1. Food Chain
    1. What is Trophic Level?
    2. Type of Food Chains
    3. Ecological Pyramids
    4. Types of Pyramids
  2. Food Web
    1. Energy Flow in the Ecosystem
    2. 2nd law of themodynamics
    3. Connection: Trophic level vs Energy flow
    4. Energy=Unidirectional
    5. Nutrients/Material=Cyclic
  3. DDT and Biomagnification
  4. Biotic Potential.
  5. Carrying Capacity
  6. Interspecies relations
  7. Predation vs Parasitism
  8. Free download IGNOU Material
  9. Mock questions

Food Chain

  • For example, plants are eaten by insects, who are eaten by frogs, these frogs are eaten by fish, who are eaten by human beings.
  • Plant > insect > frog > fish > human.
  • This sequence is known as Food chain.

What is Trophic Level?

  • Each step in the food chain is called a “Trophic level”.
  • In this food chain: Plant > Insect > Frog > Fish > human= five steps.
  • The trophic level to which an organism belongs, indicates how far it is away from plants in the food chain.
  1. green plants (producers)= trophic level I
  2. herbivores (primary consumers)= trophic level II
  3. carnivores (secondary consumers)=trophic level III
  4. top carnivores (tertiary consumers)= trophic level IV
  • Herbivores (deer, cow etc.) are always @second trophic level.
  • Higher trophic levels are made up of carnivores.
  • An organism cannot always be assigned to one specific trophic level.
  • E.g. man (omnivore), so he is both a herbivore (@2nd trophic level) and as a carnivore he can be (@>2nd trophic level)
Type of Food Chains
Food Chain Description
  1. Grazing
Starts with green plants.e.g. Grass> grasshoppers> birds> hawks.
  1. Parasitic
Also starts with green plants but here smaller organisms depending on larger organism. e.g. Grass> cow> lice.
  1. Detritus* /saprophytic
Starts with dead plant/animal/ waste material> decomposers.

*Detrius = dry leaves, dead plant, animal remains, dead skin cells, other organic waste (urine, excreta).

Ecological Pyramids

  • You already know about the food chain: grass –> Grasshoppers –>birds –>hawk.
  • Each step or level is known as Trophic level.
  • If we draw these trophic levels in a verticle fashion, a pyramid is created. This pyramid is known as Ecological pyramid.
  • The Autotrophs (green plants) = base of the pyramid
  • The top carnivores (lion tigers) = tip (apex) of this pyramid.

Types of Pyramids

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

food chain pyramids

Food Web

  • Food Chain:–> Grass> grasshopper >frog > fish > man
  • Food chain assume the isolated linear line. But in real life, frog could be eating other insects apart from grasshopper. Fish could be eating smaller fishes apart from frog. So there exists an interconnected Network of (Food Chain + Food Chain + Food chain…).
  • This interconnected network of food chains = food web.

Why is Food web important?

  • Suppose in a jungle, the deer species is wiped out due to some human poaching/ diseases / natural disaster.
  • Then its predators (lion,tiger) can feed on other animals such as fox, wolf, crane, peacock etc. until deer population grows again.
  • Thus, food web= help living organism survive in case of minor or major setbacks in their food chain.
  • food web=provides stability to the ecosystem.

Energy Flow in the Ecosystem

  • So far you know what is food chain and what is food web.
  • You also know what is Trophic level. (each step in a food chain).
  • When energy is passed from one tropic level to next trophic level, some part of energy is wasted. So energy declines as we move up in the chain/ web / pyramid.


  1. Because some energy is wasted as body heat.
  2. No herbivore (deer, cow) can entirely digest and absorb the plant nutrients.
  3. Some energy is wasted while capturing the prey. (e.g. Tiger chasing a deer.)
  4. Some energy remains trapped e.g. Tiger only eats the flash of a deer. He doesn’t eat the skin, hair, bone and teeth of a deer.
  • So the energy stored in the skin, hair, bone and teeth of a deer =not moved up in the food chain.
  • However this energy is later utilized when scavenger (hyena) comes into picture. Because Hyena’s stomach acid is so powerful, it can digest the skin, hair, bone, teeth and other remnants of a deer corpse.

