Environmental Science

LESSON 5. Energy Flow In The Ecosystem

The Water Cycle

Water is the essence of life on the earth, without which living organisms cannot survive. The hydrologic cycle is a continuous natural process of transportation of water from the reservoirs and oceans to the atmosphere, then to the land surface, and finally back to the reservoirs and oceans. During this process of transportation, water takes the form of liquid, solid and gas at different stages of the cycle. Water, in the form of liquid in reservoirs and oceans, evaporates and converts itself into gaseous state to form clouds; water then precipitates in the form of liquid as rain, and as solid in the form of snow and hail. When it rains, the water runs along the ground and flows into rivers or falls directly into the sea. A part of the rainfall is intercepted by the plant leaves, buildings and other objects on the land surface. A part of the rainwater that falls on land percolates into the ground. This is stored underground throughout the rest of the year. Water is drawn up from the ground by plants along with the nutrients from the soil. The water is transpired from the leaves as water vapour and returned back to the atmosphere which will fall back as rainfall. This process of movement of water in a hydrologic cycle involves the dynamic aspects of water.

 The Carbon cycle

Carbon is a building block of both plant and animal tissues. The carbon, which occurs in organic compounds, is included in both the abiotic and biotic parts of the ecosystem.  In the atmosphere, carbon occurs as carbon dioxide (CO₂). The atmosphere is a minor reservoir of carbon dioxide, while the oceans are major reservoirs of CO₂ with 50 times the concentration as compared to air. About  2700 billion tons of carbon dioxide is available in the atmosphere and about 6600 billion tons in the biosphere, vegetation, and soil and about 1,36,000 billion tons in the oceans. In the presence of sunlight, plants take up carbon dioxide from the atmosphere through their leaves. The plants combine  carbon dioxide with water, which is absorbed by their roots from the soil. In the presence of sunlight they are able to form carbohydrates that contain carbon. This process is known as photosynthesis. Plants use this complex mechanism for their growth and development. In this process, plants release oxygen into the atmosphere on which animals depend for their respiration. Plants therefore help in regulating and monitoring the percentage of Oxygen and Carbon dioxide in the earth's atmosphere. All of mankind thus depends on the oxygen generated through this cycle. It also keeps the CO₂ at acceptable levels. Plants, human beings and animals release carbon dioxide during  respiration and also as excreted waste. The dead bodies of plants and animals also return carbon to the soil thereby completing the  processes of the carbon cycle.

The Oxygen Cycle

Oxygen is an important element for life in the biosphere. It is taken up from the air by plants, human beings and animals for respiration. However it is returned back to the atmosphere by the plants during photosynthesis. There exists a link between Oxygen Cycle and Carbon Cycle. Afforestation generally increases the oxygen levels in our atmosphere thereby plays an important role in our lives.

The Nitrogen Cycle

The air in troposphere, the air which we breathe, consists, by volume, of about 78 % nitrogen, 21 % oxygen, 1 percent argon and 0.03 % carbon dioxide. Also, present are traces of other gases, most of which are inert. Nitrogen is fixed either by the physical process of lightening or biologically by some bacteria and / or blue green algae. When animals defecate, this waste material is broken down by worms and insects mostly beetles and ants. This material is thus broken down further into nutrients that plants can absorb and use for their growth. Thus nutrients are recycled back from their growth. Thus nutrients are recycled back from animals to plants. Similarly the bodies of dead animals are also broken down into nutrients that are used by the plants for their growth. Thus the nitrogen cycle on which life is dependent is completed.

Nitrogen fixing bacteria and fungi in soil  gives this important element to plants, which absorb it as nitrates. The nitrates are a part of the plant's metabolism, which help in forming new plant proteins. This is used by animals that feed on the plants. The nitrogen is then transferred to carnivorous animals when they feed on the herbivores.  When we think of food webs, we usually think of the large mammals and other large forms of life. But we need to understand that it is the unseen small animals, plants and microscopic forms of life that are of great value for the functioning of the ecosystem. Circulation of nitrogen between living components and soil/ atmosphere is mediated by a group of micro-organisms which convert one form of nitrogen into another.

The Energy Cycle

The energy cycle is based on the flow of energy through the ecosystem. Flow of energy in an ecosystem takes place through the food chain and it is the energy flow which keeps the ecosystem going. The flow of energy is unidirectional or one way flow. According to the first law of thermodynamics, energy can neither be created nor be destroyed but it can only be transformed from one for to another. Energy from sunlight is converted by plants themselves into growing new plant material which includes leaves, flowers, fruit, branches, trunks and roots of plants. Since plants can grow by converting the sun's energy directly into their tissues, they are known as producers in the ecosystem. The solar energy captured by the green plants (producers) gets converted into biochemical energy of plants and later into that of consumers. The plants are used by herbivorous animals as food, which gives them energy. A large part of this energy is used up for day to day function of these animals such as breathing, digesting food, supporting growth of tissues, maintaining blood flow and body temperature. Energy is also used for activities such as looking for food, finding shelter, breeding and bringing up young ones. The carnivores in turn depend on herbivorous animals on which they feed. Thus the different plant and animal species are linked to one another through food chains. Each food chain has three or four links. However as each plant or animal can be linked to several other plants or animals, these inter-linked chains can be depicted as a complex food web. This is thus called the 'web of life' that shows that there are thousands of interrelationships in nature.

The energy in the ecosystem can be depicted in the form of a food pyramid or energy pyramid. The amount of energy present at each trophic level is considered for this type of pyramid. The food pyramid has a large base of plants called 'producers'. The pyramid has a narrower middle section that depicts the number and biomass of herbivorous animals, which are called 'first order consumers'. The apex depicts the small biomass of carnivorous animals called 'second order consumers'. Main is one of the animals at the apex of the pyramid. Thus to support mankind, there must be a large base of herbivorous animals and an even greater quantity of plant material. At every successive trophic level, there is a huge loss of energy in the form of heat, respiration etc. Thus, at each next higher level only 10 % of the energy passes on. Hence, there is a sharp decline in energy level of each successive trophic level as we move from producers to top carnivores. Therefore, the energy pyramid is always upright.

Integration of cycles in Nature

The various cycles described above are a part of global life processes. These biogeochemical cycles have specific features in each of the ecosystems. All theses cycles are closely interlinked in some form or other. These cycles are however linked to those of adjacent ecosystems. All ecosystems regulate themselves and maintain themselves under a set of environmental conditions. Their characteristics are specific to the plant  and animal communities in the region. This is related to the geographical features of the area, the climate and the chemical composition of the soil. Together the cycles are responsible for maintaining life on earth. If mankind disturbs these cycles beyond the limits that nature can sustain, they will eventually break down and lead to a degraded earth on which man will not be able to survive.