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Energy flow in the ecosystem, Practice Problems and FAQs

Oxygen, water and food are the three major components that are needed for the survival of humans. We have a biological clock that reminds us to eat food at the correct intervals everyday. If we go without eating food at regular intervals, our body feels lethargic and weak, especially if it is required to perform a physical activity. In fact, when we get used to specific intervals between meals, stretching the interval even by a couple of minutes leaves us cranky due to hunger. Do you know the reason behind this? It is because during the long hours between meals, our body uses most of the energy that is stored in the body. Any physical or metabolic activity of the body requires energy which is produced by the breakdown of simple sugars, such as glucose, within the cell by the process of cellular respiration. During periods of prolonged starvation, the glycogen stored in the muscles is readily converted to its monomer glucose, which then undergoes cellular respiration in order to produce energy. So the body needs more energy after working and for that we have to take in more food. So it is clear that we get energy from our food. 


Humans, being omnivores, eat both plants and animals to obtain energy. This indicates that some amount of energy must be present in the body of those animals and plants which we eat. Right? Then only it can be transferred from their body to us. Where do they get energy from?

Herbivorous animals obtain energy from the plants they eat and carnivorous animals obtain energy from the other animals that they feed upon. But, where do plants get the energy from? They obtain it from the ultimate source of energy on earth, the sun!! 

But do you think that plants utilise all the sunlight that falls on them? Is all the utilised energy transferred from the plants to the animals? What happens to this energy when it reaches the animals that are present in the higher trophic levels? We know that there is an energy flow between the living organisms, but how does that happen? Through this article we are going to find the answers to all these questions. 

Table of contents:

Ecosystem functions

The structural and functional unit of nature is called the ecosystem, which is a stable system formed due to interactions between the abiotic and biotic components. Air, water, soil, light etc, are the abiotic or non-living components and producers, consumers and decomposers are the biotic components. The ecosystem is self sustainable. 

                                               Fig: Ecosystem 

There are four functional aspects of the ecosystem. They are as follows: 

  • It sustains life systems and ecological processes by energy flow
  • Synthesis of organic matter by producers, i.e, productivity
  • Decomposition of dead organic matter
  • Cycling of minerals and nutrients 

How is life sustained in an ecosystem? We know that energy is necessary for sustaining life. Energy is needed for us to do all kinds of work. We need energy even for the basic functions of the body even when we are asleep. If a single organism requires so much energy, imagine the requirements of a large ecosystem!

The source of energy for all ecosystems on earth is the sun. But there is an exception for this, which is the deep sea hydro-thermal ecosystem. Less than 50 percent of the incident solar radiation is photosynthetically active radiation (PAR). PAR is the portion of the light spectrum that can be utilised by plants. PAR refers to the light of wavelength ranging from 400 to 700 nm. 

                                                   Fig: PAR in the light spectrum

But not all the PAR is captured. Plants and photosynthetic bacteria capture 2-10 percent of the PAR and this small amount of energy sustains the entire living world. 

                            Fig: PAR captured by the plants and photosynthetic bacteria

All organisms are directly or indirectly dependent for their food on plants. The energy of the PAR that is absorbed by the producers (green plants and photosynthetic microbes) is utilised by them to prepare organic matter or food in the form of carbohydrates (glucose). Thus, producers convert solar energy into chemical energy that is stored in the food they prepare. This energy is transferred to other organisms when they directly feed on the producers or feed on other organisms which feed on the producers. It is very important to know how the solar energy captured by plants flows through different organisms of an ecosystem. Let’s see how.

Energy flow in the ecosystem

The movement of energy from the external environment through successive trophic levels and back to the external environment is called energy flow in the ecosystem. It is common to all ecosystems and is one of the most fundamental processes. Every single biotic component of the ecosystem needs energy to survive. 

The ultimate source of energy for all ecosystems is sunlight. The sunlight enters the ecosystem through the process of photosynthesis by producers. All other organisms depend on the producers for this energy. Based on the source of energy the organisms are grouped into different trophic levels. 

                                                      Fig: Flow of energy 

Trophic level

Trophic level can be defined as the position of an organism in a food chain at which it obtains food. The different trophic levels in an ecosystem includes the following:

  • Producers
  • Herbivores 
  • Primary carnivores
  • Secondary carnivores

                                          Fig: Different trophic levels


Producers are the organisms which can prepare their own food by the process of photosynthesis (using light energy) or chemosynthesis (using chemical energy). In a terrestrial ecosystem such as forest major producers are herbaceous and woody plants. While in the aquatic ecosystem, phytoplankton are the major producers. Producers form the 1st trophic level in an ecosystem.

