Functions of an ecosystem

Functions of an ecosystem & Ecological Succession [UPSC]

In this article, I want to walk you through the structure and functions of an ecosystem for the UPSC Exam.

Ecosystem is defined as the interaction between Biotic and Abiotic components, OR interaction of the organism with the surrounding environment.

Functions of an ecosystem

The functional attributes of the ecosystem keep the components running together. Ecosystem functions are natural processes or exchange of energy that takes place in various plant and animal communities of different biomes of the world.

It can be studied under the following three heads-

  • Energy flow
  • Nutrient cycling (biogeochemical cycles)
  • Ecological succession or ecosystem development

Energy flow

Energy is the basic force responsible for all metabolic activities. The flow of energy from producer to top consumers is called unidirectional energy flow.

The study of Trophic level interaction in an ecosystem gives an idea about the energy flow through the ecosystem.

Trophic level interaction

Trophic level interaction deals with how the members of an ecosystem are connected based on nutritional needs.

Following trophic levels can be identified in a food chain.

  • Autotrophs: They are the producers of food for all other organisms of the ecosystem. They are largely green plants and convert inorganic material in the presence of solar energy by the process of photosynthesis into chemical energy (food). The total rate at which the radiant energy is stored by the process of photosynthesis in the green plants is called Gross Primary Production (GPP). This is also known as total photosynthesis or total assimilation. From the gross primary productivity, a part is utilized by the plants for their metabolism. The remaining amount is stored by the plant as Net Primary Production (NPP) which is available to consumers.
  • Herbivores: The animals which eat the plants directly are called primary consumers or herbivores e.g. insects, birds, rodents and ruminants.
  • Carnivores: They are secondary consumers if they feed on herbivores and tertiary consumers if they use carnivores as their food. e.g. frog, dog, cat and tiger.
  • Omnivores: Animals that eat both plant and animals e.g. pig, bear and man
  • Decomposers: They take care of the dead remains of organisms at each trophic level and help in recycling the nutrients e.g. bacteria and fungi.

Energy flows through the trophic levels from producers to subsequent trophic levels. This energy always flows from lower {producer) to higher (herbivore, carnivore, etc.) trophic level. It never flows in the reverse direction that is from carnivores to herbivores to producers.

There is a loss of some energy in the form of unusable heat at each trophic level so that energy level decreases from the first trophic level upwards.

The trophic level interaction involves three concepts namely:

  1. Food Chain
  2. Food Web
  3. Ecological Pyramid

Food Chain

Transfer of food energy from green plants (producers) through a series of organisms with repeated eating and being eaten is called a food chain. e.g.

Grasses → Grasshopper → Frog → Snake → Hawk/Eagle

Each step in the food chain is called a trophic level. In the above example grasses are 1st, and the eagle represents the 5th trophic level.

food chain

Types of Food Chains

In nature, two main types of food chains have been distinguished:

  1. Grazing food chain
  2. Detritus food chain
Grazing food chain

The consumers who start the food chain, utilizing the plant or plant part as their food, constitute the grazing food chain. This food chain begins from green plants at the base and the primary consumer is an herbivore.

For example, in the terrestrial ecosystem, the grass is eaten up by caterpillar, which is eaten by lizard and lizard is eaten by a snake.

In Aquatic ecosystem phytoplankton’s (primary producers) is eaten by zooplanktons which is eaten by fishes and fishes are eaten by pelicans.

Detritus food chain

The food chain starts from dead organic ‘matter of decaying animals and plant bodies to the microorganism and then to detritus feeding organisms called detrivores or decomposers and to other predators.

The distinction between these two food chains is the source of energy for the first level consumers. In the grazing food chain, the primary source of energy is living plant biomass while in the detritus food chain the source of energy is dead organic matter or detritus.

Grazing food chain vs Detritus food chain

Food Web

Trophic levels in an ecosystem are not linear rather they are interconnected and make a food web. Thus, the food web is a network interconnected food chains existing in an ecosystem. One animal may be a member of several different food chains. Food webs are more realistic models of energy flow through an ecosystem.

food web

The flow of energy in an ecosystem is always unidirectional. The quantity of energy flowing through the successive trophic levels decreases as shown by the reduced sizes of boxes in figure (see below diagram). At every step in a food chain or web the energy received by the organism is used to sustain itself and the leftover is passed on to the next trophic level.

pyramid of energy

Ecological Pyramid

Ecological pyramids are the graphic representations of trophic levels in an ecosystem. They are pyramidal and they are of three types: The producers make the base of the pyramid and the subsequent tiers of the pyramid represent herbivore, carnivore and top carnivore levels.

The pyramid consists of several horizontal bars depicting specific trophic levels which are arranged sequentially from primary producer level through herbivore, carnivore onwards. The length of each bar represents the total number of individuals at each trophic level in an ecosystem.

