Why Flow of Materials in Ecosystem is Cyclic but the 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 Per Cent 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 per cent. The studies of transfer of energy in different food chains in a large number of ecosystems have revealed a uniform pattern of transfer of energy, which is given by 10 per cent law.

The 10 per cent 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 per cent law, only 10 per cent 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 per cent 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. Let us take one example to understand the 10 per cent law more clearly.

Suppose 1000 joules of light energy emitted by the sun falls on the plants (called producers). We know that the plants convert only one per cent (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. Please note that the ten per cent 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 per cent law to the food chain: Plants------ > Herbivores ------> Carnivores. 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.

Applying the ten per cent 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. We will now solve some problems based on ten per cent law.

Sample 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:

(i) 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 per cent law does not apply at this stage).

(ii) The plants are eaten up by deer. Now, according to 10 per cent law, 10% of 200 j, that is, 20 J of energy will be available in deer as flesh food.

(iii) 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.

Sample 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 per cent law to the above food chain:

(i) According to ten per cent 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.

(ii) The energy available to snakes will be 10% of 10 J which is 1 J.

(iii) And finally, the energy available to peacocks will be 10% of 1 J, which is 0.1 J.