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Ecosystem – CBSE Notes for Class 12 Biology

An ecosystem referred to as a geographic area where plants, animals, and other organisms, as well as weather and landscape, work together to form a bubble of life. Ecosystems contain biotic or living, parts, as well as abiotic factors, or non-living parts.

Ecosystem – Structure and Function

Structure:

Generally ecosystems consist of two basic components.

  1. Abiotic component: It includes basic in-organic (soil, water, oxygen, calcium carbonates, phosphates etc.) and organic compounds. It also includes physical factors such as moisture, wind currents and solar radiation. Radiant energy of sun is the only significant energy source for any ecosystem.
  2. Biotic components: It Includes producers, consumers and decomposers.
  • Producer: These are the autotrophic, chlorophyll-bearing organisms, which produce their own food.
  • Consumers: A consumer which gets nutrition by eating plants is called Primary consumers (herbivore) (eg) Rabbit, deer and cow. The Secondary Consumer: (carnivores) is an animal that eats the flesh of herbivores (eg) cats and dogs. Tertiary Consumers: are the type of carnivores, which prey upon other carnivores. (eg) Lion, tiger and vulture.
  • Decomposers: Decomposers attack the dead remains of producers and consumers and degrade the complex organic substances into simpler compounds to derive their nutrients. The decomposers play very important role in maintaining the dynamic nature of ecosystem.

Function

The components of the ecosystem are seen to function as a unit when you consider the following aspects:

(i) Productivity

(ii) Decomposition

(iii) Energy flow and

(iv) Nutrient cycling

Productivity

Productivity refers to the amount of organic matter accumulated in any unit time. It is of following types.

  1. Primary productivity: Green plants absorb solar energy and store it in organic form as chemical energy. This forms the first and basic form of energy storage and is known as primary productivity. It is the rate at which the organic material is formed by photosynthesis per unit area of surface per unit time.
  2. Secondary productivity: It refers to consumers or heterotrophs. The consumers utilize the food materials during the process of respiration. The rate at which the food energy is assimilated is called secondary productivity.
  3. Net productivity: This refers to the rate of storage of organic matter which is not used by heterotrophs. These may be equivalent to the net primary production minus consumption by the heterotrophs.

Decomposition

Decomposition is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water, simple sugars and mineral salts.

Energy Flow

Energy flow is the amount of energy that moves through a food chain. The energy input, or energy that enters the ecosystem, is measured in Joules or calories. Accordingly, the energy flow is also called calorific flow.

Except for the deep sea hydro-thermal ecosystem, sun is the only source of energy for all ecosystems on Earth. Of the incident solar radiation less than 50 per cent of it is photosynthetically active radiation (PAR).

Plants capture only 2-10 per cent of the PAR and this small amount of energy sustains the entire living world.

The energy flow takes place via the food chain and food web. During the process of energy flow in the ecosystem, plants being the producers absorb sunlight with the help of the chloroplasts and a part of it is transformed into chemical energy in the process of photosynthesis.

This energy is stored in various organic products in the plants and passed on to the primary consumers in the food chain when the herbivores consume (primary consumers) the plants as food. Then conversion of chemical energy stored in plant products into kinetic energy occurs, degradation of energy will occur through its conversion into heat.

Then followed by the secondary consumers. When these herbivores are ingested by carnivores of the first order (secondary consumers) further degradation will occur. Finally, when tertiary consumers consume the carnivores, energy will again be degraded. Thus, the energy flow is unidirectional in nature.

Moreover, in a food chain, the energy flow follows the 10 percent law. According to this law, only 10 percent of energy is transferred from one trophic level to the other; rest is lost into the atmosphere. This is clearly explained in the following figure and is represented as an energy pyramid.

Ecological Pyramids

The ecological pyramids represent the trophic structure and also trophic function of the ecosystem. Each bar of the pyramid represents a different trophic level. The different types of ecological pyramids include:

Pyramid of Numbers: This represents the number of organisms in each trophic level, irrespective of their size.

Pyramid of Biomass: This represents the total mass of organisms at each trophic level.

Pyramid of Productivity: It is the total amount of energy present at each trophic level and the total energy lost between each trophic level.

Ecological Succession

Ecological succession, the process by which the structure of a biological community evolves over time. Two different types of succession—primary and secondary—have been distinguished. The final stage of succession is a climax community, which is a very stable stage that can endure for hundreds of years.

Succession of Plants

The process of plant succession consists of nine steps. The nine steps are: (1) Nudation (2) Migration (3) Germination (4) Ecesis (5) Colonisation and Aggregation (6) Competition and Co-action (7) Invasion (8) Reaction and (9) Stabilisation.

Plant succession may be of two kinds:

  • Primary succession: It begins in areas which have previously been unoccupied by plants, such as open water, bare rock, or sand.
  • Secondary succession: This kind of succession begins wherever the existing vegetation has been destroyed without denuding the area of soil. It usually starts after forest fires, cutting of the trees, flood and erosions. It is also of common occurrence in abandoned agricultural lands. A single case of plant succession at a particular kind of habitat is usually referred to as a sere, and the various stages of a sere are called seral stages.

Nutrient Cycling

The nutrient cycle is a system where energy and matter are transferred between living organisms and non-living parts of the environment. This occurs as animals and plants consume nutrients found in the soil, and these nutrients are then released back into the environment via death and decomposition. Mineral cycles include the carbon cycle, sulfur cycle, nitrogen cycle, water cycle, phosphorus cycle, oxygen cycle, among others that continually recycle along with other mineral nutrients into productive ecological nutrition.

Carbon Cycle

The carbon cycle describes the process in which carbon atoms continually travel from the atmosphere to the Earth and then back into the atmosphere. Since our planet and its atmosphere form a closed environment, the amount of carbon in this system does not change.

Fossil fuel also represent a reservoir of carbon. Carbon cycling occurs through atmosphere, ocean and through living and dead organisms.

According to one estimate 4 × 1013 kg of carbon is fixed annually in the biosphere through photosynthesis. A considerable amount of carbon returns to the atmosphere as CO2 through respiratory activities of the producers and consumers.

Phosphorus Cycle

The phosphorus cycle is the process by which phosphorus moves through the lithosphere, hydrosphere, and biosphere. Phosphorus is essential for plant and animal growth, as well as the health of microbes inhabiting the soil, but is gradually depleted from the soil over time.

Phosphorus is a major constituent of biological membranes, nucleic acids and cellular energy transfer systems. Many animals also need large quantities of this element to make shells, bones and teeth. The natural reservoir of phosphorus is rock, which contains phosphorus in the form of phosphates.

Ecosystem Services

Ecosystem services are the many and varied benefits to humans provided by the natural environment and from healthy ecosystems.

These ecosystems, functioning in healthy relationship, offer such things like natural pollination of crops, clean air, extreme weather mitigation, human mental and physical well-being.

Robert Constanza and his colleagues have very recently tried to put price tags on nature’s life-support services. Researchers have put an average price tag of US $ 33 trillion a year on these fundamental ecosystems services, which are largely taken for granted because they are free. This is nearly twice the value of the global gross national product GNP which is (US $ 18 trillion).

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