Transfer of Energy in Food Chains – Energy Flow in Ecosystem

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How is energy transferred throughout a food chain?

The Sun Provides Energy For Maintaining All Life On Earth

All the organisms (plants and animals) depend on the sun for their constant need of energy, and upon earth for the materials which enter into their body. We will now describe how energy received from the sun flows in the various trophic levels of an ecosystem in the form of chemical energy of food.

Transfer Of Energy In Food Chains

The food chain in a community actually represents a stepwise transfer of food and the energy contained in food. The food and energy enter the living components of the ecosystem through the process of photosynthesis. This is because photosynthesis is a process which combines the substances like carbon dioxide, water and sunlight energy to form food like carbohydrates and converts light energy of the sun into chemical energy of carbohydrates. This food and energy is then transferred from the producer organisms to herbivores and from herbivores to carnivores, through the food chain. Let us discuss this flow of energy in detail.

First Step

The green plants (or producers) have a mechanism for trapping solar energy (sun’s energy)-with the help of their green pigment called chlorophyll. The green plants after trapping the solar energy, convert it into chemical energy which is stored as carbohydrates in the plants. Thus, the initial point where energy from the environment enters into the living components of ecosystem (like plants and animals) is the process of preparation of food by green plants through photosynthesis.

On an average, about 1% of the sun’s energy falling on the leaves is used by the plants in the process of photosynthesis and stored as chemical energy of food. The plants utilise the energy stored in them for their metabolic activities like respiration and growth (tissue building). Some of the energy is, however, not utilized and it is released as unusable heat into the community environment (see Figure).

Second Step

The plants (or producers) are eaten up by herbivores. The chemical energy stored in plant food is transferred with food to herbivores. The herbivores utilize this energy for their various metabolic activities like respiration and also for their growth. Some of the energy, however, remains unutilized which is released by the herbivores as heat energy to the environment (see Figure).

Third Step

The herbivores are eaten up or consumed by carnivores. The chemical energy stored in the flesh of herbivores is transferred with food (or flesh) to the carnivores. The carnivores utilize this energy for their various metabolic activities like respiration and also for their growth. Some of the energy, however, remains unutilized by the carnivores and it is released as heat energy into the environment (see Figure). This process of the transfer of energy is repeated with large carnivores or top carnivores (who eat small carnivores), and so on.

It should be noted that some of the energy from producers and consumers (like plants, herbivores and carnivores) is also utilized for the life processes of micro-organisms called decomposers. The decomposers, in turn, release the unutilized energy as heat into the environment (see Figure). It is obvious from the above discussion that the energy which remains unutilized by producers, consumers (herbivores and carnivores) and decomposers is lost into the environment as heat. It is called community heat.

We should remember the following points about the transfer of energy in the ecosystem :

1. Energy is not created in the ecosystem. Energy is only converted from one form to another. For example, light energy coming from the sun is converted into chemical energy of food like carbohydrates by the process of photosynthesis. Thus, photosynthesis converts light energy into chemical energy.

2. There is a continuous transfer of energy from one trophic level of organisms to the next trophic level in a food chain. For example, producers like plants transfer energy to the herbivorous animals like deer, and the herbivorous animals like deer transfer energy to carnivorous animals like lion, so that there is a continuous transfer of energy in the food chain : Plants → Deer → Lion. This transfer of energy takes place in the form of chemical energy of food.

3. At each trophic level of organisms, some of the energy is utilized by the organisms for their metabolic activities like respiration, and for growth.

4. A part of the energy at each trophic level (like producers, herbivores and carnivores) is utilized for the functioning of decomposers.

5. There is a loss of energy at each energy transfer in various trophic levels of organisms which goes into the environment and remains unutilized. In other words, we can say that the amount of energy available at each successive trophic level is less than the energy available at the producer level. Thus, when we move from the first trophic level of producers (plants) to second trophic level of herbivores and third trophic level of carnivores, the amount of energy available gradually decreases. This is because at each trophic level, energy is lost as heat energy which goes into the environment.

