a dark coloured, amorphous, partiaily decomposed organic matter rich in cellulose, lignin, tannins, resin, etc. and is highly resistant against microbial action. It undergoes decomposition at an extremely slow rate. Humus is slightly acidic, colloidal and functions as reservoir of nutrients.

(ii) Mineralisation. It is the release of inorganic substances (e.g., CO2, H2O, minerals) from organic matter during the process of decomposition. They are formed alongwith simple and soluble organic substances when digestive enzymes are poured over organic matter by saprotrophic microbes.

Factors Affecting Decomposition

The rate of decomposition of detritus is controlled by a number of factors.

(i) Chemical Nature of Detritus. Decomposition of detritus is slow if it contains lignin, chitin, tannins (phenolics) and cellulose. It is rapid if detritus possesses more of nitrogenous compounds (like proteins, nucleic acids) and water soluble reserve carbohydrates.

(ii) Soil pH. Detrivores are fewer in acidic soils. Microbial activity is also low in such soils. Therefore, rate of decomposition of organic matter is slow in acidic soils. Partially decomposed organic matter piles up over such soils. Detrivores are abundant in neutral and slightly alkaline soils, while decomposer microbes are rich in neutral and slightly acidic soils.

(iii) Temperature. At a temperature of more than 25°C, decomposers are very active in soils having good moisture and aeration. In humid tropical regions, it does not take more than 3 – 4 months for complete decomposition of detritus. However under low temperature conditions (>10°C) of soils, the rate of decomposition is very slow even if moisture and aeration are optimum.

(iv) Moisture. An optimum moisture helps in quicker decomposition of detritus. Reduction in moisture reduces the rate of decomposition as in areas of prolonged dryness like tropical deserts where, otherwise, the temperature is quite high. Excessive moisture also impedes decomposition. Temperature and soil moisture are the most important climatic factors that regulate decomposition through their effects on the activities of soil microbes.

(v) Aeration. It is required for activity of decomposers and detrivores. A reduced aeration will slow down the process of decomposition.

energy flow

 Energy Flow  

Flow of incident energy is shown below

It means plant capture only 2-10% of the PAR and this small amount of energy sustains the entire living world. All organisms depends upon plants (directly or indirectly) for energy.

Food Chain

It is a sequence of living organisms due to interdependence in which one organism consumes another.

Trophic Level :

Organisms occupy a place in the natural surroundings or in a community, according to their feeding relationship with other organism.

Every position is called trophic level.

It is based on the source of their nutrition.

The ultimate source of energy used by all living organisms is the sunlight which is entrapped by green plants, and utilized for the synthesis of complex organic substances (carbohydrates) during photosynthesis.

The energy trapped in organic substances by autotrophs is passed on to different living organisms through food.

Exchange of both energy and materials thus occurs through food.

The sequence of populations or organisms or trophic levels in an ecosystem through which food and its contained energy flows constitutes a food chain.

The number of trophic levels in a food chain is equal to the number of steps involving the transfer of food from one organism to the other.

Decomposers are not included in food chain as they operate at all trophic levels.

1. Producers: They are autotrophs, synthesizing complex organic substances from simple inorganic substances like CO2 and H2O during photosynthesis. The sunlight provides the energy for the process, the solar energy is converted into chemical energy and is stored in different complex organic substances like carbohydrates, proteins, lipids, etc.

2. Consumers: They are heterotrophic organisms incapable of synthesizing their own food. They depend upon plants (producers) for their food requirement directly or indirectly. The consumers are of different types:

(i) Primary consumers: They are herbivores which feed upon plants or plant products, e.g., rabbit, deer, field mouse, cow, elephant, small fish, tadpoles, several insects, zooplanktons like Paramecium, Daphnia, etc. These are called Key industry animals as they convert plant matter into animal matter.

(ii) Secondary consumers : They do not feed upon plants directly; instead feed upon herbivores, so are primary carnivores e.g., fox, jackal, frog, fish, several birds, etc.

(iii) Tertiary consumers: They are larger camivores which feed upon smaller carnivores e.g., wolf feeding upon fox, snake feeding on frog. These carnivores may also become prey to still larger carnivores. The latter are termed top carnivores e.g., tiger, lion, shark, crocodile, eagle, etc. A food chain may vary in length but usually consists of 4 or 5 steps or trophic levels. A few common food chains are given below :

Fig. : Diagramatic representation of trophic levels in an ecosystem

Types of Food Chain

(i) Grazing Food Chain (GFC)/predator food chain

(a) Major conduit for energy flow in aquatic ecosystems.

