(2023) Environment Revision Part 3 : Environment ncert ,shankar ias ,pmf ias notes

 Ecological Succession

  • 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.
  • Succession is a universal process of directional change in vegetation, on an ecological time scale.
  • Succession occurs due to large scale changes or destruction (natural or manmade).
  • The process involves a progressive series of changes with one community replacing another until a stable, mature, climax community develops.

  • The first plant to colonize an area is
  •  called the pioneer community.
  • The final stage of succession is called the climax community.
  • A climax community is stable, mature, more complex and long-lasting.
  • The stage leading to the climax community is called successional stages or seres.
  • Each transitional community that is formed and replaced during succession is called a stage in succession or a seral community.
  • Succession is characterized by the following: increased productivity, the shift of nutrients from the reservoirs, increased diversity of organisms, and a gradual increase in the complexity of food webs.
  • Succession would occur faster in area existing in the middle of the large continent. This is because here seeds of plants belonging to the different seres would reach much faster.

Primary Succession

  • Primary succession takes place an over where no community has existed previously.
  • Such areas include rock outcrops, newly formed deltas and sand dunes, emerging volcano islands and lava flows, glacial moraines (muddy area exposed by a retreating glacier), etc.
  • In primary succession on a terrestrial site, the new site is first colonised by a few hardy pioneer species that are often microbes, lichens and mosses.
  • The pioneers over a few generations alter the habitat conditions by their growth and development.

  • Lichen are plant-like organisms that consist of a symbiotic association of algae (usually green) or cyanobacteria and fungi.
  • Fungi provide shelter, water and minerals to the algae and, in return, the alga provides food.
  • The pioneers through their death any decay leave patches of organic matter in which small animals can live.
  • The organic matter produced by these pioneer species produce organic acids during decomposition that dissolve and etch the substratum releasing nutrients to the substratum.
  • Organic debris accumulates in pockets and crevices, providing soil in which seeds can become lodged and grow.
  • The new conditions may be conducive to the establishment of additional organisms that may subsequently arrive at the site.
  • As the community of organisms continues to develop, it becomes more diverse, and competition increases, but at the same time, new niche opportunities develop.
  • The pioneer species disappear as the habitat conditions change and invasion of new species progresses, leading to the replacement of the preceding community.
Secondary Succession
  • Secondary succession is the sequential development of biotic communities after the complete or partial destruction of the existing community.
  • A mature or intermediate community may be destroyed by natural events such as floods, droughts, fires, or storms or by human interventions such as deforestation, agriculture, overgrazing, etc.
  • This abandoned land is first invaded by hardy species of grasses that can survive in bare, sun-baked soil.
  • These grasses may be soon joined by tall grasses and herbaceous plants. These dominate the ecosystem for some years along with mice, rabbits, insects and seed-eating birds.
  • Eventually, some trees come up in this area, seeds of which may be brought by wind or animals.
  • And over the years, a forest community develops. Thus, an abandoned land over a period becomes dominated by trees and is transformed into a forest.
Difference Between Primary and Secondary Succession
  • Unlike in the primary succession, the secondary succession starts on a well-developed soil already formed at the site. Thus, secondary succession is relatively faster.
Autogenic and Allogenic Succession
  • When succession is brought about by living inhabitants of that community itself, the process is called autogenic succession, while change brought about by outside forces is known as allogenic succession.
  • Autogenic succession is driven by the biotic components of an ecosystem.
  • Allogenic succession is driven by the abiotic components (fire, flood) of the ecosystem.

  • Grasses have one good trick to monopolise a place. In the dry season the grasses dry up and cause fires which destroy other plant species and their seeds.
  • Also, grasslands develop in regions with scanty rainfall where plant growth cannot be achieved.
  • Though forests form the climax community in most of the ecosystems, in the grassland ecosystem grasses form the climax community. Thanks to fire and lack of water.
  • Grasslands are almost irreversible once deforestation in water-scarce areas gives way to grasslands.
Autotrophic and Heterotrophic succession
  • Succession in which, initially the green plants are much greater in quantity is known as autotrophic succession; and the ones in which the heterotrophs are greater in quantity is known as heterotrophic succession.
Succession in Plants
  • Succession that occurs on land (dry areas) where moisture content is low for e.g. on a bare rock is known as xerarch.
  • Succession that takes place in a water body, like ponds or lake is called hydrarch.
  • Both hydrarch and xerarch successions lead to medium water conditions (mesic) – neither too dry (xeric) nor too wet (hydric).
  • With time the xerophytic habitat gets converted into a mesophyte (plat needing only a moderate amount of water).
Succession in Water
  • In primary succession in water, the pioneers are the small phytoplankton, and they are replaced with time by free-floating angiosperms, then by rooted hydrophytes, sedges, grasses and finally the trees.
  • The climax again would be a forest. With time the water body is converted into land.
  • Another important fact is to understand that all succession whether taking place in water or on land, proceeds to a similar climax community – the mesic.


