Energy Flow in Ecosystem, Food Chains, Food Webs, Ecosystem Productivity and Ecological Pyramids

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In the last article, we discussed the basics of Ecosystem- the concept, types, structure and associated concepts.

In this article, we will discuss the following:

  • What are food chains, Trophic levels and food webs?
  • How does the Energy flow in an ecosystem?
  • What is Ecosystem Productivity?
  • What are Ecological Pyramids? What are the different types of Ecological Pyramids?

What are food chains and food webs?

A food chain describes how energy and nutrients move through an ecosystem. It basically shows who eats what in the ecosystem.

Food Chains are generally classified into two: A grazing food chain and detritus food chain

  1. Grazing food chain: It includes eating of a plant by an animal and eating of an animal by another. The food chain always starts with the green plants.


  1. Detritus Food Chain: It starts from dead organic matter (detritus) and goes through decomposers (bacteria, fungi) to the detritus feeders (earthworms) and then to carnivores feeding on them.


The food chains show simple and isolated feeding relationships. Such simple relationships do not or rarely occur in ecosystems. In reality, all the food chains are interconnected. All of the interconnected and overlapping food chains in an ecosystem form a food web.

What are the Trophic Levels?

Each organism occupies a particular position in a food chain depending on its feeding relationship with other organisms. This position is known as the trophic level.  It is important to note that the amount of energy decreases at successive trophic levels.

When an organism dies it is converted to detritus. It then serves as an energy source for decomposers.

How does the Energy Flow in an Ecosystem?

In simplest terms, the behaviour of energy in an ecosystem is called energy flow. The flow of energy in the ecosystem is always unidirectional (linear). The energy flow obeys the first and second law of thermodynamics.

Laws of Thermodynamics (Rudolf Clausius and William Thomson)

  • 1st Law: Energy cannot be created or destroyed but can be transformed from one form to another.
  • 2nd Law: When energy is converted into different forms, its capacity to perform useful work diminishes.


To understand the above statements, let us trace the route of energy flow in the ecosystem

  • The various types of biotic components -autotrophs, heterotrophs (discussed in the earlier article)- are intimately interlinked with one another by a mutual interdependence on food energy.
  • The ecosystem derives all the energy, required for its functioning, from the Sun. The green plants then use this solar energy to convert carbon dioxide and water into glucose by the process of photosynthesis. Thus, solar energy gets converted to chemical energy. All organisms are dependent for their food on producers, either directly or indirectly.
  • The food energy flows in the ecosystem through the process of eating and being eaten. Example: The rabbit (herbivore) eats the plants and is being eaten by a fox (carnivore) and the fox is then eaten by a lion (top carnivore). So, you find a unidirectional flow of energy from the sun to producers and then to consumers.

  • In this entire process, the living organisms continuously use up a certain amount of energy to perform bodily functions (respiration, digestion, movement etc)- and in the process releases heat.
  • Energy is always lost when transferred from one organism to another. At each step up the food chain, only 10% of the energy is passed on to the next level, while approximately 90% of the energy is lost as heat.

What is Ecosystem Productivity?

The productivity of an ecosystem is the amount of organic matter accumulated in any unit time. It is of two types: Primary Productivity and Secondary Productivity.

Primary productivity: It is the rate at which biomass or organic matter is produced per unit area over a time period by plants during photosynthesis.

It can be divided into gross primary productivity (GPP) and net primary productivity (NPP).

  1. Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis.
  2. A certain amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP).

Secondary Productivity: Secondary productivity is defined as the rate of formation of new organic matter by consumers. Net primary productivity is the available biomass for the consumption to heterotrophs (herbivores and decomposers).

The highest levels of productivity occur at the boundaries between land and water. Examples: alluvial plains, estuaries, coral reefs. On the other hand, deserts and deep oceans are very low in productivity.

What are the Ecological Pyramids? What are the different types of Ecological Pyramids?

Ecological pyramids are the diagrammatic representations of the feeding relationship and energy transfers in the ecosystem. They are of three types: Pyramid of Numbers, Pyramid of Biomass and Pyramid of Energy.

  1. Pyramid of Numbers: It represents the relationship between number of individuals of primary producers and consumers at different trophic levels. It can be upright or inverted.
  • In an upright pyramid of numbers, the number of individuals decrease from lower to higher trophic level. Example: In a grassland ecosystem.
  • In an inverted pyramid of number, the number of individuals increase from lower to higher trophic levels. Example: A large number of birds feeding on a single tree.

  1. Pyramid of Biomass: Biomass id the total mass (in g. dry weight) of living organisms in an area at a particular point of time. A pyramid of biomass is obtained when the rectangles used to construct the pyramid represent the masses of organisms at each trophic level.
  • Pyramid of biomass can be upright (e.g. in grassland ecosystem) or inverted. Inverted pyramids can occur in aquatic ecosystems. This happens because in aquatic ecosystems producers are tiny phytoplankton. Thus, consumer biomass exceeds producer biomass.

  1. Pyramid of Energy: It represents the total amount of energy used at each trophic level per unit of area per unit of time. It thus shows how energy flows from one trophic level to another in an ecosystem. A pyramid of energy is always upright.

Check your progress with UPSC Previous year Questions

Q) With reference to the food chains in the ecosystem, which of the following kinds of organisms is/are known as decomposer/decomposers? (2013)

  1. Virus
  2. Fungi
  3. Bacteria

Select the correct answer using the code below

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


Q) With reference to food chains in the ecosystem, consider the following statements (2013)

  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


Q) Which of the following is the correct sequence of ecosystems in the order of decreasing productivity? (2013)

  1. Oceans, lakes, grasslands, mangroves
  2. Mangroves, oceans, grasslands, lakes
  3. Mangroves, grasslands, lakes, oceans
  4. Oceans, mangroves, lakes, grasslands


Q) Which one of the following is the correct sequence of a food chain? (2014)

  1. Diatoms-Crustaceans-Herrings
  2. Crustaceans-Diatoms-Herrings
  3. Diatoms-Herrings-Crustaceans
  4. Crustaceans-Herrings-Diatoms




Q1-  b

Q2 – a

Q3- c

Q4- a


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