(b) Always begins with producers

(c) Sun is the only source of energy

(d) Size of organisms commonly increase at higher trophic levels

Fig. : Energy flow through different trophic levels

(ii) Detritus Food Chain (DFC)

Death of an organism is the begining of DFC.

(a) In terrestrial ecosystems, a much larger fraction of energy flows through the DFC than through the GFC.

(b) Source of energy is detritus not sun.

(c) Composed of a long chain of detritus eating organisms (detritivores)

(d) In some ecosystems (e.g., Tropical rain forest) more energy flows in this chain than grazing food chain.

(e) DFC may be connected with the GFC at some levels, as some of the organisms of DFC are prey to certain GFC animals and in a natural ecosystem, some organisms like cockroaches, crows etc. are omnivores.

(iii) Parasitic food chain/Auxiliary food chain

Size of the organisms finally reduces at higher trophic level (parasite).

e.g., Tree herbivore birds lices and bugs.

Terrestrial food chains

1. Grass Rabbit Cat Wolf Tiger

2. Grass Grasshopper Frog Snake Peacock Falcon.

3. Vegetation Insect Predator bird Hawk.

Fig. : A food chain - 1. Producer-Grass 2. Primary consumer-Grasshopper

3. Secondary consumer-Frog 4. Tertiary consumer-Snake 5. Quaternary top consumer-Eagle.

Aquatic food chains

1. Phytoplanktons Zooplanktons Crustaceans Predator insects Small fish Large fish.

2. Phytoplanktons Zooplanktons Crustaceans Predator insects Kingfisher Stork.

Food web

In ecosystem, linear food chains as shown above seldom exist, because every organism has alternate source of food.

An animal may have preference for a particular prey, but if the latter has a small population, it may feed upon some other prey.

Single animal may be eaten by different animals and thus different food chains get interconnected and one animal may be a link in more than one food chain.

The network of interconnected food chains at different trophic levels in a biotic community is termed food web.

Occurence of food webs provides stability to ecosystem.

Food webs operate because of taste preference for particular food and unavailability of food.

One animal may feed upon organism of ever different trophic level like -Snakes may feed upon mice (herbivore) and frogs (carnivore), jackals are both carnivores and scavengers.

Only 10% of the gross productivity of producers is entrapped by herbivores for their body building.

Similarly 10% of the herbivore productivity is available for raising productivity of primary carnivores.

Higher carnivores are also able to retain only 10% of energy present in primary carnivores.

It is called 10% law (Lindemann, 1942).

It is due to this fact, the number of trophic levels in the GFC is restricted as the energy transfer follows this law.

Respiratory loss gradually increases in successive trophic levels. It is 20%, 30% and 60% respectively at producer, consumer and top carnivore level.

Standing State or Standing Quality: Amount of all the inorganic substances present in an ecosystem per unit area at a given time.

Standing Crop: Amount of living material present in different trophic levels at a given time. It is commonly expressed as the number of organisms per unit area.

ecological pyramids


An ecological pyramid is the graphic representation of, trophic levels of a food chain.

Ecological pyramids were developed by Charles Elton (1927) and are, therefore, also called Eltonian pyramids.

Three types of ecological pyramids are recognized, viz., (i) pyramid of number, (ii) pyramid of biomass, and (iii) pyramid of energy, giving graphic representation to three important parameters at different trophic levels in food chain respectively, number of individuals, amount of biomass and amount of energy.

(i) Pyramid of number:

In most ecosystems, the number of producers is maximum.

During transfer of food at any trophic level, only 10% of the food present in one trophic level becomes part of the next trophic level.

90% of the food is either lost in wastage or broken down during cellular respiration for providing energy for various life activities.

Producers, thus can support fewer herbivores and herbivores can support still fewer carnivores and so on.

Thus the number of top carnivores is too small to support any other trophic level and don't act as prey to any other organism.

Fig. : Pyramid of numbers in a grassland ecosystem.

(For example, only three top-carnivores are supported in an

ecosystem based on production of nearly 6 millions plants)

Pyramid of numbers, though upright in most cases, like a pond or a grassland but may not be so always.