  • Homeostasis is the maintenance of stable equilibrium, especially through physiological (through bodily part functions. E.g. Cooling your body through sweating processes.
  • Organisms try to maintain the constancy of its internal environment despite varying external environmental conditions that tend to upset their homeostasis.


  • Some organisms can maintain homeostasis by physiological (sometimes behavioural – migrating to tree shade) means which ensures constant body temperature, constant osmotic concentration, etc.
  • All birds and mammals and a very few lower vertebrate and invertebrate species are indeed capable of such regulation (thermoregulation and osmoregulation).
  • The ‘success’ of mammals is largely due to their ability to maintain constant body temperature and thrive whether they live in Antarctica or the Sahara Desert.
  • Plants, on the other hand, do not have such mechanisms to maintain internal temperatures.


  • An overwhelming majority of animals and nearly all plants cannot maintain a constant internal environment. Their body temperature changes with the ambient temperature.
  • In aquatic animals, the osmotic concentration of the body fluids changes with that of the ambient water osmotic concentration. These animals and plants are simply conformers.
Why these conformers had not evolved to become regulators?
  • Thermoregulation is energetically expensive for many organisms. This is particularly true for small animals like shrews and hummingbirds.
  • Heat loss or heat gain is a function of surface area. Since small animals have a larger surface area relative to their volume, they tend to lose body heat very fast when it is cold outside; then they have to expend much energy to generate body heat [a lot of food goes into heat generation] through metabolism.
  • This is the main reason why very small animals are rarely found in polar regions.


  • The organism can move away temporarily from the stressful habitat to a more hospitable area and return when a stressful period is over
  • Every winter the famous Keoladeo National Park (Bhartpur) in Rajasthan hosts thousands of migratory birds coming from Siberia and other extremely cold northern regions.


  • In bacteria, fungi and lower plants, various kinds of thick-walled spores are formed which help them to survive unfavourable conditions – these germinate on the availability of suitable environment.
  • In higher plants, seeds and some other vegetative reproductive structures serve as means to tide over periods of stress besides helping in dispersal.
  • In animals, the organism, if unable to migrate, might avoid the stress by escaping in time. The familiar case of bears going into hibernation during winter is an example of an escape in time.
  • Some snails and fish go into aestivation to avoid summer-related problems – heat and desiccation.
  • Under unfavourable conditions, many zooplankton species in lakes and ponds are known to enter diapause, a stage of suspended development.
  • In ecology, the term homeostasis applies to the tendency for a biological system to resist changes.
  • Ecosystems are capable of maintaining their state of equilibrium.
  • They can regulate their own species structure and functional processes.
  • This capacity of the ecosystem of self-regulation is known as homeostasis.
  • For example, in a pond ecosystem, if the population of zooplankton increases, they consume a large number of the phytoplankton and as a result, food would become scarce for zooplankton.
  • When the number of zooplanktons is reduced because of starvation, the phytoplankton population start 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, negative feedback mechanism induced by the limiting resource (here its scarcity of food) 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.

  1. Energy flow through the food chain

  1.  Trophic Levels

  • A trophic level is the representation of energy flow in an ecosystem.
  • The trophic level of an organism is the position it occupies in a food chain.
  • Trophic level interaction deals with how the members of an ecosystem are connected based on nutritional needs.
Trophic levels
AutotrophsGreen plants (Producers)
HeterotrophsHerbivore (Primary consumers)
HeterotrophsCarnivores (Secondary consumers)
HeterotrophsCarnivore (Tertiary consumers)
HeterotrophsTop carnivores (Quaternary consumers)
  • Energy flows through the trophic levels from producers to subsequent trophic levels is unidirectional.
  • Energy level decreases from the first trophic level upwards due to loss of energy in the form of heat at each trophic level.
  • This energy loss at each trophic level is quite significant. Hence there are usually not more than four-five trophic levels (beyond this the energy available is negligible to support an organism).
  • The trophic level interaction involves three concepts namely
  1. Food Chain,Food Web, Ecological Pyramids

