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Energy Flow through an Ecosystem: Food Chain, Food Web

Functions of Ecosystem
  1. Ecological succession or ecosystem development (previous post)
  2. Homeostasis (or cybernetic) or feedback control mechanisms (previous post)
  3. Energy flow through the food chain
  4. Nutrient cycling (biogeochemical cycles) (next post)

Energy Flow Through an Ecosystem – Trophic Levels

(Trophe = Nourishment)

  • 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

Pic Credits

Trophic levels
Autotrophs Green plants (Producers)
Heterotrophs Herbivore (Primary consumers)
Heterotrophs Carnivores (Secondary consumers)
Heterotrophs Carnivore (Tertiary consumers)
Heterotrophs Top 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
  2. Food Web
  3. 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.

Grazing food chain

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.

Detritus 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).
Q. With reference to the food chains in ecosystems, which of the following kinds of organism is/are known as decomposer organism/organisms?
  1. Virus
  2. Fungi
  3. Bacteria

Select the correct answer using the codes given below.

  1. 1 only
  2. 2 and 3 only
  3. 1 and 3 only
  4. 1, 2 and 3


  • Fungi and Bacteria are decomposers. They breakdown organic matter into simple inorganic substances.
  • Virus represents dormant life. They are metabolically inactive as long as they are outside a host body. They are not decomposers. They invade host cells and use their nucleus (DNA machinery) to carry out their life processes.
  • Protists are unicellular organisms, such as paramecium and euglena. They’re not decomposers either.

Answer: b) 2 and 3 only

Q. Which one of the following is the correct sequence of a food chain?
  1. Diatoms-Crustaceans-Herrings
  2. Crustaceans-Diatoms-Herrings
  3. Diatoms-Herrings-Crustaceans
  4. Crustaceans-Herrings-Diatoms


  • 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.

Answer: (a) Diatoms-Crustaceans-Herrings. Tough question.

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.

Food Web

Picture Credits

Q. With reference to food chains in ecosystems, consider the following statements:
  1. A food chain illustrates the order in which a chain of organisms feed upon each other.
  2. Food chains are found within the populations of a species.
  3. A food chain illustrates the numbers of each organism which are eaten by others.

Which of the statements given above is/are correct?

  1. 1 only
  2. 1 and 2 only
  3. 1, 2 and 3
  4. None


  • 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).

Answer: a) 1 only

Types of Biotic Interactions in a Food Web

‘0’ is no effect; ‘–’ is detrimental; ‘+’ is beneficial.

Type of interaction Species Effect Comments
1 2
Negative Interactions
Amensalism 0 One 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.
Competition Adversely affects both species.
  • Competition occurs when two populations or species, both need a vital resource that is in short supply.
Positive Associations
Commensalism + 0 One 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
Neutralism 0 0 Neither species affects the other
  • True neutralism is extremely unlikely.
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