The number, biomass and energy of organisms gradually decrease with each step from the producer level to the consumer level and the diagrammatic representation assumes a pyramid shape.

The ecological pyramids are of three categories-

  1. Pyramid of numbers,
  2. Pyramid of biomass, and
  3. Pyramid of energy or productivity

Pyramid of Number:

This represents the number of organisms at each trophic level. For example, in grassland the number of grasses is more than the number of herbivores that feed on them and the number of herbivores is more than the number of carnivores. In some instances, the pyramid of number may be inverted, i.e herbivores are more than primary producers as you may observe that many caterpillars and insects feed on a single tree.

pyramid-of-numbers upright

Pyramid of Biomass:

This represents the total standing crop biomass at each trophic level. Standing crop biomass is the amount of the living matter at any given time. It is expressed as gm/unit area or kilo cal/unit area. In most of the terrestrial ecosystems the pyramid of biomass is upright. However, in the case of aquatic ecosystems the pyramid of biomass may be inverted e.g. in a pond phytoplankton are the main producers, they have very short life cycles and a rapid turnover rate (i.e. they are rapidly replaced by new plants). Therefore, their total biomass at any given time is less than the biomass of herbivores supported by them.

pyramid of biomass

Pyramid of Energy:

This pyramid represents the total amount of energy at each trophic level. Energy is expressed in terms of rate such as kcal/unit area /unit time or cal/unit area/unit time. E.g. in a lake autotroph energy is 20810 kcal/m/year. Energy pyramids are never inverted.

pyramid of energy
Ecological Efficiency

It is clear from the trophic structure of an ecosystem that the amount of energy decreases at each subsequent trophic level. This is due to two reasons:

  1. At each trophic a part of the available energy is lost in respiration or used up in metabolism.
  2. A part of the energy is lost at each transformation, i.e. when it moves from lower to higher trophic level as heat.

It is the ratio between the amount of energy acquired from the lower trophic level and the amount of energy transferred from the higher trophic level is called ecological efficiency.

Lindman in 1942 defined these ecological efficiencies for the 1st time and proposed a 10% rule e.g. if autotrophs produce 100 cal, herbivores will be able to store 10 cal. and carnivores 1 cal. However, there may be slight variations in different ecosystems and ecological efficiencies may range from 5 to 35%.

When energy moves between trophic levels, 10% of the energy is made available for the next level. (The exception is the transition from the sun to producers, in which case only 1% of the energy is retained.)

When a consumer eats a plant, it gains energy from the plant. That energy is used for growth, reproduction, and other biological processes. Some of that energy is also lost through heat loss. Thus, when a predator eats that consumer, all of the energy the consumer gained from the plant is not available to the predator: it has been used and lost.

As we move up an energy pyramid or a trophic level, we can see that less and less of the original energy from the sun is available. Roughly ten percent of the previous trophic level’s energy is available to the level immediately higher up. This is called the 10% Rule.

law of trophic efficiency
Significance of studying food chains
  • It helps in understanding the feeding relations and interactions among different organisms of an ecosystem.
  • It explains the flow of energy and circulation of materials in ecosystems.
  • It helps in understanding the concept of bio-magnification in ecosystems.

Trophic level and Pollutants

Pollutants especially non-degradable ones move through the various trophic levels in an ecosystem. non-degradable pollutants mean materials, which cannot be metabolized by the living organisms.

Example: chlorinated hydrocarbons.

We are concerned about these phenomena because together they enable even small concentrations of chemicals in the environment to find their way into organisms in high enough dosages to cause problems.

Movement of these pollutants involves two main processes:

  • Bio-accumulation
  • Bio-magnification
bioaccumulation and biomagnification


Bioaccumulation refers to the increase in concentration of a pollutant in an organism.

It usually occurs when an organism ingests a particular substance at a faster rate than it can metabolize or excrete.

While most organisms are contaminated as a result of consuming contaminated food, others, aquatic species, in particular, absorb contaminants directly from the water, in what is known as bioconcentration.

Example- phytoplankton and other microscopic organisms absorbing pollutants like DDT, lead, and mercury, and storing it in their tissues.

Bioconcentration is the intake and retention of a substance in an organism entirely by respiration from water in aquatic ecosystems or from the air in terrestrial ones.


Biomagnification is also called Bioamplification. It is simply the increase in the concentration of a substance in a food chain, not an organism. Persistent organic pollutants (POPs) are compounds that biomagnify. Persistent organic pollutants (POPs) are chemical substances that persist in the environment. These substances bio-accumulate through the food web and pose risk not only to humans but also to other living organisms because of their adverse
effects. These pollutants consist of pesticides (such as DDT), industrial chemicals (such as polychlorinated biphenyls, PCBs) and unintentional by-products of industrial processes (such as dioxins and furans).