In some cases it may be inverted, i.e., the number of the organisms at each successive trophic level is higher than that in preceeding one and the size decreases gradually at each successive level; e.g., a large-sized tree (producer) may support and provide nourishment to several birds (herbivores).

The number of ectoparasites like mites, ticks, lices, -bugs etc., dependent upon birds for nourishment is much larger than birds.

The number thus increases at each successive level.

The pyramid of number may become spindle-shaped for a tree as herbivorous birds are usually eaten by eagle or falcon.

The number of eagles is much less than that of birds feeding upon tree.

The number of organisms thus increases at lower trophic levels and finally decreases at higher trophic levels.

(ii) Pyramid of biomass :

Biomass is the amount of living matter (expressed as weight) at any particular trophic level at a given time.

Pyramid of biomass in terrestrial ecosystems is usually upright.

For upright pyramid, total biomass of plants (producers) in a specific area is more than that of herbivores (primary consumers) and it gradually decreases at each successive trophic level.

It is least in top carnivores. It is upright for tree and grassland ecosystems.

Fig. : Pyramid of biomass. Showing a sharp decrease in biomass at higher trophic levels

In aquatic ecosystem, the pyramid of biomass may be inverted, e.g., biomass of zooplanktons is higher than that of phytoplanktons as life span of former is longer and the latter multiply much faster though having shorter life span.

A number of generations of phytoplanktons may thus be consumed by single generation of zooplanktons.

Biomass of fish may still be larger as fish are larger in size with longer life span and a number of generations of zooplanktons can be consumed by fishes.

However during transfer, only 10% of the biomass of one generation is passed on to next trophic level.

Fig. : Inverted pyramid of biomass in Pond Ecosystem

(iii) Pyramid of energy:

The pyramid ofenergy is always upright because the flow of energy is unidirectional from producer to consumer level.

The energy content is maximum in producers.

The energy decreases at each trophic level of food chain, as part of the energy is lost as heat and major part of energy is liberated during respiration for use in various activities.

Only 10% of the energy of previous trophic level is received by next trophic level, as proposed by 10 per cent law of Lindeman (1942).

Just to illustrate 1,000 calories of solar energy is needed to produce 10 calories of energy stored in a plant (if plants trap 1 % solar energy).

Herbivores, feeding upon plant, will retain 1 cal of available stored energy and carnivores feeding upon them will gain only 0.1 cal of usable energy.

Fig. : An ideal pyramid of energy. Observe that primary producers convert
only 1 % of the energy in the sunlight available to them into NPP

Limitations of ecological pyramid

(i) It does not take into account the same species belonging to 2 or more trophic levels, e.g., insectivorous plants.

(ii) It assumes a simple food chain and does not accommodate a food web.

(iii) Saprophytes, decomposers, microbes and detrivores are not given any place in ecological pyramids.

Ecological succession


Biotic community is seldom static.

Its composition changes with time due to interactions between biotic and abiotic components.

This change is orderly and sequential, parallel with the changes in the physical environment.

These changes lead finally to a community that is in near equilibrium with the environment and is called climax community.

Such gradual and fairly predictable changes in the species composition of a given area are collectively called ecological succession.

During succession, some species colonise an area and their populations become more numerous, whereas populations of other species decline and even disappear.

The first biotic community that develops in a bare area is termed as pioneer community, e.g., lichens on bare rock.

The pioneer community is followed by a specific orderly sequence of series of plant communities called transitional communities or seral communities.

The entire series of communities is known as sere.

The last community in biotic succession which is relatively stable and in harmony with environment of that area is termed' as climax community.

It is also called as climatic climax.

All successional processes leads to a similar mesic climax.

Succession is hence a process that starts where no living organisms are there.

Types of Biotic Succession

(A) Depending upon the nature of habitat of its start, it is of two types, i.e., xerosere, starting on terrestrial habitat and hydrosere, starting on aquatic habitat. Xerosere is further of different types, i.e., lithoserestarting on barren rock and psammosere-starting on sand, halosere-in salt marshes (i.e., in a physiologically dry soil).

(B) Depending upon the type of nudity of the area, it is of two types:

(i) Primary succession. It starts at barren area, never having vegetation of any type. Cooled volcanic lava, sand dunes, igneous rocks, newly exposed sea or newly submerged terrestrial habitats in water, etc., are the areas where primary succession starts. It is very difficult for the pioneer community to get established in these areas and thus it takes a very long time.

