Ecosystem Class 12 Notes: CBSE Biology Chapter 12

Summary - Class 12 Biology Ecosystem Notes (Chapter 12)

Ecosystem: The ecosystem is the functional unit of nature in which living organisms interact with each other as well as their surrounding physical environment.

Biosphere: The biosphere is a global ecosystem that includes all of the world's local ecosystems.

Types of the Ecosystem:

1.  Aquatic Ecosystem: Forest, grassland, desert, etc.

2. Terrestrial Ecosystem: Pond, wetland, lake, estuary, river, etc.

These are the natural ecosystems.

An ecosystem can be artificial or man-made like the croplands, aquarium, etc.

12.1: Ecosystem- Structure and Function

Biotic Factors: This includes all the living organisms in an ecosystem.

Abiotic Factors: This includes all the nonliving factors in an ecosystem like sunlight, humidity, wind, etc.

The characteristic physical structure of an ecosystem depends on the interaction of biotic and abiotic factors.

Stratification: It is the vertical and also the horizontal distribution of different varieties that occupy different levels. For example, trees occupy the top vertical strata or layers of a forest, shrubs are the second, and herbs and grasses occupy the bottom layers.

For an Ecosystem to Function as a Unit the Following Factors are Important:

1. Productivity

2.Decomposition

3. Energy flow

4. Nutrient cycling

The Function of an Ecosystem is that the Conversion of Inorganic into Organic Material by the Subsequent Steps:

• Use of the energy of the sun by the autotrophs.

• Consumption of the autotrophs by various heterotrophs.

• Decomposition and mineralisation of all the dead organic interest release the nutrients back into the soil for reuse by the autotrophs.

• These events are repeated again and again.

As we move along the trophic levels energy moves in a unidirectional manner. Energy dissipates and is lost as heat to the environment. Example: Pond ecosystem.

Abiotic Factors: 

Water with all dissolved inorganic and organic nutrients, soil rich in nutrients at the rock bottom of the pond, solar input, day-length, temperature, and other climatic factors.

Biotic Factors:

Producers or Autotrophs: Algae, phytoplankton, floating, submerged, and marginal plants in the pond.

Consumers or Heterotrophs: Zooplankton, free-living, and bottom-dwelling forms.

Decomposers: Fungi, bacteria, and flagellates are abundant in the bottom of the pond.

12.2 Productivity:

A constant input of solar power is the basic requirement for any ecosystem.

Primary Production: It is the amount of biomass produced per unit area in a given time by plants during photosynthesis. It is expressed in terms of weight (g₋2) or energy (kcal m₋2).

Productivity: It is the rate of biomass production. It is expressed as g₋2

yr₋1 or kcal m₋2 yr₋1.

Gross Primary Productivity (GPP): GPP of an ecosystem is the rate of production of organic matter during photosynthesis.

Net Primary Productivity (NPP): Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP).

GPP – R = NPP

The available biomass for the consumption of heterotrophs or herbivores and decomposers is the net primary productivity.

The rate of formation of new organic matter by consumers is termed secondary productivity.

The primary productivity of an ecosystem depends on the plant species and various environmental factors and therefore, varies from place to place.

For the whole biosphere, the annual net primary productivity is approximately 170 billion tons (dry weight) of organic matter. Despite occupying nearly ${ 70 }$% of the earth’s surface, the productivity of the oceans is only around 55 billion tons.

12.3: Decomposition:

Decomposition: It is the process of breaking down complex organic matter into simpler inorganic substances such as carbon dioxide, water, and nutrients by the action of decomposers.

Detritus: Dead remains of plants and animals such as leaves, barks, flowers, faecal matter of animals, etc. constitute detritus.

Detritivores: Organisms that feed on detritus are called detritivores. Earthworms, fungi, etc. are examples of detritivores.

Steps in Decomposition:

1. Fragmentation: It is the breakdown of detritus into smaller particles by the action of detritivores.

2. Leaching: It's the phenomenon by which water-soluble nutrients sink to the soil horizon and precipitate as salts that aren't available.