2nd law of themodynamics

food chain pyramid energy

  • When energy changes from one form to another, some of the energy is Lost. Suppose a jungle receives 1000 calories worth Sunlight everyday.
  • Most of the sunlight is not absorbed; some is reflected back to space.
  • Out of the sunlight that is absorbed, only a small portion is utilized by green plants
  • Out of which the plant uses up some for respiration, therefore only 100 calories are stored as energy rich materials.
  • Now a deer, eats the green plant containing 100 cal of food energy.
  • The deer uses some of it for its own metabolism and stores only 10 cal as food energy.
  • A tiger eats the deer gets an even smaller amount of energy.
  • Thus usable energy decreases from sunlight to producer(green plant) to herbivore(deer) to carnivore(tiger).
  • Therefore, the energy pyramid will always be upright.

Connection: Trophic level vs Energy flow

  • The loss of energy (explained in previous para) limits the number of trophic levels in the ecosystem.
  • Hence trophic levels are seldom more than five.
  • This also explains why the steps in a food chain are limited to four or five.


flow of energy nutrients in food chain

  • Movement of Energy is Unidirectional (one-way).
  • Because initial Solar energy trapped by an autotroph (plant) does not go back to Sun.
  • Similarly, energy that passes from herbivore (deer) to carnivore (lion) does not pass back to herbivore (deer).
  • This unidirectional (one-way) and continuous energy flow=prevents the collapse of Ecosystem.
  • (otherwise, if lion ate deer and another rabbit ate lion then what will happen hahaha!)


  • The Movement of nutrients/materials is cyclic.
  • e.g. grasshopper eats grass and dies.
  • Its body is decomposed and again absorbed by some other plant/grass.
  • In other words: The Decomposers break down the fecal matter, excretory products and dead bodies of all plants and animals into inorganic materials.
  • And this inorganic material is again reused by autotrophs (green plants).
  • Ok here is a question? If you want to make the movement of nutrients unidirectional then what should be done? Ans. Eliminate the decomposers.

Entry of DDT and other pesticides, damages this smooth cycle of nutrient movement. Let’s check how:

DDT and Biomagnification

  • DDT=dichloro-diphenyl trichloroethane.
  • It is a “broad” spectrum pesticide (meaning it can kill variety of insects, not just one or two.)
  • It lasts for a relatively long time (nearly 20 years) after one application.
  • DDT is cheap to produce, been extensively used across world.
  • Problem: DDT cannot be metabolized (digested) by any living organism.


  • Suppose DDT was sprayed on water.
  • There are millions of planktons in the water. Each of them absorbed 5ppm of DDT. Since they cannot digest DDT, it remains unprocessed in their body.
  • A fish eats 8 such planktons, so it gets 8 x 5 =40 ppm of DDT in its body.
  • A bird eats 50 such fishes, so bird gets 40 x 50 =2000 ppm of DDT in its body.
  • So the amount of DDT keeps increasing as we move up in the food chain. This phenomenon is known as bio magnification.
  • Bio magnification is bad because it produces variety of diseases in the species + its offsprings (babies) via teratogenesis.
  • Teratogenesis = formation of defective embryo.

To sumup

  • Biomagnification is a man-induced process.
  • Biomagnification occurs with non-degradable chemicals/substances/pollutants only.
  • For example Pesticides, anti-knocking agents used in internal combustion engines, paints, artificial perfumes etc.
  • Biomagnification doesn’t occur with degradable chemicals/substances for example sugar, tea, cheese, milk etc. because they can be digested by (some) living organisms.

Biotic Potential.

  • Unlimited food + ideal environmental conditions =a species can produce offspring @ maximum rate. This is known as Biotic potential.
  • It can be high or low, depending on how many offspring (children) can a species produce in ideal conditions.
Biotic potential Example Why?
High Bacteria, insects, mice, rabbits produce a large number of offspring in a short time.
low larger species like elephants, tigers and humans Gestation period is long. So they produce only a few offspring species during their lifetime, even if ideal food and environment conditions are given.