(ii) Secondary succession. It starts at the habitats which become barren secondarily by the destruction of earlier vegetation under the influence of natural calamaties like forest fire, volcanic eruptions, earthquakes, landslides, floods, etc., or due to overgrazing and leaving the crop fields uncultivated for a long period. Such habitats are fertile, rich in organic matter and even occupied by certain organisms. Secondary succession, thus, starts quickly and climax community is established in a short span.

(C) Autogenic and Allogenic Succession : When a community replaces the other due to the modification of the environment by the community itself (internal factors) the succession is called autogenic. On the contrary, when a community replaces the other, largely due to the forces other than the effects of communities on the environment, the succession is said to be allogenic.

(D) Autotrophic and Heterotrophic Succession : Autotrophic succession is characterised by early dominance of autotrophic organisms and begins in predominantly inorganic environment. On the contrary, heterotrophic succession is characterised by early dominance of heterotrophs and begins in a predominantly organic environment.

Process of Succession

Major steps in a primary autotrophic succession are as follows :

1. Nudation: Exposure of an area.

2. Migration: The process of dispersal of seeds, spores and other structure of propagation of the species to bare area is known as migration.

3. Germination: It occurs when conditions are favourable.

4. Ecesis: Successful germination of propagules and its establishment in a bare area is known as ecesis.

5. Colonisation and Aggregation: After ecesis, the individuals of the species increase in number as the result of reproduction.

6. Competition and Co-action : Due to limited resources, species show both inter and intraspecific competition. This results into elimination of unsuitable and weaker plants.

7. Invasion: Various other types of plants try to establish in the spaces left by the elimination of previous plants due to competition.

8. Reaction: The newly arrived plants interrupt with the existing ones. As the result of reaction, environment is modified and becomes unsuitable for the existing community which sooner or later is replaced by another community.

9. Stabilisation: The process when the final climax community becomes more or less stabilised for a longer period of time and it can maintain itself in equilibrium with the climate of the area. As compared to transitional communities, the climax community has larger size of individuals, complex organization, complex food chains and food webs, more efficient energy use and more nutrient conservation.

Succession on bare rock

Succession starting on bare rock is termed lithosere. The various seral stage in lithosere are as follows.

1. Lichen stage.

The pioneer lichens on such habitats are usually crustose lichens, e.g., Graphis, Rhizocarpon.

The propagules of these lichens settle and get established on wet rock surface soon after rainfall.

They can tolerate desiccation and high temperature.

The acidic substance produced by lichens corrodes the rock surface forming small depressions and release minerals needed for the growth of lichens.

The dead and decaying organic matter of the lichens along with sand particles, brought by wind, to get collected in depressions and forms a little bit of soil by mixing with weathered rock particles.

The habitat becomes suitable for foliose lichens like Parmelia.

Foliose lichens compete with crustose lichens and slowly replace the latter due to their larger size.

They increase shading of rocks, more accumulation of organic matter and formation of larger depressions.

This accelerates the process of soil formation and makes the habitat more suitable for next seral stage, the moss stage.

2. Moss stage.

Due to interaction of foliose lichens, the habitat becomes suitable for hardy mosses (e.g., Tortula, Grimmia) to grow.

Mosses, being larger in size and having gregarious habit, shade the lichens and replace them.

Their rhizoids can penetrate deeper.

Growth of mosses leads to accumulation of more soil and organic matter which can retain moisture for a longer span and soon the habitat is occupied by moisture loving mosses (e.g., Hypnum, Bryum).

Fig. : Different stages in xerosere starting on rock (lithosere)

2. Annual grass stage.

During rainy season, the compact mat formed by mosses on weathered rock retain sufficient moisture and the habitat thus becomes suitable for germination of seeds of annual grasses and hardy herbs, e.g., Aristida, Poa, Eleusine, etc.

Their roots penetrate deeper and cause more weathering of rocks.

They replace mosses, grow for a couple of months and their death and decay results in increased organic matter.

This mixes with weathered rock particles to form soil and thus process of soil formation continues.

3. Perennial grass stage.

Due to increasing moisture and soil in rock crevices, annual grasses are replaced by perennial grasses.

These grasses like Heteropogon, Cymbopogon, etc., spread very fast due to the presence of runners and rhizomes.