Flow of Materials in Ecosystem is Cyclic but Flow of Energy is Unidirectional

The materials like water, carbon (as carbon dioxide) and nitrogen (as minerals) are taken up by the plants from soil, air and water bodies, etc., and made into food. This food is then passed on to the animals like herbivores and carnivores in a food chain. After the death and decay of plants and animals, the materials like water, carbon and nitrogen present in their bodies are returned to soil, air and water, from where they were taken originally.

These materials can then be reused for the growth of new plants. In this way, the same materials are used again and again, the materials are not lost from the environment. So, the flow of materials like water, carbon and nitrogen, etc., in the ecosystem is said to be cyclic. This is not so in the case of energy.

The flow of energy in the ecosystem is unidirectional (or one-directional). The energy enters the plants (from the sun) through photosynthesis during the making of food. This energy is then passed on from one organism to another in a food chain. Energy given out by the organisms as heat is lost to the environment, it does not return to be used by the plants again. This makes the flow of energy in ecosystem ‘unidirectional’. Thus, the flow of energy in the ecosystem is said to be unidirectional because the energy lost as heat from the living organisms of a food chain cannot be reused by plants in photosynthesis.

Ten Percent Law

During the transfer of energy through successive trophic levels in an ecosystem, there is a loss of energy all along the path. No transfer of energy is 100 percent. The studies of transfer of energy in different food chains in a large number of ecosystems has revealed a uniform pattern of transfer of energy, which is given by 10 percent law. The 10 percent law which was given by Lindeman in the year 1942, is one of the most useful generalisations about the magnitude of loss of energy in food chains.

According to ten percent law, only 10 percent of the energy entering a particular trophic level of organisms is available for transfer to the next higher trophic level. All the energy transfers in food chains follow the 10% law which in simple terms means that the energy available at each successive trophic level is 10 percent of the previous level.

Thus, there is a progressive decline (gradual reduction) in the amount of energy available as we go from producer level to the higher trophic levels of organisms. Figure 24. Raymond Lindeman : Let us take one example to understand the 10 percent law more clearly. The scientist who gave ten percent law for energy transfers in food chains.

Suppose 1000 joules of light energy emitted by the sun falls on the plants (called producers). We know that the plants convert only one percent (1%) of the light energy falling on them into chemical energy of food. So, the energy which will be available in plant matter as food will be only 1% of 1000 joules, which comes to 10 joules.

The remaining 1000 – 10 = 990 joules of light energy or solar energy which is not utilized by the plants is reflected back into the environment (see Figure). Please note that the ten percent law will not apply at this stage. It will apply only in the transfer of energy in the food chain.

We will now apply the 10 percent law to the food chain : Plants → Herbivores → Carnivores (represented by plants, deer and lion in Figure). The plants or first trophic level has 10 joules of energy in it. Now, according to ten per cent law, only 10% of 10 joules of energy (which is 1 joule) will be available for transfer at the next trophic level, so that the herbivore (deer) will have only 1 joule of energy stored as food at the second trophic level (see Figure).

Applying the ten percent law again we find that 10% of the remaining 1 joule (which is 0.1 joule) will be transferred to third trophic level of carnivore (lion). So, the energy available in the lion as food will be only 0.1 joule (see Figure). We will now solve some problems based on ten percent law.

Example Problem 1.
Calculate the amount of energy available to lion in the following food chain if plants have 20000 J of energy available from the sun :
Plants → Deer → Lion
Solution:

1. Plants can trap only 1% of the sun’s energy falling on them. Now, 1% of 20000 J is 200 J, so the plants have actually 200 J of energy available in them as food (The 10 percent law does not apply at this stage).
2. The plants are eaten up by deer. Now, according to 10 percent law, 10% of 200 J, that is, 20 J of energy will be available in deer as flesh food.
3. The deer will transfer 10% of its 20 J energy to the lion. Thus, the food energy available to the lion will be 10% of 20 J which comes to 2 J.