An apt example of biomagnification will be when small fish eat contaminated microscopic organisms, and big fish eat the small fish. So, first, the pollutants are transferred from microscopic organisms to the small fish that feed on them, and then to the big fish that feed on these small fish. As the burden of pollutants is passed from one organism to another, it gets amplified, and thus, biomagnification is also known as bioamplification. In essence, bio-magnification is similar to bioaccumulation but is descriptive of higher-level biological processes, not individual.


For bio-magnification to occur

  • The pollutant must be long-lived
  • Mobile, soluble in fats
  • Biologically active

If a pollutant is short-lived, it will be broken, down before it can become dangerous. If it is not mobile, it will stay in one place and is unlikely to be taken up by organisms.

If the pollutant is soluble in water, it will be excreted by the organism. Pollutants that dissolve in fats, however, may be retained for a long time.

Biotic Interaction

The biological community of an area or ecosystem is a complex network of interactions. The interaction that occurs among different individuals of the same species is called intraspecific interaction while the interaction among individuals of different species in a community is termed as interspecific interaction.

Interactions between organisms belonging to the same trophic level often involve competition. Individuals of the population may compete for food, space and mates. For example, if a mouse has been eaten by a cat, other cats competing for this resource would have one less mouse to prey on. The snake another predator of the mice would also have fewer mice to eat during the night if the cat has succeeded. Direct competition though, between the cat and snake is not much as they prey at different times. They also eat a variety of different foods. So, competition may be intraspecific as well as interspecific.

Interspecific relationships may be direct and close as between a lion and deer or indirect and remote as between an elephant and a beetle. This is because interactions between two species need not be through direct contact. Due to the connected nature of ecosystems, species may affect each other through intermediaries such as shared resources or common enemies. Specific terms are applied to interspecific interactions depending upon whether
the interaction is beneficial, harmful or neutral to individuals of the species.

Biotic Interaction

S. No.Type

Species 1

Species 2

















(+) Benefited(-) Harmed
(0) Neither benefited nor harmed

Types of Biotic Interaction Interactions

From the table you can see that in certain types of interspecific associations at least one of the species is harmed by the other. Such associations are termed as negative, in a case where both the associated species are benefited is a positive association and when the associated species are neither benefited nor harmed represents a neutral interaction.

  • Amensalism: This is a negative association between two species in which one species harms or restricts the other species without itself being adversely affected or harmed by the presence of the other species. Organisms that secrete antibiotics and the species that get inhibited by the antibiotics are examples of amensalism.
    For example, the bread mould fungi Penicillium produces penicillin an antibiotic substance that inhibits the growth of a variety of bacteria. Penicillium benefits apparently by having greater availability of food when the competition because of the bacteria is removed.
  • Predation: In this type of interaction predator captures, kills and eats an animal of another species called the prey. The predator naturally benefits from this relationship. while the prey is harmed. Predators like leopards, tigers and cheetahs use speed, teeth and claws to hunt and kill their prey.
  • Parasitism: In this type of interaction, one species is harmed and the other benefits. Parasitism involves parasite usually a small size organism living in or on another living species called the host from which the parasite gets its nourishment and often shelter. The parasite is benefited, and the host is harmed.
    Many organisms like animals, bacteria and viruses are parasites of plants and animals. Plants like dodder plants (Cuscuta) and mistletoe (Loranthus) are parasites that live on flowering plants. Tapeworm, roundworm, malarial parasite, many bacteria, fungi, and viruses are common parasites of humans.
  • Competition: This is an interaction between two populations in which both species are harmed to some extent. Competition occurs when two populations or species, both need a vital resource that is in short supply. The vital resource could be food, water, shelter, nesting site, mates or space. Such competition can be:
    • interspecific competition-occurring between individuals of two different species occurring in a habitat and intraspecific competition-occurs between individuals of the same species. Intraspecific competition occurs between members of the same species and so it is very intense.
  • Commensalism: In this relationship one of the species benefits while the other is neither harmed nor benefited. Some species obtain the benefit of shelter or transport from another species. For example, suckerfish, remora often attaches to a shark utilizing its sucker which is present on the top side of its head. This helps the remora get protection, a free ride as well as a meal from the leftover of the shark’s meal. The shark does not however get any benefit, nor is it adversely affected by this association.
    Another example of commensalisms is the relationship between trees and epiphytic plants. Epiphytes live on the surface of other plants like ferns, mosses and orchids and use the surface of trees for support and for obtaining sunlight and moisture. The tree gets no benefit from this relationship nor are they harmed.
  • Mutualism: This is a close association between two species in which both the species benefit. For example, of protocooperation of the sea anemone, a cnidarian gets attached to the shell of hermit crabs for benefit of transport and obtaining new food while the anemone provides camouflage and protection utilizing its stinging cells to the hermit crab.
    However, some mutualisms are so intimate that the interacting species can no longer live without each other as they depend totally on each other to survive. Such close associations are called symbiosis. An example of such close mutualistic association is that of termite and their intestinal flagellates. Termites can eat wood but have no enzymes to digest it.