4. Shrub stage.

The habitat occupied by perennial grasses soon become suitable for invasion of xerophytic shrubs like Zizyphus, Rubus, Rhus, Capparis,etc.

These shrubs soon get established in such habitats, replacing the perennial grasses.

As shrubs are larger in size, their roots penetrate deeper, causing more fragmentation of rock and hence more accumulation of soil.

5. Climax stage.

Shrubs are soon replaced by hardy trees.

Soon the atmosphere becomes more moist due to large amount of water transpired by large sized plants and ultimately the community, which is relatively more stable, occupies the habitat.

This is termed climax community.

The nature of climax community is determined by the climate of that area.

Succession in aquatic habitat

The succession starting in aquatic habitat like freshly formed pond is termed hydrosere.

The various seral stages are as follows:

1. Plankton stage.

The pioneers of hydrosere are the phytoplanktons, the minute microscopic autotrophic organisms like diatoms, unicellular, colonial or filamentous green algae and blue green algae (cyanobacteria).

The spores of these organisms reach the newiy formed pond through wind or animals.

They multiply rapidly.

The pond water containing large number of phytoplanktons becomes suitable habitat for zooplanktons which feed upon phytoplanktons, thus maintaining the balance.

The organic matter formed by death and decay of planktons, particulary zooplanktons mixes with clay and silt at the bottom of pond to form soft mud.

The habitat becomes suitable for the growth of next stage.

2. Submerged stage.

With the formation of soft mud at the bottom of pond, the habitat becomes suitable for the growth of anchored and submerged plants like Hydrilla, Najas, Potamogeton, etc.

They are anchored in mud and form dense growth.

More silt gets deposited around plants.

The accumulated silt along with decomposition of organic matter formed by decay of submerged plants makes the substrate fertile and raises the bottom level.

3. Floating stage.

In the pond, where the water becomes shallow, anchored plants with leaves floating (e.g., Nelumbo, Nuphar, Nymphaea), start growing and replace the plants of submerged stage which migrate to deeper waters in a pond.

These plants have subterranean stems like rhizomes and corms.

With the growth of these plants, water becomes richer in minerals and organic matter and the habitat becomes suitable for free floating plants like Lemna,Azolla, Spirodela, Wolffia, Eichhornia, etc.

The bottom level is further raised due to growth of free floating plants and accumulation of dead and decaying remains of these plants.

Water becomes much shallower at the periphery of pond.

It becomes almost a marshy habitat.

4. Reed swamp stage.

The periphery of the pond, where the water is very shallow due to interaction of free floating stage, becomes suitable for the growth of amphibious plants like Typha, Sagittaria, Phragmites, Scirpus etc.

These plants lose large quantity of water during transpiration and produce large amount of organic matter.

5. Sedge or marsh meadow stage.

The peripheral area of pond built up by plants of reed swamp stage is invaded by Cyperus, Carex (Sedge), Juncus and grasses like Themeda and Dichanthium and herbs like Campanula, Caltha, Polygonum, etc.

6. Scrub/Woodland stage.

The peripheral area of the pond occupied by marsh meadow stage is invaded by shrubs which can grow in water-logged soil and tolerate bright sunlight e.g., Cornus (Dogwood), Cephalanthus (button brush).

This further invite invasion of trees capable of bearing bright sunlight and water logging e.g., Populus (cottonwood), Alnus (Alder).

Fig. : Diagramatic representation of primary succession on pond

7. Climax Forest.

The woodland stage trees are replaced by new trees which grow to greater heights.

The nature of climax community is determined by climate, e.g., rain forest in moist tropical area and mixed coniferous or deciduous forest in temperate area.

Mixed tropical forest include trees like Quercus (oak), Ulmus (elm), Acer (maple) and gymnosperms like Abies (fir), Taxus (yew) and Picea (spruce).

Major trends during ecological succession

The major structural and functional attributes of ecological succession are:

(i) Increase in species diversity.

(ii) Increase in structural complexity.

(iii) Increase in organic matter.

(iv) Decrease in net community production.

(v) Food chain relationship becomes complex.

(vi) Niche become specific and narrower.

(vii) Stability increases.

(viii) More immobilization of nutrients (mineral nutrients fixed in biota).

(ix) Increased energy efficiency

Nutrient cycle

Nutrient Cycling 

The essential elements or inorganic substances are provided by earth and are required by organisms for their body building and metabolism, so they are called biogenetic nutrients or bio-geochemicals.

