Functions of Ecosystem

In an ecosystem there are two processes proceeding simultaneously:

1 ) Energy flow and

2 ) Biogeochemical cycle

The energy flow is in a single direction and is non-cyclic where as Biogeochemical flow is cyclic (Any mineral cycle)

Biogeochemical flow

Energy flow

Although matter circulates, energy cannot be reused indefinitely. Solar energy is converted by the photosynthetic producers (plants) into chemical energy in the form of plant carbohydrates. Herbivores consume the plant carbohydrates and so this chemical energy is transferred to them. Carnivores consume herbivores. So the energy is circulated further to the next trophic level. In these animals, this chemical energy is converted mostly into mechanical energy (work done) and heat. The heat is lost to the atmosphere at each trophic level. At the cell level, chemical energy in the food molecules is transferred into usable energy and stored as ATP (adenosine triphosphate) molecules. The body cells use the energy stored in ATP for synthesis of new chemical compounds and their transport and for mechanical work. It is estimated that 90% of the energy is used up at each trophic level and only 10% of it is transferred to the next trophic level. Finally, at the last trophic level (decomposer) no energy is left for recycling. Hence, energy flows from sun through producers to consumers in a single direction only. The decreasing energy level at each step sets the limit of trophic levels to a maximum of 4 or 5. It is the continual trapping of light energy by green plants that makes good this loss and maintains the uninterrupted flow energy in an ecosystem.

It is found that there is maximum energy at the producer (plant) level and as you go further and further the energy in food goes on decreasing. Therefore, the herbivores get more energy rich food, than carnivores.

Biogeochemical cycles

Organic materials synthesised by the producers are eaten and assimilated by the consumers. With the help of decomposers, all the organic materials in the bodies of the consumers are eventually broken down into inorganic materials. These are then rebuilt into organic compounds by the synthetic activities of the consumers. Thus, matter circulates in nature. Though it may constantly change it's form, there is no overall loss or gain.

The cyclic flow of nutrients between non-living environment (soil, rocks, air, water) and living organisms is known as biogeochemical cycle. The major nutrient element i.e. carbon, hydrogen, oxygen and nitrogen, which form about 95% mass of the living organism, are circulated again and again between living and non-living components of the ecosystem.

Let us study the carbon cycle and the nitrogen cycle

Carbon cycle

The atmosphere contains a pool of CO₂. CO₂ is removed from the pool by the photosynthetic activities of plants. It is released back into the environment by respiration which is carried out by all organisms including those micro-organisms (decomposing bacteria) responsible for the decay of dead plants and animals. Not all dead material decays. Sometimes dead plants and animals form fossil fuels such as peat, coal, petroleum and petroleum-based gases. Man uses these materials as sources of energy and when they are burned CO₂ is returned to the CO₂ pool.

The Carbon Cycle

The amount of carbon dioxide in the atmosphere is maintained by a balance between the processes that withdraw carbon dioxide from it (photosynthesis) and those, that add carbon dioxide to it (respiration and combustion). The red arrows represent the flow of carbon dioxide.

Nitrogen cycle

The nitrogen cycle is complex as it depends on the activities of various bacteria. Nitrate in the soil is built up by plants into protein which is then passed on to animals, which eat the plants. The nitrogen from the protein is returned to the environment as ammonia (ammonification), either from excretory matter or through the action of decomposing bacteria on dead bodies. Various nitrifying bacteria convert the ammonia back to nitrate (nitrification). The nitrifying bacteria play an important role in replenishing the environment with nitrate upon which plants are dependent for their protein. Another class of bacteria called denitrifying bacteria liberate free nitrogen from nitrate which leads to the pool of nitrogen gas being built up in the atmosphere. Some of this nitrogen is constantly removed by nitrogen fixing micro-organisms, which build it up into nitrate and ultimately protein.

The Nitrogen cycleThe amber arrows represent the flow of nitrogen