Carrying Capacity

  • It means the maximum population that an environment can support and sustain.
  • When the carrying capacity is reached, birth rates equal death rates and the population should maintain a steady state equilibrium.

Interspecies relations

Interaction Species A Species B Example
  1. Mutualism
Enjoys Enjoys Bees and flowers (pollination)
  1. Commensalism
Enjoys Neutral Orchid plant growing on a mango tree.
  1. Predation
Enjoys Harmed Tiger eating deer.
  1. Parasitism
Enjoys Harmed Tapeworm in human digestive tract.
  1. Competition
Harmed Harmed Jungle Deers vs domestic goats grazing in same area.
  1. Amensalism
Neutral Harmed bread mold Penicillium secretes penicillin and it destroys other bacteria.

Btw, what’s the difference between

Predation vs Parasitism



Tiger killing deer. Tapeworm in human digestive tract.
done only for food. Done for food + shelter.
Prey (deer) killed immediately Host not killed immediately
Predator is (usually) big in size. Smaller.
Predator’s biotic potential is usually very low. (tiger, lion etc.) Parasite’s biotic potential is usually higher.

Free Download IGNOU EnB Material

This article is based on the first four chapter of IGNOU’s Certificate course in Environmental Studies: Click me to download (Size 30MB)
You already have NIOS material on EnB. This IGNOU set has lot of overlapping with its coverage, but still a good read nonetheless.

Mock questions

Q1. Correct statements about Bio magnification

  1. It is a natural phenomenon occurring since stone-age.
  2. It is associated with biodegradable chemicals.
  3. Bio Magnification protects biodiversity.
  1. Only 1 and 2
  2. Only 2 and 3
  3. Only 1 and 3
  4. None.

Q2. Correct statements

In an ecosystem,

  1. The flow of energy is cyclic
  2. The flow of nutrients is unidirectional
  1. Only 1
  2. Only 2
  3. Both
  4. None

Q3. Correct Statements about Trophic levels:

  1. Each step in a food chain is called a “Trophic level”.
  2. Carnivores always occupy trophic level 1 or 2.
  3. Herbivores always occupy trophic level 3 or above.
  4. Every species can be assigned a single unique trophic level.
  1. Only 1 and 2
  2. Only 1 and 3
  3. Only 1, 2 and 3
  4. Only 1

Q4. Which among the following is/are example(s) of bio magnification?

  1. Tiger kills a cow. A (Human) poacher poisons the corpse of cow. Tiger comes back and eats it. Tiger dies.
  2. A factory discharges untreated chemical affluent into a pond. A Man regularly eats fishes caught from the pond. Man gets cancer after some years.
  1. Only 1
  2. Only 2
  3. Both
  4. None

Q5. An ecosystem would collapse:

  1. If flow of energy is not unidirectional.
  2. If flow of nutrient is not cyclic.
  1. Only 1
  2. Only 2
  3. Both
  4. None

Q6. The usable energy decreases as we move up in the food chain. Which among the following explains this phenomenon?

  1. Newton’s third law of motion.
  2. Einstein’s law of general relativity.
  3. Second law of thermodynamics.
  4. None of above.

Q7. When an ecosystem has reached its carrying capacity It means.

  1. Birth rate = death rate
  2. Birth rate < death rate
  3. Birth rate > death rate
  4. Birth rate greater than or less than death rate

Q8. Which among the following is/are example(s) of commensalism?

  1. An orchid growing on a eucalyptus tree
  2. Tapeworm in the intestinal tract of a child.
  3. Plasmodium in Human blood.
  4. Bees drinking nectar of a flower.
  1. Only 2 and 3
  2. Only 1 and 4
  3. Only 1, 2 and 4
  4. Only 1

Q9. Correct statement:

  1. Biotic potential of tigers is higher than rabbits.
  2. Biotic potential of rabbits is higher than tigers.
  3. Both equal
  4. None of above.

Q10. If one species benefits by harming another species. It may be

  1. Parasitism
  2. Predation
  1. Only 1
  2. Only 2
  3. Either 1 or 2
  4. None.