                                                 Fig: Producers 

All other organisms depend on plants (directly or indirectly) for their food needs. Hence they are called consumers or heterotrophs. 

Herbivores or Primary consumers

If animals feed on the producers, the plants, they are called herbivores or primary consumers. Some common herbivores are insects, birds and mammals like deer in the terrestrial ecosystem. In aquatic ecosystems, molluscs such as clams, crustaceans, and protozoans serve as primary consumers. This is the 2nd trophic level of the food chain.

                                            Fig: Herbivores

Primary carnivores or Secondary consumers

Animals which feed on the primary consumers or herbivores are called primary carnivores or secondary consumers. For example, foxes which feed on deer, frogs which feed on grasshoppers, small fishes which feed on zooplankton (small molluscs, crustaceans, etc) are secondary consumers. They occupy the third trophic level.

                                       Fig: Primary carnivores 

Secondary carnivores or Tertiary consumers

Those animals that depend on the primary carnivores for food are labelled secondary carnivores or tertiary consumers. For example, a snake feeding on a frog is an example of a secondary carnivore or tertiary consumer. Tertiary consumers form the 4th trophic level of a food chain.

             Fig: Secondary carnivore

Top carnivores or Quaternary consumers

These are the apex predators which feed on the secondary carnivores or tertiary consumers and are not eaten by any other animal. For example, a tiger which feeds on the fox, an eagle which feeds on the snake, are examples of top carnivores or quaternary consumers. The trophic level of carnivorous animals does not go beyond this 5th trophic level.

                                   Fig: Tertiary or Top carnivores 

Detritivores and decomposers

The 6th trophic level is occupied by organisms which feed on the dead and decayed remains of plants or animals. These can be detritivores such as earthworms, millipedes, termites, etc. which help in the fragmentation and pulverisation of dead organic matter. This level is also occupied by decomposer organisms such as bacteria and fungi which help in breaking down the complex organic compounds in the dead decaying matter into simpler inorganic substances that can be released into the environment. 

                      Fig: Detritivores and decomposers

Food chain 

Food chain is the chronological series of organisms in which there is a transfer of food energy between the organisms through a series of repeated events of eating and being eaten. Food chains can be of different types based on the the first trophic level involved - 

  • Grazing food chain
  • Detritus food chain 

Grazing food chain (GFC)

This is the most commonly occurring food chain and is also known as the predator food chain as predation is involved at every level. Such a food chain begins with producers occupying the first trophic level. It involves 3-5 types of consumers and ends with top carnivores which are not further preyed upon. Energy flows from one trophic level to the next as producers are eaten by herbivores (primary consumer) which are further eaten by primary carnivores (secondary consumer), secondary carnivores (tertiary consumers) or top carnivores (quaternary consumers). 

The trophic level occupied by an organism is not absolute and can vary from one chain to another. For example, a snake can act as a secondary consumer when it feeds on a mouse (primary consumer) that feeds on rice grains. On the other hand, it can also act as a tertiary consumer when it feeds on a frog (secondary consumer) that eats grasshoppers (primary consumer) that feed on leaves (producers). In fact top carnivores can also act as tertiary consumers in certain food chains.

Omnivores can operate at various trophic levels. For example, when a rat feeds on grains it acts as a herbivore or primary consumer but when it feeds on smaller animals, it acts as a secondary consumer or primary carnivore.

                                                     Fig: Grazing food chain 

Detritus food chain (DFC) 

The food chain in which the primary source of energy is the freshly deposited decomposing dead organic matter or detritus is called detritus food chain. This food chain starts with the dead organic matter and surprisingly more energy passes through the detritus food chain than the grazing food chain which starts with producers. 

There are various types of detritus feeders:

  • Scavengers - These form the first level of the detritus food chain. These feed on corpses of animals and leave large fragments. Scavengers include crows, kites, vultures, hyenas, etc.
  • Detritivores - These are animals which feed on freshly deposited detritus and fragments left over by scavengers and help in fragmenting and pulverising it. These include millipedes, earthworms, beetles, etc.
  • Decomposers - These are microscopic organisms which help in chemically breaking down the complex organic compounds in freshly deposited detritus or fragmented detritus to simpler inorganic minerals or turning them into organic humus.