    However, their intestine contains certain flagellate protists (protozoans) that have the necessary enzymes to digest the cellulose of the wood eaten by termites and convert them into sugar. The flagellates use some of this sugar for their metabolism while enough is left for the termite. Both termite and flagellates cannot survive without each other. Another familiar example of symbiosis is seen in the pollination of flowers where flowering plants are cross-pollinated by the bees which benefit by getting nectar from the plants and both
    cannot survive without the other.
  • Neutralism: Neutralism describes the relationship between two species which do interact but do not affect each other. It is to describe interactions where the fitness of one species does not affect what so ever on that of others. True neutralism is extremely unlikely and impossible to prove. When dealing with the complex networks of interactions presented by ecosystems, one cannot assert positively that there is no competition between or benefit to either species. Since true neutralism is rare or nonexistent, its usage is often extended to situations where interaction is merely insignificant or negligible.

Allelopathy refers to the chemical inhibition of one species by another. The “inhibitory” chemical is released into the environment where it affects the development and growth of neighboring plants.

Allelopathic chemicals can be present in any part of the plant. They can be found in leaves, flowers, roots, fruits, or stems. They can also be found in the surrounding soil. Target species are affected by these toxins in many different ways. The toxic chemicals may inhibit shoot/root growth, they may inhibit nutrient uptake, or they may attack a naturally occurring symbiotic relationship thereby destroying the plant’s usable source of a nutrient.

Not all plants have allelopathic tendencies. Some, though they exhibit these tendencies, maybe displaying aggressive competition of a non-chemical form. Much of the controversy surrounding allelopathy is in trying to distinguish the type of competition being displayed. In general, if it is of a chemical nature, then the plant is considered an allelopathic.


In ecosystems flow of energy is linear but that of nutrients is cyclical. This is because energy flows downhill i.e. it is utilized or lost as heat as it flows forward the nutrients on the other handcycle from dead remains of organisms released back into the soil by detrivores which are absorbed again i.e. nutrient absorbed from soil by the root of green plants are passed on to herbivores and then carnivores.

The nutrients locked in the dead remains of organisms and released back into the soil by detrivores and decomposers. This recycling of the nutrients is called biogeochemical or nutrient cycle (Bio = living, geo = rock chemical = element). There are more than 40 elements required for the various life processes by plants and animals. The entire earth or biosphere is a closed system i.e. nutrients are neither imported nor exported from the

There are two important components of a biogeochemical cycle

  1. Reservoir pool – atmosphere or rock, which stores large amounts of nutrients.
  2. Cycling pool or compartments of cycle-They are relatively short storages of carbon in the form of plants and animals.

Types of Nutrient Cycle

Based on the replacement period a nutrient cycle is referred to as a Perfect or Imperfect cycle.

A perfect nutrient cycle is one in which nutrients are replaced as fast as they are utilized. Most gaseous cycles are generally considered as perfect cycles.

In contrast sedimentary cycles are considered relatively imperfect, as some nutrients are lost from the cycle and get locked into sediments and so become unavailable for immediate cycling.

Based on the nature of the reservoir, there, are two types of cycles namely the Gaseous and Sedimentary cycle.

  • Gaseous Cycle – where the reservoir is the atmosphere or the hydrosphere.
  • Sedimentary Cycle – where the reservoir is the earth’s crust.

Carbon Cycle

carbon cycle

The source of all carbon is carbon dioxide present in the atmosphere. It is highly soluble in water; therefore, oceans also contain large quantities of dissolved carbon dioxide.

The global carbon cycle consists of the following steps


Green plants in the presence of sunlight utilize CO2 in the process of photosynthesis and convert the inorganic carbon into organic matter (food) and release oxygen. Apart from the food made through photosynthesis is used by plants for their metabolism and the rest is stored as their biomass which is available to various herbivores, heterotrophs, including human beings and microorganisms as food.

Annually 4-9 x1013 kg of CO2 is fixed by green plants of the entire biosphere. Forests acts as reservoirs of CO2 as carbon fixed by the trees remain stored in them for long due to their long-life cycles. A very large amount of CO2 is released through forest fires.


Respiration is carried out by all living organisms. It is a metabolic process where food is oxidized to liberate energy, CO2 and water. The energy released from respiration is used for carrying out life processes by living organisms (plants, animals, decomposers etc.).

Thus, CO2 is released into the atmosphere through this process.


All the food assimilated by animals or synthesized by plants is not metabolized by them completely. A major part is retained by them as their biomass which becomes available to decomposers on their death. The dead organic matter is decomposed by microorganisms and CO2 is released into the atmosphere by decomposers.


Burning biomass releases carbon dioxide into the atmosphere.

Impact of human activities

The global carbon cycle has been increasingly disturbed by human activities particularly since the beginning of the industrial era. Large scale deforestation and ever-growing consumption of fossil fuels by growing numbers of industries, power plants and automobiles are primarily responsible for increasing the emission of carbon dioxide.