All the three subdivisions of the earth contribute these elements as such or in the form of compounds.

However, as the earth is a closed system for matter, the supply of biogenetic nutrients is finite or limited.

It is estimated that amount of living matter (made up of biogenetic nutrients) contained in all the past and present organisms is several times more than the total mass of earth.

This is possible only if biogenetic nutrients do not remain locked up in the body of organisms but are released, during excretion and after death, back into the nonliving world so that they can be used again and again.

Circulation or exchange of biogenetic nutrients between the living and the nonliving components is called cycling of matter or biogeochemical cycling (at global scale).

However, the whole of biogenetic nutrients are not always in circulation.

The nutrients occur in two states-reservoir pool and cycling pool.

(i) Reservoir pool is the reservoir of biogenetic nutrients from which they are very slowly transferred to the cycling pool, e.g., metal phosphates, nitrogen gas of the atmosphere.

(ii) Cycling Pool is the pool of biogenetic nutrients which are repeatedly exchanged between the biotic and abiotic components of the biosphere.

Environmental factors, e.g., soil, moisture, pH, temperature etc. regulate the rate of release of nutrients into the atmosphere.

The function of the reservoir is to meet with the deficit which occurs due to imbalance in the rate of influx and efflux.

Bio-geochemical cycles: These are of two types (1) Gaseous cycle (2) Sedimentary cycle

1. In gaseous cycles, materials involved in circulation are gases. Four most abundant elements present in living organisms, i.e., C(CO2), H (water vapours), O and N have predominantly gaseous cycles. The main reservoir pool for gaseous cycles is atmosphere or hydrosphere (water). Gaseous cycles are quick and are relatively perfect systems as the elements remain in circulation more or less uniformly.

2. In sedimentary cycles, biogenetic materials involved in circulation are non-gaseous. The reservoir pool for these cycles is lithosphere, e.g., phosphorus, calcium, sulphur. Recycling of sulphur involves both gaseous and sedimentary phase (mixed type). Sedimentary cycles are very slow and are less perfect systems as the elements may get locked and go out of circulation for longer duration .

Different biogeochemical cycles are discussed below:

1. The Carbon Cycle:

Carbon is constituent of almost all organic compounds of the cell such as carbohydrates, proteins, lipids, enzymes, nucleic acids, hormones, etc., and thus may be considered basis of life.

Carbon constitutes 49% of dry weight of organisms and is next only to water.

Infact, 71 % of total global carbon is found dissolved in oceans.

This oceanic reservoir regulates the amount of CO2 in atmosphere.

According to an estimate 4 × 1013 kg of carbon is fixed in the biosphere through photosynthesis annually.

Carbon in the atmosphere is present as CO2, in hydrosphere as dissolved CO2 or carbonic acid or bicarbonates and in lithosphere as fossil fuels or carbonates and graphite in rocks.

Carbon present in lithosphere is not readily available, as it becomes available to the living world only when it is either burnt or changed chemically.

There is however, regular exchange of carbon between atmosphere and hydrosphere and is readily available to the living world in the form of free or dissolved carbon dioxide.

Carbon passes into living components mainly during photosynthesis in the form of CO2 by autotrophs, but atmosphere and hydrosphere do not get depleted of their carbon content because of its return through two major processes: (i) Biological-respiration by living organisms and decomposition of organic matter. (ii) Non-biological-combustion of carbon containing fuel that releases CO2 in the atmosphere.

Human activities have significantly influenced the carbon cycle.

Rapid deforestation and massive burning of fossil fuel for energy and transport have significantly increased the rate of release of CO2 into the atmosphere.

Fig. : Carbon cycle

2. The Oxygen Cycle :

Oxygen is constituent of important biomolecules in the cell and is also required for respiration in aerobes.

It is present in natural gaseous form (molecular O2) in atmosphere and constitutes about 21% of the air. In combined form it is component of CO2, water and number of oxidised salts.

Terrestrial organisms take oxygen directly from the air and aquatic organisms take oxygen either from water present in it, in diffused or dissolved state or directly from the atmosphere to be used in respiration.

It is returned to the atmosphere in the form of CO2 and H2O.

Fig. : Oxygen cycle

Carbon dioxide and water produced during respiration are used by plants during photosynthesis and molecular oxygen is released into the atmosphere for reuse in respiration.