Both decomposers and detritivores can be eaten by smaller animals which in turn can be eaten by larger animals. 

                                               Fig: Detritus food chain

Fundamental laws governing energy flow

First law of thermodynamics

It states that energy can neither be created nor be destroyed. It can only be transferred from one form to another. Hence the total energy of the universe is constant. During photosynthesis, there is a transformation of light energy (sunlight) into chemical energy (food).

Second law of thermodynamics

According to this law, the energy transformation is not hundred percent efficient. So, some energy is dissipated as heat energy into the surroundings, while transforming from one form to another. Hence while energy gets transferred from one trophic level to another, some part of the energy is lost as heat, which cannot be reused. Therefore, energy flow in an ecosystem is unidirectional or one way flow from producers through herbivores to carnivores. So the energy can be used only once and cannot be recycled.

Law of 10% energy transfer in ecosystem 

The 10 percent law is followed in the energy transfer between trophic levels. This law states that only 10 percent of the energy is transferred from one trophic level to the next higher trophic level. The remaining 90 percent energy is lost as heat or is used in metabolic activities. This law was proposed by Linderman in 1942.

For example, say a plant receives 20000 kJ of energy from the sun. 10% of it will be transferred to herbivores such as a deer, i.e, 2000 kJ. A fox feeding on this deer will receive only 10% of 2000kJ, that is, 200 kJ. Only 10% of this 200 kJ, that is, 20kJ will be transferred to a tiger feeding on the fox.

                                        Fig: 10 percent law of energy transfer

Only 10% is passed to the next trophic level because almost 90% of the energy is lost to the environment as heat. Here the second law of thermodynamics is applied in the ecosystem. 

So every organism receives energy through the food chain. But what happens to the energy once the organism dies? Once the organism dies, the body of the organism is decomposed by decomposers. They secrete digestive enzymes that break down the complex organic compounds in the dead and decaying organic matter into simple, inorganic materials, which are subsequently absorbed by them. The energy trapped within the dead remains of organisms is absorbed by the detritivores and decomposers when they feed on these remains and is partly used for metabolic activities and partly lost to the environment as heat released during respiration . The detritus food chain does not follow the 10% law of energy transfer. 

The energy available for the individuals at higher trophic levels will be gradually declining. Hence after 5th trophic level, the available energy will be insufficient for survival. This is the reason why the trophic levels are limited. 

Pyramid of energy

The graphical representation of the relationship between the organisms at different trophic levels of a food chain in terms of biomass, numbers and energy is called an ecological pyramid. There are different types of ecological pyramids and the one which shows the transfer of energy from one trophic level to the next higher trophic level is called the pyramid of energy. The energy pyramid represents energy transfer in a grazing food chain. It represents the total energy at each trophic level of a food chain and exhibits that there is loss of energy at successive trophic levels as we go from lower trophic levels to higher ones because energy flow follows the 10% law of energy transfer. Thus, maximum energy is available at the level of producers which form the broad base of the pyramid. Minimum energy is available at the highest trophic level which is located at the apex of the pyramid. Thus energy pyramids are always upright. 

                                            Fig: Ecological pyramid of energy 

Energy flow models

The energy flow models connect the trophic levels with one another, displaying the energy inputs and losses at each level. According to the premise that plants and animals can be grouped into trophic levels and that both plants and animals are subject to the same laws of thermodynamics, it was Linderman (1942) who first proposed such a model. He emphasised that net primary production and the efficiency at which food energy is converted into biomass dictate the amount of energy at the trophic level.

The flow of energy through the different trophic levels can be explained in two models as follows: 

  • Single channel energy flow model
  • Y- shaped model of energy flow

Single channel energy flow model

In this system the energy flow is unidirectional or one way flow. Nutrients like carbon, nitrogen, phosphorus, sulphur etc. are reused in the food chain and move in a cyclic manner. But energy in the food chain is not reused when it moves from the producers to the herbivores and then to the carnivores. 

According to this model, the energy is never reverted back to the lower trophic level, when it passes from lower to higher trophic level. Hence organisms at higher trophic levels always depend on the organisms at lower trophic levels for their energy requirements. 

Since the energy flow is unidirectional, if the primary energy source, that is, sun gets destroyed, then the ecosystem will collapse. 