Food Chain

  • Transfer of food energy from green plants (producers) through a series of organisms with repeated eating and being eaten link is called a food chain. E.g. Grasses → Grasshopper → Frog → Snake → Hawk/Eagle.
  • Each step in the food chain is called trophic level.
  • A food chain starts with producers and ends with top carnivores.
  • The trophic level of an organism is the position it occupies in a food chain.
  • Types of Food Chains: 1) Grazing food chain and 2) Detritus food chain

Grazing food chain

  • The consumers which start the food chain, utilising the plant or plant part as their food, constitute the grazing food chain.
  • For example, in a terrestrial ecosystem, the grass is eaten by a caterpillar, which is eaten by lizard and lizard is eaten by a snake.
  • In Aquatic ecosystem phytoplankton (primary producers) are eaten by zooplanktons which are eaten by fishes and fishes are eaten by pelicans.

Detritus food chain

  • This type of food chain starts from organic matter of dead and decaying animals and plant bodies from the grazing food chain.
  • Dead organic matter or detritus feeding organisms are called detrivores or decomposers.
  • The detrivores are eaten by predators.
  • In an aquatic ecosystem, the grazing food chain is the major conduit for energy flow.
  • As against this, in a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain than through the grazing food chain.
  • Bacterial and fungal enzymes degrade detritus into simpler inorganic substances. This process is called catabolism.
  • Humification and mineralisation occur during decomposition in the soil.
  • Humification leads to accumulation of a dark-coloured amorphous (formless) substance called humus that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate.
  • Being colloidal in nature, humus serves as a reservoir of nutrients.
  • The humus is further degraded by some microbes and release of inorganic nutrients occur by the process known as mineralisation.
  • Warm and moist environment favour decomposition whereas low temperature and anaerobiosis inhibit decomposition resulting in a buildup of organic materials (soils become acidic like in taiga).

  • The food chain starts with a producer and ends with a top consumer.
  • Phytoplankton are the primary producers in the oceans. They include:
  • diatoms (unicellular algae),
  • coccolithophores (unicellular, eukaryotic protist),
  • Cyanobacteria (Bluegreen algae)– Synechococcus, Prochlorococcus, Nostoc, spirogyra etc.
  • Dinoflagellates (flagellated protists).
  • Crustaceans form a very large group of arthropods which includes crabs, lobsters, crayfish, shrimp, krill and barnacles (Biology NCERT).
  • Herrings are a fish, and they eat crustaceans.

Food Web

  • Multiple interlinked food chains make a food web.
  • Food web represents all the possible paths of energy flow in an ecosystem.
  • If any of the intermediate food chains is removed, the succeeding links of the chain will be affected largely.
  • The food web provides more than one alternative for food to most of the organisms in an ecosystem and therefore increases their chance of survival.

  • A food chain illustrates the order in which a chain of organisms feed upon each other. (True)
  • Food chains are found within the populations of a species. (Man won’t eat man – so, false)
  • A food chain illustrates the numbers of each organism which are eaten by others (food web illustrates the number, not the food chain).
Types of Biotic Interactions in a Food Web
Type of interactionSpeciesEffectComments
Negative Interactions
Amensalism0One species is inhibited while the other species is unaffected.
  • The bread mould fungi Penicillium produce penicillin an antibiotic substance which inhibits the growth of a variety of bacteria.
  • A large tree shades a small plant, retarding the growth of the small plant. The small plant has no effect on the large tree.
Predation+One species (predator) benefits while the second species (prey) is harmed and inhibited.
  • Predators like leopards, tigers and cheetahs use speed, teeth and claws to hunt and kill their prey.
  • Predators help in maintaining species diversity in a community, by reducing the intensity of competition among competing prey species.
Parasitism+Beneficial to one species (parasite) and harmful to the other species (host).
  • Parasitism involves parasite living in or on another living species called the host.
  • The parasite gets its nourishment and often shelter from its host.
  • Tap worm, roundworm, malarial parasite, many bacteria, fungi, and viruses are common parasites of humans.
  • The female mosquito is not considered a parasite, although it needs our blood for reproduction. Why? Because it doesn’t live on the host.
CompetitionAdversely affects both species.
  • Competition occurs when two populations or species, both need a vital resource that is in short supply.
Positive Associations
Commensalism+0One species (the commensal) benefits, while the other species (the host) is neither harmed nor inhibited
  • Suckerfish often attaches to a shark. This helps the suckerfish 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.
Mutualism++Interaction is favourable to both species
  • Sea anemone gets attached to the shell of hermit crabs for the benefit of transport and obtaining new food while the anemone provides camouflage and protection utilizing its stinging cells to the hermit crab.
  • 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 (symbiosis is intense mutualism – E.g. coral and zooxanthellae).
Neutral Interactions
Neutralism00Neither species affects the other
  • True neutralism is extremely unlikely.