Carbon dioxide has been continuously increasing in the atmosphere due to human activities such as industrialization, urbanization and increasing use and number of automobiles. This is leading to an increasing concentration of CO2 in the atmosphere, which is a major cause of global warming.

Nitrogen Cycle

Nitrogen is an essential component of protein and required by all living organisms including human beings.

Nitrogen cycle

Our atmosphere contains nearly 79% of nitrogen but it cannot be used directly by the majority of living organisms. Broadly like carbon dioxide, nitrogen also cycles from the gaseous phase to the solid phase then back to the gaseous phase through the activity of a wide variety of organisms. Cycling of nitrogen is vitally important for all living organisms. There are five main processes which essential for the nitrogen cycle are elaborated below.

(a) Nitrogen fixation: This process involves the conversion of gaseous nitrogen into Ammonia, a form in which it can be used by plants. Atmospheric nitrogen can be fixed by the following three methods: –

  • Atmospheric fixation: Lightening, combustion and volcanic activity help in the fixation of nitrogen.
  • Industrial fixation: At high temperature and high pressure, molecular nitrogen is broken into atomic nitrogen which then combines with hydrogen to form ammonia.
  • Bacterial fixation: There are two types of bacteria
  • Symbiotic bacteria e.g. Rhizobium in the root nodules of leguminous plants.
  • Free-living or symbiotic e.g. 1. Nostoc 2. Azobacter 3. Cyanobacteria can combine atmospheric or dissolved nitrogen with hydrogen to form ammonia.

(b) Nitrification: In nitrification, a host of soil bacteria participate in turning ammonia into nitrate – the form of nitrogen that can be used by plants and animals. This requires two steps, performed by two different types of bacteria.

First, soil bacteria such as Nitrosomonas or Nitrococcus convert ammonia into nitrogen dioxide. Then another type of soil bacterium, called Nitrobacter, adds a third oxygen atom to create nitrate.

These bacteria don’t convert ammonia for plants and animals out of the goodness of their hearts. Rather, they are “chemotrophs” who obtain their energy from volatile chemicals. By metabolizing nitrogen along with oxygen, they obtain energy to power their own life processes.

The process can be thought of as a rough (and much less efficient) analog to the cellular respiration performed by animals, which extract energy from carbon-hydrogen bonds and use oxygen as the electron acceptor, yielding carbon dioxide at the end of the process.

Nitrates, the end product of this vital string of bacterial reactions – can be made artificially, and are the main ingredient in many soil fertilizers. You may actually hear such fertilizer referred to as “nitrate fertilizer.” By pumping the soil full of nitrates, such fertilizers allow plants to grow large quickly, without being dependent on the rate at which nitrogen-fixing bacteria do their jobs.

Interestingly, high-energy environments such as lightning strikes and volcanic eruptions, can convert nitrogen gas directly into nitrates but this doesn’t happen nearly enough to keep modern ecosystems healthy on its own.

(c) Assimilation: In assimilation, plants finally consume the nitrates made by soil bacteria and use them to make nucleotides, amino acids, and other vital chemicals for life.

Plants take up nitrates through their roots and use them to make amino acids and nucleic acids from scratch. Animals that eat the plants are then able to use these amino acids and nucleic acids in their own cells.

(d) Ammonification: Because there is so much nitrogen in the atmosphere, it may seem that the process could stop there but the atmosphere’s supply is not infinite, and keeping nitrogen inside plant and animal cells would eventually result in big changes to our soil, our atmosphere, and our ecosystems.

Fortunately, that’s not what happens. In a robust ecosystem like ours, anywhere that energy has been put into creating an organic chemical, there is another form of life that is waiting to extract that energy by breaking those chemical bonds.

In this case, a process called “ammonification” is performed by soil bacteria that decompose dead plants and animals. In the process, these decomposers break down amino acids and nucleic acids into nitrates and ammonia, and release those compounds back into the soil.

(e) Denitrification: In the final step of the nitrogen cycle, anaerobic bacteria can turn nitrates back into nitrogen gas.

This process, like the process of turning nitrogen gas into ammonia, must happen in the absence of oxygen. As such it often occurs deep in the soil, or in wet environments where mud and muck keep oxygen at bay.

In some ecosystems, this denitrification is a valuable process to prevent nitrogen compounds in the soil from building up to dangerous levels. Denitrification is the reverse of nitrogen fixation.

Water Cycle

Water is essential for life. No organism can survive without water. Precipitation (rain, snow, slush dew etc.) is the only source of water on the earth. Water received from the atmosphere on the earth returns back to the atmosphere as water vapor resulting from direct evaporation and through evapotranspiration the continuous movement of water in the biosphere is called water cycle (hydrological cycle). You have already studied that earth
is a watery planet of the solar system, about 2/3rd of the earth surface is covered with water. However, a very small fraction of this is available to animals and plants.

