7. Energy Flow in an Ecosystem

Corrected: Carnivores occupy the third trophic level in the food chain.

Justification: every food chain begins with the producers (green plants), which form the first trophic level. Herbivores feed on plants and form the second trophic level. Carnivores feed on herbivores, so they form the third level. For example, in grass → grasshopper → frog → snake, the frog (a carnivore) is at the third level.

Corrected: The flow of energy in an ecosystem is considered to be a ‘one way’ transport. (The flow of nutrients is cyclic.)

Justification: energy comes from the sun, passes from producers to consumers, and part of it is lost as heat at every level; it never returns to the sun. Nutrients, on the other hand, return from dead organisms to the soil, air and water through decomposers, and are used again by plants. So energy flows one way, while nutrients cycle.

Corrected: Plants in an ecosystem are called producers. (Herbivores are the primary consumers.)

Justification: green plants make their own food by photosynthesis using sunlight, carbon dioxide and water, so they are autotrophs or producers. Animals that feed directly on plants, such as the grasshopper, squirrel and elephant, are the primary consumers.

• The sun is the source of energy in an ecosystem. Green plants store a part of the solar energy in the form of food.
• This energy passes from producers to herbivores, carnivores and apex consumers, and finally to decomposers.
• At every level a large part of the energy is used for life processes and lost as heat. Decomposers also dissipate energy as heat.
• This heat energy cannot be used again by plants, and no part of the energy ever returns to the sun.
• Therefore energy moves only in one direction, from the sun through the food chain; its flow is ‘one way’.

• All living organisms need nutrients such as carbon, oxygen and nitrogen for growth and life processes. These cycles keep supplying them.
• The amount of each element on earth is limited, so it must be continuously returned from organisms to the environment and taken up again.
• If one part of a cycle becomes faster or slower, the balance is upset. For example, excess burning of fuels raises the CO₂ in the air and causes global warming; cutting forests reduces the oxygen released and the CO₂ absorbed; loss of soil bacteria reduces nitrates and soil fertility.
• Therefore equilibrium in these cycles is necessary to keep the ecosystem stable and life on earth healthy.

• Plants take nutrients such as carbon dioxide, water, nitrates and minerals from the air, water and soil (abiotic components).
• These nutrients pass to herbivores and carnivores through the food chain (biotic components).
• When organisms die, decomposers break down their bodies and wastes into simple substances, which mix with the air, water and soil.
• Plants absorb these substances again. Thus the same nutrients move again and again from the abiotic to the biotic components and back; their flow is cyclic.

The circulation of carbon from the atmosphere to living organisms and, after their death, back to the atmosphere is the carbon cycle.

• From air to plants: green plants take carbon dioxide from the air and, in the presence of sunlight and chlorophyll, convert it into glucose by photosynthesis. They also make proteins and fats.
• Through the food chain: herbivores eat plants, carnivores eat herbivores, and so on; the carbon compounds pass from one level to the next.
• Back to air: all organisms release carbon dioxide by respiration. After death, decomposers break down their bodies and wastes and release carbon dioxide.
• Long-term storage: remains buried for millions of years form coal and petroleum. Burning these fossil fuels, wood and forest fires, and volcanoes return carbon to the air. Oceans dissolve and release carbon dioxide.

Nitrogen forms 78% of the atmosphere and is needed to make proteins and nucleic acids, but most organisms cannot use free nitrogen. The circulation of nitrogen through its different compounds is the nitrogen cycle. Its main steps are:

1. Nitrogen fixation: free nitrogen of the air is converted into compounds such as nitrates. This is done by nitrogen-fixing bacteria (Rhizobium in the root nodules of leguminous plants, Azotobacter in soil), by lightning, and industrially in fertiliser factories.
2. Absorption: plants absorb nitrates from the soil through their roots and use them to make proteins. Animals get nitrogen by eating plants.
3. Ammonification: when plants and animals die, and from their excretory wastes, decomposers release ammonia.
4. Nitrification: nitrifying bacteria in the soil convert ammonia into nitrites and then into nitrates, which plants can use again.
5. Denitrification: denitrifying bacteria convert some nitrates back into free nitrogen gas, which returns to the atmosphere. The cycle is complete.

Oxygen forms 21% of the atmosphere and is also present in water and in the earth’s crust. The circulation of oxygen within the biosphere is the oxygen cycle.

• Release of oxygen: green plants and algae release oxygen into the air during photosynthesis.
• Use of oxygen: animals, humans, plants and aerobic microbes use oxygen for respiration and give out carbon dioxide and water. Oxygen is also used up in burning, decomposition, corrosion and rusting.
• Return: the carbon dioxide and water formed are used again by plants in photosynthesis, which releases oxygen once more.
• Some oxygen in the upper atmosphere is converted into ozone (O₃), which protects us from ultraviolet rays.