Ecological Pyramids

  • The pyramidal representation of trophic levels of different organisms based on their ecological position (producer to final consumer) is called as an ecological pyramid.
  • The pyramid consists of a number of horizontal bars depicting specific trophic levels. The length of each bar represents the total number of individuals or biomass or energy at each trophic level in an ecosystem.
  • The food producer forms the base of the pyramid and the top carnivore forms the tip. Other consumer trophic levels are in between.
  • The ecological pyramids are of three categories:
  1. Pyramid of numbers,
  1. Pyramid of biomass, and
  1. Pyramid of energy or productivity.

Pyramid of Numbers

  • Pyramid of numbers represents the total number of individuals of different species (population) at each trophic level.
  • Depending upon the size, the pyramid of numbers may not always be upright, and may even be completely inverted.
  • It is very difficult to count all the organisms, in a pyramid of numbers and so the pyramid of number does not completely define the trophic structure for an ecosystem.
Pyramid of numbers – upright
  • In this pyramid, the number of individuals is decreased from lower level to higher trophic level.
Pyramid of numbers – upright

  • This type of pyramid can be seen in the grassland ecosystem and pond ecosystem.
  • The grasses occupy the lowest trophic level (base) because of their abundance.
  • The next higher trophic level is primary consumer – herbivores like a grasshopper.
  • The individual number of grasshoppers is less than that of grass.
  • The next energy level is a primary carnivore like rats.
  • The number of rats is less than grasshoppers, because, they feed on grasshoppers.
  • The next higher trophic level is secondary carnivore like snakes. They feed on rats.
  • The next higher trophic level is the top carnivore like Hawk.
  • With each higher trophic level, the number of individual decreases.
Pyramid of numbers – inverted
  • In this pyramid, the number of individuals is increased from lower level to higher trophic level. E.g. Tree ecosystem.
Pyramid of numbers – inverted
Pyramid of Biomass
Pyramid of Biomass

  • Pyramid of biomass is usually determined by collecting all organisms occupying each trophic level separately and measuring their dry weight.
  • This overcomes the size difference problem because all kinds of organisms at a trophic level are weighed.
  • Each trophic level has a certain mass of living material at a particular time called the standing crop.
  • The standing crop is measured as the mass of living organisms (biomass) or the number in a unit area.
Pyramid of Biomass – upright
  • For most ecosystems on land, the pyramid of biomass has a large base of primary producers with a smaller trophic level perched on top.
  • The biomass of producers (autotrophs) is at the maximum. The biomass of next trophic level i.e. primary consumers is less than the producers. The biomass of next higher trophic level i.e. secondary consumers is less than the primary consumers. The top, high trophic level has very less amount of biomass.
Pyramid of Biomass – upright
Pyramid of Biomass – Inverted

  • In contrast, in many aquatic ecosystems, the pyramid of biomass may assume an inverted form. (In contrast, a pyramid of numbers for the aquatic ecosystem is upright)
  • This is because the producers are tiny phytoplankton that grows and reproduces rapidly.
  • Here, the pyramid of biomass has a small base, with the consumer biomass at any instant exceeding the producer biomass and the pyramid assumes an inverted shape.
Pyramid of Biomass (Inverted) in an aquatic ecosystem