Water Cycle

Water is not evenly distributed throughout the surface of the earth. Almost 95 % of the total water on the earth is chemically bound to rocks and does not cycle. Out of the remaining 5%, nearly 97.3% is in the oceans and 2.1% exists as polar ice caps. Thus only 0.6% is present as freshwater in the form of atmospheric water vapors, ground and soil water.

The driving forces for the water cycle are 1) solar radiation 2) gravity. Evaporation and precipitation are the two main processes involved in the water cycle. These two processes alternate with each other Water from oceans, lakes, ponds, rivers and streams evaporates by the sun’s heat energy. Plants also transpire huge amounts of water. Water remains in the vapor state in air and forms clouds that drift with the wind. Clouds meet with the cold air in the mountainous regions above the forests and condense to form rain precipitate which comes down due to gravity.

On an average 84% of the water is lost from the surface of the through oceans by evaporation. While 77% is gained by it from precipitation. Water runoff from lands through rivers to oceans makes up 7% which balances the evaporation deficit of the ocean. On land, evaporation is 16% and precipitation is 23%.

Sedimentary Cycle

Magnesium, phosphorus and calcium circulate by means of the sedimentary cycle. The sedimentary cycle normally does, not cycle through the atmosphere but follows a basic pattern of flow through erosion, sedimentation, mountain building, volcanic activity and biological transport through the excreta of marine birds.

Phosphorus Cycle

Phosphorus Cycle

Phosphorus is an important element for all forms of life. As phosphate (PO4), it makes up an important part of the structural framework that holds DNA and RNA together. Phosphates are also a critical component of ATP, the cellular energy carrier as they serve as an energy release for organisms to use in building proteins or contracting muscles. Like calcium, phosphorus is important to vertebrates; in the human body, 80% of phosphorous is found in teeth and bones.

Phosphorus plays a central role in aquatic ecosystems and water quality. Phosphorus occurs in large amounts as a mineral in phosphate rocks and enters the cycle from erosion and mining activities. This is the nutrient considered to be the main cause of excessive growth (eutrophication) of rooted and free-floating microscopic plants in lakes.

While obviously beneficial for many biological processes, in surface waters an excessive concentration of phosphorus is considered a pollutant. Phosphate stimulates the growth of plankton and plants, favoring weedy species over others. Excess growth of these plants tends to consume large amounts of dissolved oxygen, potentially suffocating fish and other marine animals, while also blocking available sunlight to bottom-dwelling species. This is known as eutrophication.

The main storage for phosphorus is in the earth’s crust. On land phosphorus is usually found in the form of phosphates. By the process of weathering and erosion phosphates enter rivers and streams that transport them to the ocean.

In the ocean once, the phosphorus accumulates on continental shelves in the form of insoluble deposits. After millions of years, the crustal plates rise from the seafloor and expose the phosphates on land. After more time, weathering will release them from rock and the cycle’s geochemical phase begins again.

Humans can alter the phosphorus cycle in many ways, including in the cutting of tropical rain forests and through the use of agricultural fertilizers. Rainforest ecosystems are supported primarily through the recycling of nutrients, with little or no nutrient reserves in their soils. As the forest is cut and/or burned, nutrients originally stored in plants and rocks are quickly washed away by heavy rains, causing the land to become unproductive.

Agricultural runoff provides much of the phosphate found in waterways. Crops often cannot absorb all of the fertilizer in the soils, causing excess fertilizer runoff and increasing phosphate levels in rivers and other bodies of water. At one time the use of laundry detergents contributed to significant concentrations of phosphates in rivers, lakes, and streams, but most detergents no longer include phosphorus as an ingredient.

Sulphur Cycle

sulfur cycle

Sulfur is one of the components that make up proteins and vitamins. Proteins consist of amino acids that contain sulfur atoms. Sulfur is important for the functioning of proteins and enzymes in plants, and in animals that depend upon plants for sulfur. Plants absorb sulfur when it is dissolved in water. Animals consume these plants, so that they take up enough sulfur to maintain their health.

Most of the earth’s sulfur is tied up in rocks and salts or buried deep in the ocean in oceanic sediments. Sulphur can also be found in the atmosphere. It enters the atmosphere through both natural and human sources. Natural recourses can be for instance volcanic eruptions, bacterial processes, evaporation from water, or decaying organisms. When Sulphur enters the atmosphere through human activity, this is mainly a consequence of
industrial processes where sulphur dioxide (SO2) and hydrogen sulphide (H2S) gases are emitted on a wide scale.

When sulphur dioxide enters the atmosphere, it will react with oxygen to produce Sulphur trioxide gas (SO3), or with other chemicals in the atmosphere, to produce Sulphur salts. Sulphur dioxide may also react with water to produce Sulphur acid (H2SO4). Sulphur acid may also be produced from dimethyl-sulfide, which is emitted to the atmosphere by plankton species.