The oxygen in the atmosphere thus remains in state of natural dynamic equilibrium.

Oxygen is also added to the atmosphere in terms of carbon dioxide, water, sulphur dioxide, nitrogen oxides, etc., during burning or combustion of wood, coal, petroleum, natural gases.

Oxides are also produced during microbial oxidation. Chemical or biological oxidation of oxides releases molecular oxygen.

3. The Nitrogen Cycle :

Nitrogen is component of amino acids, proteins, enzymes, nucleic acids and nucleotides, which are essential structural and functional components of living organisms.

Main source of nitrogen is air, as about 3/5 of the total air is molecular nitrogen (N2).

Sufficient required amount of nitrites and nitrates are made available to the plants through a number of processes involving microorganisms either from air or from organic nitrogen locked up in dead remains of living organisms.

Fig. Nitrogen Cycle

(a) Nitrogen fixation. The conversion of molecular nitrogen into nitrogenous compounds like ammonium salts or oxides of nitrogen is termed nitrogen fixation, which is of three types:

(i) Biological nitrogen fixation

(ii) Atmospheric nitrogen fixation

(iii) Industrial nitrogen fixation

(b) Decay and decomposition of dead remains of living organisms and of their excretory products.

Plants mainly absorb nitrogen in the form of nitrate ions which are reduced to NH3 in plant cells.

NH3 is used up in synthesis of amino acids.

Animals synthesize proteins and other nitrogenous compounds from amino acids obtained through digestion of plant proteins.

During protein metabolism, a number of nitrogenous waste products like ammonia, urea and uric acid are produced which are excreted and nitrogen present in them is recycled.

A bulk of organic nitrogen remains locked up in plants and animals.

The nitrogen present in dead remains of plants and animals in the form of organic compounds is converted into amino acids by a number of saprophytic micro-organisms.

Anaerobic breakdown of proteins is termed putrefaction.

Amino acids are converted into ammonium salts by ammonifying bacteria (Bacillus sp.) and some fungi.

Some ammonium salts are directly absorbed by plants.

However, most of these are converted into nitrites by nitrifying bacteria as plants prefer absorption of these in comparison to ammonium salts.

Ammonium salts are converted into nitrites by Nitrosomonas and Nitrococcus and nitrites are converted into nitrates by Nitrobacter, Nitrocystis and Penicillium species.

Some nitrates and nitrites in the soil are lost through leaching and precipitation and become constituent of rocks, going out of cycle.

This nitrogen is slowly released for recycling in nature after an interval of millions of years during weathering of these rocks only when these get exposed.

Some nitrogen present in nitrates is released to the atmosphere as molecular nitrogen by certain denitrifying bacteria (e.g., Thiobacillus denitrificans, Pseudomonas aeruginosa) in water logged soils or other areas having anaerobic environment.

These utilize O2 of the nitrates for their requirement and thus N2 is released in molecular form and escapes in the atmosphere in gaseous state.

4. The Water Cycle:

Water is the only substance on the surface of earth that exists in all the three states of matter.

Water is essential for photosynthesis and above all O2 of the atmosphere is derived from photolysis of water during photosynthesis, which is absolutely essential for the existence of animal life.

Water or hydrological cycle is made up of two overlapping cycles-the larger global cycle, not involving living organisms and the smaller local or biological water cycle, involving exchange between environment and living organisms.

(i) Global water cycle. Water is continuously lost from oceans, lakes, rivers and moist soil through evaporation. Clouds are formed by cooling and condensation of water vapours. The clouds move along the land over long distances, get cooled and precipitate as rain, snow or hail. Rain water may directly fall into ocean and may return to the atmosphere again through evaporation.

(ii) Local or biological cycle. It involves the entry of water into living organisms and its return to the atmosphere.

5. Phosphorus cycle:

Phosphorus has a natural reservoir in rocks as phosphates.

There is no respiratory release of phosphorus into atmosphere.

Atmospheric inputs of phosphorus through rainfall are much smaller than carbon inputs and gaseous exchange of phosphorus between organisms and environment are negligible.

Phosphorus present in insoluble form in soil is converted into soluble form by chemicals secretions of microorganisms and plant roots.

Dissolved phosphate is absorbed by plants and is used to built organic compounds like phospholipids, nucleotides, nucleic acids, etc.

