7. Energy Flow in an Ecosystem

Energy flow in an ecosystem

 We have already learnt the classification of living organisms according to the mode of nutrition into producers, consumers, saprotrophs (saprophytes) and decomposers. Consider the various trophic levels of the ecosystem around you as given below.

Food chain and food web

Interactions go on continuously between producers, consumers and saprophytes. There is a definite sequence in these interactions which is called the food chain. Each chain consists of four, five or more links. An ecosystem consists of many food chains that are interconnected at various levels. Thus a food web is formed.

An organism may be the prey for many other organisms. For example, an insect feeds upon leaves of various plants but the same insect is the prey for different animals like frog, wall lizard, birds, etc. If this is to be shown in a figure, it will form an intricate web instead of a linear food chain. Such an intricate network is called a ‘Food Web’. Generally, food[1]webs are found everywhere in nature.

The energy pyramid

Trophic level

Each level in the food chain is called a trophic level. A trophic level is the step at which the organism obtains its food in the chain. The amount of matter and energy gradually decreases from producers at lowest level to top consumers at the highest level

Figure 7.4 shows the energy transfer that takes place at each trophic level. There are different levels of energy exchange in the food chain. The initial quantity of energy goes on decreasing at every level of energy exchange. Similarly, the number of organisms also decreases from the lowest level to the highest level. This pattern of energy exchange in an ecosystem is called a ‘Pyramid of energy’.

 After the death of apex consumers, their energy becomes available to the decomposers. Fungi and other micro- -organisms decompose the bodies of dead animals. They are called decomposers. In the process of obtaining food from the remains of organisms, decomposers convert them into simple carbon compounds. These substances easily mix with air, water and soil from where they are again absorbed by plants and incorporated into the food chain.

You can now see that due to the food web formed by the various modes of nutrition, energy and various nutrients circulate continuously in the ecosystem. The sun is the most important source of energy in any ecosystem. Green plants of the ecosystem store some of the solar energy in the form of food. Before reaching the decomposers, this energy is passed on from one trophic level to the next. Decomposers dissipate some amount of energy in the form of heat. However, no part of the energy ever returns to the sun. Hence, such passage of energy is referred to as ‘one way’ transport.

Bio-geo-chemical cycle

Though the energy flow in an ecosystem is one way, the flow of nutrients is cyclical. All organisms need nutrients for their growth. Study the various components in the figure given alongside and explain the bio-geo-chemical cycle in your own words. The cyclical flow of nutrients within an ecosystem is called the bio-geo-chemical cycle.

Nutrients, necessary for the growth of organisms are continuously transferred from abiotic to biotic factors and biotic to abiotic factors within an ecosystem. This cycle operates continuously through the medium of the biosphere formed by the lithosphere, atmosphere and hydrosphere. The recycling of biological, geological and chemical sources of nutrients in this process is a complex process and depends upon the level of energy transfer in the ecosystem.

The gaseous cycle is a speedier cycle than the sedimentation cycle. For example, if CO2 has accumulated in an area, it is quickly dispersed with the wind or absorbed by plants. Climatic changes and human activities seriously affect the speed, intensity and equilibrium of these cycles Hence, various aspects of these cycles are extensively studied nowadays.

The carbon cycle

The circulation and recycling of carbon from the atmosphere to living organisms and after their death back to the atomsphere is called the carbon cycle. Abiotic carbon atoms are circulated and recycled into biotic form mainly through photosynthesis and respiration. Hence, the carbon cycle is one of the important bio-geo-chemical cycles.

Plants convert carbon dioxide into carbohydrates by the process of photosynthesis. Similarly, they produce carbon compounds like proteins and fats, too. Herbivores feed upon plants. Carnivores feed upon herbivores. In this way, biotic carbon is transported from plants to herbivores, from herbivores to carnivores and from carnivores to apex consumers.

Eventally, after death, all types of consumer, are decomposed by decomposers like bacteria and fungi and carbon dioxide is released again into the atmosphere and is used again by living organism. In this way, carbon is continuously passed on from one living organism to another. After the death of living organisms, carbon goes to the atmosphere and is again taken up by living organisms.

The oxygen cycle

Oxygen forms 21% of the amosphere. It is also present in the hydrosphere and lithosphere. Circulation and recycling of oxygen within the biosphere is called the oxygen cycle. This cycle, too, includes both the biotic and abiotic components. Oxygen is continuously produced as well as used up in the atmosphere.

Oxygen is highly reactive and it readily reacts with other elements and compounds. As oxygen is found in various forms like molecular oxygen (O2 ), water (H2 O), carbon dioxide (CO2 ), inorganic compounds, etc. the oxygen cycle of the biosphere is extremely complex. Oxygen is released in the process of photosynthesis whereas it is used up in processes like respiration, combustion, decomposition, corrosion, rusting, etc.

The nitrogen cycle

Nitrogen forms 78% i.e. the maximum portion of the atmosphere. It is necessary for the maintenance of the cycle of nature. The circulation and recycling of nitrogen gas into the form of different compounds through various biotic and abiotic processes in nature is called the nitrogen cycle.

All organisms participate in the nitrogen cycle. It is an important component of proteins and nucleic acids. As compared to other elements, it is inactive and does not easily combine with other elements. Most organisms cannot use the free form of nitrogen.

Important processes of the nitrogen cycle

  1. Nitrogen fixation: Conversion of nitrogen into nitrates and nitrites through atmospheric, industrial and biological processes. 2. Ammonification: Release of ammonia through decomposition of dead bodies and excretory wastes of organisms. 3. Nitrification: Conversion of ammonia into a nitrite and then nitrate. 4. Denitrification: Conversion of nitrogen compounds into gaseous nitrogen.