All these particles will settle back onto the earth or react with rain and fall back onto earth as acid deposition. The particles will then be absorbed by plants again and are released back into the atmosphere so that the Sulphur cycle will start over again.

Homeostasis of Ecosystem

Ecosystems are capable of maintaining their state of equilibrium. They can regulate their species structure and functional processes. This capacity of the ecosystem of self-regulation is known as homeostasis.

In ecology the term applies to the tendency for a biological system to resist changes. For example, in a pond ecosystem, if the population of zooplankton increased, they would consume a large number of the phytoplankton and as a result soon zooplankton would be a short supply of food for them. As the number of zooplankton is reduced because of starvation, the phytoplankton population starts increasing. After some time, the population size of zooplankton also increases, and this process continues at all the trophic levels of the food chain.

Note that in a homeostatic system, a negative feedback mechanism is responsible for maintaining stability in an ecosystem.


However, the homeostatic capacity of ecosystems is not unlimited as well as not everything in an ecosystem is always well regulated. You will learn about the scope and limitations of homeostatic mechanisms when you gain more knowledge about ecosystems. Humans are the greatest source of disturbance to ecosystems.


Biotic communities are dynamic in nature and change over a period of time. The process by which communities of plant and animal species in an area are replaced or changed into another over a period of time is known as ecological succession. Both the biotic and abiotic components are involved in this change. This change is brought about both by the activities of the communities as well as by the physical environment in that particular area.

The physical environment often influences the nature, direction, rate and optimal limit of changes. During succession both the plant and animal communities change. There are two types of successions

  • Primary succession
  • Secondary succession

Primary Succession

Primary succession takes place an over a bare or unoccupied area such as rocks outcrop, newly formed deltas, and sand dunes, emerging volcano islands and lava flows as well as glacial moraines (muddy area exposed by a retreating glacier) where no community has existed previously. The plants that invade the first bare land, where the soil is initially absent are called pioneer species. The assemblage of pioneer plants is collectively called a pioneer community. A pioneer species generally show a high growth rate but a short life span.

Primary Succession

Primary succession is much more difficult to observe than secondary succession because there are relatively very few places on earth that do not already have communities of organisms. Furthermore, primary succession takes a very long time as compared to secondary succession as the soil is to be formed during primary succession while secondary succession starts in an area where the soil is already present.

The community that initially inhabits a bare area is called a pioneer community. The pioneer community after some time gets replaced by another community with different species combinations. This second community gets replaced by a third community. This process continues sequence-wise in which a community replaced previous by another community.

Each transitional (temporary) community that is formed and replaced during succession is called a stage in succession or a seral community. The terminal (final) stage of succession forms the community which is called a climax community. A climax community is stable, mature, more complex and long-lasting. The entire sequence of communities in a given area, succeeding each other, during the course of succession is termed sere.

The animals of such a community also exhibit succession which to a great extent is determined by plant succession. However animals of such successional stages are also influenced by the types of animals that can migrate from neighboring communities. A climax community as long as it is undisturbed, remains relatively stable in dynamic equilibrium with the prevailing climate and habitat factors.

Succession that occurs on land where moisture content is low, e.g. on bare rock is known as xerarch. Succession that takes place in a water body, like ponds or lakes is called hydrarch.

Secondary Succession

Secondary succession is the development of a community which forms after the existing natural vegetation that constitutes a community is removed, disturbed or destroyed by a natural event like hurricane or forest fire or by human-related events like tilling or harvesting the land.

Secondary Succession

A secondary succession is relatively fast as, the soil has the necessary nutrients as well as a large pool of seeds and other dormant stages of organisms.

Autogenic Succession:

The succession in which organisms themselves bring change in the environment during succession is called autogenic succession. The organisms cause a change in the soil. These changes include the accumulation of organic matter in the form of humus or litter alteration of soil nutrients and change in pH of soil. The structure of the plants themselves can also change the community.

For example, larger species like trees produce shade on to the developing forest floor. It destroys the light-requiring species. Shade-tolerant species-establish in the area.

Allogenic Succession:

The succession in which external environmental factors cause a change in the environment during succession is called allogenic succession. Soil erosion, leaching or the deposition of silt can change the soil. Similarly, clays can alter the nutrient content and water relationships in the ecosystems. Animals also play an important role in allogenic changes. They act as pollinators, seed dispersers and herbivores. They can also increase the nutrient content of the soil in certain areas.

Autotrophic Succession:

Succession that begins in a predominantly inorganic environment and is characterized by early and continued dominance by autotrophic organisms is called the autotrophic succession. The primary and secondary successions both come under the autotrophic succession and are widespread in nature.