Phosphorus moves from plants to animals of different trophic levels in ecosystem through food chains.

The phosphorus present in plants and animals (organic form) is returned to the soil (inorganic form) through decomposition of excreta of animals and dead remains of plants and animals by micro-organisms; to be reused by plants.

The phosphorus present in bones and teeth of animals is resistent to decay and thus remain out of the cycle for a long time.

Some amount of phosphorus washed down into sea, entering the food chain of aquatic ecosystem comes back to the soil.

Phosphorus in sea water is absorbed by sea weeds and finally passes into fish and seabirds through food chains.

Phosphorus rich faeces (guano) are deposited on land by sea-birds.

However, significant amount of phosphorus is lost in deep sediments, remaining out of the cycle for a long time.

It becomes available when deep sea strata is brought to surface through mining or some natural disturbances.

Some phosphate in soil combines with metals like aluminium, calcium and iron to form their insoluble salts which are not readily available to plants for absorption.

It thus appears that phosphorus is being lost from the available pool faster than it is returned to it.

6. Sulphur cycle:

Main source of sulphur to the plants are sulphates and to some extent elemental sulphur present in soil, water and rocks.

Sulphur absorbed from soil is incorporated into amino acids and subsequently into proteins by plants.

Organic sulphur is transferred to animals of different trophic levels through food chains.

Some animals may get sulphur from water as well.

Organic sulphur present in dead remains of plants and animals and animal excreta is converted into sulphates during decomposition by bacteria and fungi under aerobic conditions which are added to the soil and water for reuse by plants.

SO2 is also released into atmosphere through combustion of fossil fuels.

It gets dissolved in water to form H2SO4 during rainfall which combines with certain metals in soil to form sulphates.

Some bacteria (e.g., Beggiatoa) and fungi convert H2S and element sulphur into sulphates which are recycled through plants in the biosphere.

Rocks may also be erroded by wind and the materials including sulphur are blown into air as dust.

These materials are transferred to soil during rain.

Sulphates are also added to the soil and air by volcanic erruptions.

Sulphates from the rocks are also brought to the soil by rain water running over them.

Some sulphates seep into the soil and others reach ponds and lakes or carried by rivers to sea.

In sea, it may get deposited in sedimentary rocks and move out of the cycle.

It may get back into cycle through food chains or geological disturbances.

Ecosystem services


The products of ecosystem processes e.g. healthy forest ecosystems purify air and water, mitigate droughts and floods, cycle nutrients, generate fertile soils, provide wildlife habitat, maintain biodiversity, pollinate crops, provide storage site for carbon and also provide aesthetic, cultural and spiritual values.

– Researchers like Robert Constanza et. al., have put an average price tag of US $ 33 trillion a year on these fundamental ecological services (i.e. nearly twice the value of a global GNP -US $ 18 trillion).

– Out of total cost, soil formation accounts for about 50%, recreation and nutrient cycling less than 10% each, climate regulation and habitat for wildlife are about 6% each.

Some Important Definitions

1. Ecosystem: Sum total of interactions between living and non-living components which is capable of independent existence.

2. Stratification: Vertical distribution of different species occupying different levels in the community.

3. Gross primary productivity: Rate of organic matter synthesized by producers per unit area per unit time.

4. Net primary productivity: Rate of organic matter built up or stored by producers in their bodies per unit time and area.

5. Secondary productivity: Rate of increase in energy containing organic matter or biomass by heterotrophs or consumers per unit time and area.

6. Community productivity: Rate of net synthesis of built up of organic matter by a community per unit time and area.

7. Ecological efficiency: Percentage of energy converted into biomass by a higher trophic level over the energy of food resources available at the lower trophic level.

8. Decomposition: Breakdown of complex organic matter into inorganic substances with the help of decomposers.

9. Humification: Process of formation of humus from detritus.

10. Mineralisation: Release of inorganic substances from organic matter during the process of decomposition.

11. Food chain: Sequence of living organisms due to interdependence in which one organism consumes another.

12. Standing state: Amount of all the inorganic substances present in an ecosystem per unit area at a given time.

13. Standing crop: Amount of living material present in different trophic levels at a given time.

14. Ecological pyramid: Graphic representation of trophic levels of a food chain.

15. Nutrient cycling: Movement of nutrient elements through the various components of an ecosystem.

16. Ecological succession: Gradual and fairly predictable changes in the species composition of a given area.