The above results can be shown more clearly as follows :

Example Problem 2.
Consider the following food chain :
Grass → Mice → Snakes → Peacocks
If in this chain, 100 J of energy is available at the producer level, then calculate the energy transferred to the peacocks as food. State the law used in the calculations.
Solution:
The producer level in this food chain is grass, so 100 J of energy is available in grass as food. We have now to apply 10 percent law to the above food chain :

1. According to ten percent law, 10% of the energy of grass will be available as food in mice. Thus, the energy available to mice will be 10% of 100 J, which is 10 J.
2. The energy available to snakes will be 10% of 10 J which is 1 J.
3. And finally, the energy available to peacocks will be 10% of 1 J, which is 0.1 J.

The above conclusions can now be depicted as follows :

Why the Number of Trophic Levels in a Food Chain is Limited

At each trophic level in a food chain, a large portion of the energy is utilized for the maintenance of organisms which occur at that trophic level and lost as heat. As a result of this, organisms in each trophic level pass on less and less energy to the next trophic levels, than they receive. The longer the food chain, the less is the energy available to the final member of food chain. Food chains generally consist of three or four steps (three or four organisms) because after that the energy available for the next organism will be so small that it will be insufficient to sustain the life of that organism. There are, however, some food chains containing five steps (or five organisms) but there are rarely more than five steps (or five organisms) in a food chain. We will now discuss the accumulation of harmful chemicals in food chains.

Accumulation of Harmful Chemicals in Food Chains

The accumulation of harmful chemicals such as pesticides in the living organisms like plants, animals and humans (men) unknowingly, through the food chain, is called bioconcentration of pesticides. This happens as follows.

Pesticides are poisonous chemical substances which are sprayed over crop plants to protect them from pests (harmful small animals) and diseases. These chemical pesticides mix up with soil and water. From

soil and water, these pesticides are absorbed by the growing plants alongwith water and other minerals. When herbivorous animals eat plant food, then these poisonous chemical pesticides go into their bodies through the food chain. And when the carnivorous animals eat herbivores, then the pesticides get transferred to their bodies. Man being an omnivore, eats plant food as well as herbivores. So, the pesticides present in plant food and herbivores also get transferred to the man’s body through food.

Thus, pesticides enter the food chain at the producer level (plant level) and in the process of transfer of food through food chains these harmful chemicals get concentrated at each trophic level. The increase in concentration of harmful chemical substances like pesticides in the body of living organisms at each trophic level of a food chain is called biological magnification.

Pesticides are non-biodegradable chemicals, so they get accumulated at each trophic level. Since humans occupy the top level in any food chain, so the maximum amount of harmful chemical pesticides gets accumulated in our bodies. This damages our health gradually. Tests have shown that the most commonly used pesticide DDT is accumulating in our bodies through the food chains. Please note that the pesticides present in our food grains (like wheat, rice), fruits, vegetables, and meat cannot always be removed by washing, etc. Let us solve one problem now.

Example Problem.
Which of the following will have the maximum concentration of harmful chemicals in its body ?
Peacock, Frog, Grass, Snake, Grasshopper
Solution:
In order to answer such questions, we should first write the food chain involving the given organisms. The organism which occurs at the highest trophic level (on the extreme right side) in the food chain will have the maximum concentration of harmful chemicals in its body. In this case, grass is eaten by grasshopper; grasshopper is eaten by frog; frog is eaten by snake and finally snake is eaten by peacock. So, the food chain will be :
Grass → Grasshopper → Frog → Snake → Peacock
In this food chain, since peacock occurs at the highest trophic level (on the extreme right side), therefore, peacock will have the maximum concentration of harmful chemicals in its body.

How do Our Activities Affect the Environment

We (human beings) are an important part of the environment. Our activities change the environment around us. And the changes in environment then affect us. We will now discuss two environmental problems caused by our activities : depletion of ozone layer, and disposal of domestic wastes (or household garbage).