The given figure represents a single channel energy flow model. At each successive trophic level there is a huge loss of energy. The line at the bottom of the diagram shows the amount of energy lost at each transfer point when the total solar input is around 3,000 Kcal per square metre per day. Here, 

I - total solar energy input

LA - Light absorbed by plant cover

PG - Gross primary production

A - Total assimilation

PN - Net primary production

P - Secondary production

NU - Energy not used or stored

NA - Energy not assimilated by consumers

R - Respiratory loss

                                                Fig: Single channel energy flow model

Explanation of single channel model

This model depicts the energy flow in three trophic levels in a linear food chain. The boxes represent the trophic levels which include producers, herbivores and carnivores. The pipelines represent the energy flow in and out of the trophic levels. The size of the box depicts the energy stored in the form of biomass at that trophic level. When the pipelines get narrower, it shows that there is a loss of energy at every successive trophic level. Hence the size of the box also reduces, because there is also a corresponding decline in energy stored in biomass.

According to the first law of thermodynamics, the energy inflows and outflows of the system must be equal, and according to the second law of thermodynamics, each energy transfer is followed by a loss of energy in the form of heat energy during respiration. This heat energy is unavailable for the organisms. 

Y- shaped model of energy flow

The simultaneous working of grazing and detritus food chains in ecosystems is depicted in the Y-Shaped energy flow model or also known as the double channel model. Energy can flow through both detritus and grazing food chains and both these food chains are interconnected in the same ecosystem. For example, the animals which are part of a grazing food chain, will become a part of a detritus food chain when they die. Hence in this energy flow diagram, one arm represents the grazing food chain and another represents the detritus food chain.

It was E.P. Odum, in 1983 who created a generalised model of Y-shaped energy flow. This is applicable to both aquatic and terrestrial ecosystems. As the grazing and detritus food chains are intimately associated, it is difficult to determine the relative effect of the members involved in these food chains on the breakdown of original primary production. Thus, although in the model two separate arms emerge to represent the grazing and detritus food chains, in nature they are not completely isolated from each other. 

                                                Fig: Y-Shaped energy flow model

Explanation of Y-shaped energy flow model

In this Y shaped flow of energy, one arm represents the grazing food chain and the other arm represents the detritus food chain. They are separated sharply. In the grazing food chain, herbivores are feeding on the living plants. This will have a direct impact on the population of plants. All the plants which are not eaten by the herbivores will be available to the decomposers after death of the plants. Hence, decomposers will not be able to influence the rate of supply of their food directly. 

According to Odum the Y-Shaped energy flow model is the more realistic one than the single channel energy flow model. The reasons are as follows:

  • Since it includes both grazing and detritus pathways, it confirms the basic stratified structure of the ecosystem.
  • The separation of grazing food chain and detritus food chain happens in this model and it separates in both time and space.
  • Microconsumers and macroconsumers differ greatly in size metabolism relations. 

Importance of energy flow in ecosystem 

The flow of energy is important in the ecosystem because: 

  • Ecological balance is maintained.
  • With the help of photosynthesis, producers synthesise their food.
  • For the growth and development of plants energy is required.
  • The stored energy in the producers gets transferred when primary consumers feed on them.
  • When secondary consumers feed on the primary consumer, this energy is passed on to secondary consumers. 

Practice Problems

Q 1. Assertion : The number of trophic levels in the grazing food chain is restricted.

Reason : Energy decreases when it gets transferred from lower trophic level to the higher trophic level. 

a. Both the assertion and reason are true and the reason is the correct explanation of the assertion
b. Both the assertion and reason are true but the reason is not the correct explanation for the assertion
c. The assertion is true but the reason is false
d. Both assertion and the reason are false

Answer: A sequence of organisms where one living organism eats another, and later that organism is consumed by larger organisms is called the food chain. Different organisms occupy different trophic levels of a food chain. The 10 percent law is followed by organisms in each trophic level. According to the 10 percent law of energy transfer, the energy transferred from lower trophic level to higher trophic level is only 10%. Therefore, when the herbivore is eaten by a carnivore, the energy passed from herbivore to carnivore is only a small amount of total energy and some energy will be 'wasted' as heat or 'used up' by the carnivore. 