Heterotrophic Succession:

This succession is characterized by the early dominance of heterotrophy. It occurs in the special and rare case where the environment is predominantly organic as, for example, in a stream heavily polluted with sewage. Other examples of heterotrophic succession include the successions involved in rotting of dead trees, reduction of animal dung, invasion and destruction of plant galls. In these cases, green plants are usually not involved. Instead, the organisms in heterotrophic successions use up the energy of the dead organic matter.

In this type of succession, energy is maximum at the beginning and gradually declines as succession proceeds until the additional organic matter is added from outside. In contrast, energy flow does not necessarily decline in the autotrophic type of succession but is usually maintained or increased. In heterotrophic succession, a series of different organisms (animals, fungi, bacteria, and other forms) are involved and replace each other but the endpoint is the utilization of all the energy and dispersion of the community.

Sambhar Lake

  • The Sambhar Salt Lake is India’s largest inland saltwater body located near Jaipur in Rajasthan.
  • The lake is surrounded on all sides by the Aravali hills. 
  • It is the source of most of Rajasthan’s salt production.
  • Sambhar has been designated as a Ramsar site (recognized wetland of international importance) because the wetland is a key wintering area for tens of thousands of flamingos and other birds that migrate from northern Asia.
Sambhar lake upsc

The Indian Veterinary Research Institute (IVRI), Uttar Pradesh has confirmed avian botulism as the reason for mass mortality of birds, at Sambhar Lake in Rajasthan.

Avian Botulism
  • Avian botulism is a neuro-muscular illness caused by Botulinum (natural toxin) that is produced by a bacteria — Clostridium botulinum.
  • The bacteria are commonly found in the soil, rivers, and seawater. It affects both humans and animals.
  • The bacteria also need anaerobic (absence of oxygen) conditions and do not grow in acidic conditions.
  • It affects the nervous system of birds, leading to paralysis in their legs and wings.
  • The outbreaks of avian botulism tend to occur when average temperatures are above 21 degrees celsius, and during droughts.

Q. Which of the following are the examples of commensalism?

  1. An orchid growing as an epiphyte on a mango branch
  2. barnacles growing on the back of a whale
  3. the cattle egret and grazing cattle
  4. sea anemone and the clown fish

Select the correct answer using the code given below:

A. 1 ,2 and 4 only
B. 2 and 3 only
C. 1, 3 and 4 only
D. 1, 2, 3 and 4


  • Commensalism is the interaction in which one species benefits and the other is neither harmed nor benefited.
  • An orchid growing as an epiphyte on a mango branch, and barnacles growing on the back of a whale benefit while neither the mango tree nor the whale derives any apparent benefit, the cattle egret and grazing cattle in close association, are the classic example of commensalism.
  • The egrets always forage close to where the cattle are grazing because the cattle, as they move, stir up and flush out from the vegetation insects that otherwise might be difficult for the egrets to find and catch.
  • Another example of commensalism is the interaction between the sea anemone that has stinging tentacles and the clownfish that lives among them.
  • The fish gets protection from predators which stay away from the stinging tentacles. The anemone does not appear to derive any benefit by hosting the clownfish.

Hence, option (d) is correct.

Q. Which of the following are the examples of Mutualism?

  1. Lichens
  2. Mycorrhizae
  3. fig trees and wasp

Select the correct answer using the code given below:

A. 1 and 2 only
B. 2 and 3 only
C. 1 and 3 only
D. 1, 2, and 3


  • Mutualism is the interaction that confers benefits on both the interacting species.
  • Lichens represent an intimate mutualistic relationship between a fungus and photosynthesizing algae or cyanobacteria.
  • Similarly, the mycorrhizae are associations between fungi and the roots of higher plants. The fungi help the plant in the absorption of essential nutrients from the soil while the plant in turn provides the fungi with energy-yielding carbohydrates.
  • The most spectacular and evolutionarily fascinating examples of mutualism are found in plant-animal relationships. Plants need the help of animals for pollinating their flowers and dispersing their seeds. Plants offer rewards or fees in the form of pollen and nectar for pollinators and juicy and nutritious fruits for seed dispersers.
  • In many species of fig trees, there is a tight one-to-one relationship with the pollinator species of wasp. It means that a given fig species can be pollinated only by its ‘partner’ wasp species and no other species. 
  • Orchids show a bewildering diversity of floral patterns many of which have evolved to attract the right pollinator insect (bees and bumblebees) and ensure guaranteed pollination by it Not all orchids offer rewards. The Mediterranean orchid Ophrys employs ‘sexual deceit’ to get pollination done by a species of bee. One petal of its flower bears an uncanny resemblance to the female of the bee in size, colour, and markings. The male bee is attracted to what it perceives as a female, ‘pseudocopulates’ with the flower, and during that process is dusted with pollen from the flower. When this same bee ‘pseudocopulates’ with another flower, it transfers pollen to it and thus, pollinates the flower.

Hence, option (d) is correct.

Reference: NCERT & Shankar IAS Book (Environment)

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