Because oxygen is highly reactive and exists in many forms (O₂, H₂O, CO₂, oxides), the oxygen cycle is closely linked with the carbon cycle and is very complex. Photosynthesis and respiration keep the amounts of oxygen and carbon dioxide in the air in balance.

• Plant and protect trees: trees absorb carbon dioxide and release oxygen. I would take part in tree plantation and oppose unnecessary cutting of trees.
• Save fuel and electricity: using public transport, a bicycle or walking, and switching off lights and fans when not needed, reduces the burning of fossil fuels and the CO₂ released.
• Avoid burning waste: I would not burn dry leaves, garbage or plastic, which releases CO₂ and harmful gases.
• Compost organic waste: kitchen and garden waste can be turned into compost, so that decomposers return nutrients to the soil.
• Use fewer chemical fertilisers: use organic manure, and grow leguminous crops such as gram and peas, whose root-nodule bacteria fix nitrogen naturally.
• Keep water bodies clean: do not throw waste into rivers and lakes, so that aquatic plants and animals can carry out their part of the cycles.
• Spread awareness: tell family and friends about global warming, pollution and conservation.

Food chain: the definite sequence in which food, and the energy in it, passes from producers to consumers is a food chain. Each step is one link, or trophic level. For example: grass → grasshopper → frog → snake → eagle. A food chain usually has four, five or more links.

Food web: in nature an organism rarely eats only one kind of food or is eaten by only one kind of animal. An insect feeds on the leaves of many plants and is eaten by frogs, lizards and birds. So food chains cross and join each other at many points. The network formed by many interconnected food chains is a food web.

Inter-relationship:

• A food web is made up of many food chains. The food chain is the unit, and the food web is the complete network of these units.
• Each food chain shows only one path of energy flow, but a food web shows all the possible paths in an ecosystem.
• An organism can occupy different trophic levels in different food chains of the same web. A bird eating seeds is a primary consumer, while the same bird eating grasshoppers is a secondary consumer.
• A food web gives stability to the ecosystem. If one food chain is broken, for example if the number of rabbits falls, the jackal can still eat squirrels. A single food chain has no such alternative.
• More food chains in a web means a more balanced and stable ecosystem.

The cyclical flow of nutrients between the abiotic and biotic components of an ecosystem is called the bio-geo-chemical cycle. There are two types:

Importance:

• They continuously supply the nutrients that all organisms need for growth, repair and life processes.
• The amount of each element on earth is limited; these cycles return it from dead organisms to the environment, so it can be used again and again.
• They keep the composition of air, water and soil balanced — for example the balance of oxygen and carbon dioxide in the air, and of nitrates in the soil.
• They link the lithosphere, hydrosphere, atmosphere and biosphere, and keep ecosystems stable.
• They maintain soil fertility, which is necessary for agriculture.

The amount of energy decreases at every step as it passes from plants to apex consumers. Each organism uses most of the energy it gets for its own life processes (respiration, movement, growth), and a large part is lost as heat. Only about 10% of the energy at one level is passed on to the next. This pattern is called the pyramid of energy.

Example: in an aquatic ecosystem, if the phytoplankton hold 10,000 kcal of energy, the zooplankton that eat them get only about 1,000 kcal, the fishes about 100 kcal, and the humans who eat the fish only about 10 kcal. Because so little energy reaches the top, the number of apex consumers is always small, and food chains rarely have more than four or five links.

1. Direction: matter (nutrients) flows in a cycle; energy flows only one way.
2. Source: matter comes from the air, water and soil of the earth itself; energy comes from outside, from the sun.
3. Reuse: matter is used again and again; energy, once used, cannot be used again.
4. Loss: the total amount of matter stays constant — it only changes form. Energy is lost as heat at every trophic level.
5. Role of decomposers: decomposers return matter to the soil and air for plants to use again; the energy they release is lost as heat.
6. Example: the carbon a tiger takes in by eating a deer returns to the air as CO₂ and can be used by grass again. The energy in the grass is mostly lost as heat by the deer and the tiger, and never comes back to the grass.

Why: matter can neither be created nor destroyed, and decomposers convert dead remains back into simple substances that plants can absorb again, so matter circulates. Energy is converted into heat during every life process, and plants can use only sunlight, not heat, to make food. So energy cannot go back up the chain and must be supplied continuously by the sun.

For example, a model of the water or carbon cycle on a thermocol or cardboard base. Show the sun, clouds, trees, animals, a factory and the soil. Join them with coloured arrows made from wire or paper strips, and label each process (photosynthesis, respiration, decomposition, combustion). Keep a short chart beside the model explaining each step.