Pyramid of Energy

  • To compare the functional roles of the trophic levels in an ecosystem, an energy pyramid is most suitable.
  • An energy pyramid represents the amount of energy at each trophic level and loss of energy at each transfer to another trophic level. Hence the pyramid is always upward, with a large energy base at the bottom.
  • Suppose an ecosystem receives 1000 calories of light energy in a given day. Most of the energy is not absorbed; some is reflected to space; of the energy absorbed only a small portion is utilized by green plants, out of which the plant uses up some for respiration and of the 1000 calories; therefore only 100 calories are stored as energy-rich materials.
  • Now suppose an animal, say a deer, eats the plant containing 100 calories of food energy. The deer use some of it for its metabolism and stores only 10 calories as food energy. A lion that eats the deer gets an even smaller amount of energy. Thus, usable energy decreases from sunlight to producer to herbivore to carnivore. Therefore, the energy pyramid will always be upright.
  • Energy pyramid concept helps to explain the phenomenon of biological magnification – the tendency for toxic substances to increase in concentration progressively with higher trophic levels.
Ecological Efficiency
  • Ecological efficiency describes the efficiency with which energy is transferred from one trophic level to the next.
  • The number of trophic levels in the grazing food chain is restricted as the transfer of energy follows 10 per cent law – only 10 per cent of the energy is transferred to each trophic level from the lower trophic level.
  • The decreases at each subsequent trophic level is due to two reasons:
  • At each trophic, a part of the available energy is lost in respiration or used up in metabolism.
  • A part of the energy is lost at each transformation.
Limitations of Ecological Pyramids
  • It does not consider the same species belonging to two or more trophic levels.
  • It assumes a simple food chain, something that seldom exists in nature; it does not accommodate a food web.
  • Moreover, saprophytes (plant, fungus, or microorganism that lives on decaying matter) are not given any place in ecological pyramids even though they play a vital role in the ecosystem.
Pollutants and Trophic Level – Biomagnification
  • Pollutants move through the various trophic levels in an ecosystem.
  • Non-degradable pollutants (persistent pollutants), which cannot be broken down by detrivores, not only move through the various trophic levels but also remain in that tropic level for a very long duration.
  • Chlorinated Hydrocarbons (Organochlorides) are the most damaging non-degradable pollutants that are long-lasting.

Chlorinated Hydrocarbons (CHC)

  • CHCs are hydrocarbons in which one or more hydrogen atoms have been replaced by chlorine E.g. DDT (dichlorodiphenyltrichloroethane), endosulfan, chloroform, carbon tetrachloride, etc.
Applications of Chlorinated Hydrocarbons (CHC)
  • CHCs are used in the production of polyvinyl chloride (a synthetic plastic polymer used to make PVC pipes).
  • Chloroform, dichloromethane, dichloroethane, and trichloroethane are useful solvents.
  • These solvents are immiscible with water (not forming a homogeneous mixture when mixed with water) and effective in cleaning applications such as degreasing and dry cleaning.
  • DDT, heptachlor and endosulfan are were widely used as pesticides.
Effects of CHC
  • Dioxins (toxic by-products produced when organic matter is burned in the presence of chlorine in industrial or natural processes such as volcanic eruptions and forest fires), and some insecticides, such as DDT, are persistent organic pollutants.
  • DDT was widely used a few decades ago as an effective pesticide and insecticide.
  • It was later identified as a persistent organic pollutant, and its usage was phased out in almost all developed countries.
  • It accumulated in food chains and caused eggshell thinning in certain bird species.
  • In India, it is still being used by civic administrations as a mosquito repellent (disease vector control).
  • In India, traces of DDT spray used three decades ago can still be found on the walls of homes.
  • Crops that are grown in fields that were sprayed with DDT in the last decades show substantial traces of the insecticide.
  • DDT residues continue to be found in mammals all across the planet.
  • In Arctic areas, particularly high levels are found in marine mammals.
  • The traces of persistent organic pollutant are found in human breast milk.
  • In some species of milk-producing marine mammals, males typically have far higher levels, as females reduce their concentration by transfer to their offspring through lactation.
  • Endosulfan, one of the most widely used pesticide, is an endocrine disruptor (enhances the effect of estrogens causing reproductive and developmental damage in both animals and humans).
  • Because of its threats to human health and the environment, a global ban on the manufacture and use of endosulfan was negotiated under the Stockholm Convention in 2011.
  1. Bioaccumulat
  2. Bioaccumulation is the gradual accumulation of pollutants, chemicals (chronic poisoning) or other substances in an organism.
  • Bioaccumulation occurs when the rate of loss of the substance from the body of the organism through catabolism (breakdown of complex molecules in living organisms), or excretion is lower than the rate of accumulation of the substance.
  • As persistent organic pollutants like DDT are long-lasting, the risk of bioaccumulation is high even if the environmental levels of the pollutant are not high.


  • Biomagnification refers to progressive bioaccumulation (increase in concentration) at each tropical level with the passage of time.
  • In order for biomagnification to occur, the pollutant must have a long biological half-life (long-lived), must not be soluble in water but must be soluble in fats. E.g. DDT.
  • If the pollutant is soluble in water, it will be excreted by the organism.
  • Pollutants that dissolve in fats are retained for a long time. Hence it is traditional to measure the amount of pollutants in fatty tissues of organisms such as fish.
  • In mammals, milk produced by females is tested for pollutants since the milk has a lot of fat in.

credit- shankarias, pmfias,ncert, nios, google