Thus as the primary carnivore gets eaten by the secondary carnivore and the secondary carnivore gets eaten by the top carnivore, the amount of energy is reduced at every trophic level and the total amount of energy available at the highest trophic level, that is trophic level 4 or 5 is very less. Any organism at a higher trophic level will not receive enough energy that can support its survival. Hence the number of trophic levels is limited to 4 - 5 in both detritus and grazing food chain.

Hence the correct option is a.

Q 2. In the given image the amount of energy available for grasshoppers in the food chain is 2000 KJ. now find out the amount of energy available for the eagle.

a. 200 KJ
b. 20 KJ
c. 2 KJ
d. 0.2 KJ

Answer: According to the 10% law of energy transfer, energy flows from one trophic level to another and only 10 percent is passed from one trophic level to the next. The second trophic level is occupied by grasshoppers in the given food chain. It is given that the energy available for grasshoppers is 2000 KJ. Then, the energy transferred to the frog will be 10 percent of the 2000.

Energy transferred from grasshopper to frog = 10/100 x 2000 = 200 KJ. 

Energy transferred from frog to python = 10/100 x 200 = 20 KJ. 

Energy transferred from python to Eagle = 10/100 x 20 = 2 KJ. 

Hence the correct option is c

Q 3. With respect to DFC, which of the following statements is incorrect?

a. Detritivores meet their energy requirements from the sun
b. It is composed of detritus-eating organisms
c. Much larger energy flows through this food chain in the terrestrial ecosystems
d. They release nutrients in soil for autotrophs

Answer: Detritus food chain is also known as DFC. Dead and decaying organic material initiate the DFC. Detritivores are animals like earthworms, millipedes, etc that feed on detritus (freshly deposited dead and decaying organic matter or faeces) to obtain energy. In the process they fragment the detritus and pulverise it. Heterotrophic organisms like bacteria and fungi are decomposers. They feed on the dead and decayed materials to meet their energy requirements. For this they will convert the complex organic compounds in the detritus to simple organic and inorganic molecules. These organisms while degrading the material release nutrients into the soil which can be taken up by plants. Hence the correct option is a. 

Q 4. Assertion : The driving force or primary source of energy for most of the ecosystems on earth is the same.

Reason : Except for the deep sea hydrothermal ecosystem, the sun is the only source of energy for most ecosystems.

a. Both the assertion and reason are true and the reason is the correct explanation of the assertion
b. Both the assertion and reason are true but the reason is not the correct explanation for the assertion
c. The assertion is true but the reason is false
d. Both assertion and the reason are false

Answer: The sun is the only source of energy for all ecosystems on Earth. Of all the radiation reaching Earth, less than 50% of it is photosynthetically active radiation (PAR) and autotrophic organisms are able to make food from simple inorganic materials using this energy. So, all organisms are dependent directly or indirectly on producers (green plants, algae) for their food or energy. Hence assertion is a true statement. Deep sea hydrothermal ecosystems are an exception to the dependence of organisms on the radiation from the sun, directly or indirectly for energy. Deep sea hydrothermal ecosystems lack sunlight. It also has toxic minerals, extreme temperatures and pressures etc. The conversion of toxic minerals, sulphur compounds and heat into food and energy is done by the bacteria living in such ecosystems. This process is known as chemosynthesis. Hence both the assertion and reason are true and the reason is the correct explanation of the assertion. Hence the correct option is a. 


Q 1. What is meant by trophic structure?
Answer: The partitioning of biomass between different trophic levels is called trophic structure. The organisms are arranged into different trophic levels within the trophic structure. It also shows the feeding relationships between different organisms, within a set area and time. 

Q 2. What is standing crop?
Answer: Standing crop refers to the total amount of biomass at a particular trophic level.

Q 3. What is the importance of studying about the food chain?
Answer: Food chains show the detailed relationships in ecosystems. It can show how every organism depends on each other for their survival. It also depicts the problems caused when a producer or consumer is lost from the ecosystem.

Q 4. Who introduced the food chains?
Answer: Arab scientist Al-Jahiz introduced the food chains for the first time in the 10th century. Then it was published and popularised in a book by Charles Elton in 1927. Charles Elton introduced the concept of food web. 


Related Topics

The Ecosystem - Components, Structure and Functions

Biomass, Primary Productivity, Secondary Productivity 

Decomposition, Process of decomposition, Factors affecting decomposition 

Nutrient cycling: Carbon cycle and Phosphorus cycle 

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