3. Catabolism: It is the breakdown of detritus into simple inorganic nutrients by the action of bacterial and fungal enzymes.

4. Humification:  It is the formation and continuous deposition of a dark-coloured organic amorphous substance called humus. Humus is extremely resistant to microbial action and decomposes at a very slow rate. Because it is colloidal, it acts as a nutrient reservoir.

5. Mineralisation:  It is the process by which some microorganisms further break down humus to form simple inorganic nutrients.

• Humification and mineralization occur in the soil.

• All steps of decomposition occur simultaneously on detritus.

• Decomposition is largely an oxygen-requiring process.

• On both the chemical composition of detritus and climatic factors, the rate of decomposition is dependent. When detritus is rich in lignin and chitin, the pace of decomposition is slower, and when detritus is rich in nitrogen and water-soluble compounds like sugars, the rate of decomposition is faster.

• The most important climatic factors that play important role in decomposition are temperature and soil moisture. They affect the activities of soil microbes. 

• Decomposition is favoured in warm and moist environments.

• Decomposition is inhibited by low temperatures and anaerobiosis. This results in a build-up of organic materials.

12.4: Energy Flow:

• The Sun is the ultimate source of energy for all ecosystems on Earth (except for the deep-sea hydrothermal ecosystem).

• Photosynthetically Active Radiation (PAR): The solar radiation that can be utilized by plants for photosynthesis. Of all incident solar radiation, only ${ 50 }$% is PAR. The plants can capture only ${ 2-10 }$% of the PAR and it is this energy that sustains all life on earth.

• There is a unidirectional flow of energy that is the energy from the sun flows to the producers and then the consumers. Therefore, all organisms are directly or indirectly dependent on the sun.

• All ecosystems follow the First and the Second Law of thermodynamics.

• First Law of Thermodynamics: The First Law of Thermodynamics explains that energy can be converted from one form to another in the presence of heat, work, and internal energy, but it can neither be created nor destroyed, under any circumstances.

• The unidirectional flow of energy shows the First Law of Thermodynamics as energy from the sun is converted into chemical energy by the producers which are then utilized by the consumers. At any level, energy can neither be created nor be destroyed.

• Second Law of Thermodynamics: The Second Law of Thermodynamics states that the state of entropy of the entire universe, as an isolated system, will always increase over time. The changes in the entropy in the universe can never be negative, this is explained by the second law.

• To counteract the universal tendency toward increasing disorderliness ecosystems need a constant supply of energy to synthesize the molecules they require.

• Energy flow in the ecosystem occurs in the form of the food chain.

• Food chain: A food chain is an arrangement of species in an ecological community based on the order of predation, in which each organism feeds on the organisms below it.

• Producers: All the green plants in the ecosystem utilize solar energy for photosynthesis. They are always autotrophic.

• Consumers: All heterotrophs, whether directly or indirectly reliant on plants for nourishment, are referred to as consumers. Depending on their position in the food chain the organisms can be divided into primary, secondary, tertiary, or quaternary levels.

• Primary Consumers/Herbivores: The organisms that are directly dependent on the producers. In the above food chain, the grasshopper is the primary consumer.

• Secondary Consumers/Primary Carnivores: The organisms that are dependent on the primary consumers. The frog in the above example is a secondary consumer.

• Tertiary consumers/Secondary carnivores: The organisms that are dependent on the secondary consumers.

• Quaternary Consumers/Tertiary carnivores: The organisms that are dependent on the tertiary consumers. They usually hold the top position which is the carnivore in a food chain.

• A food chain cannot have more than a quaternary consumer because there would not be enough energy left for the next level organism.

• The detritus food chain’s (DFC) starting point is dead organic matter. It is consists of decomposers mainly fungi and bacteria which are heterotrophs too. They break down dead organic matter or detritus to meet their energy and nutrient requirements. These are also called saprotrophs in which appro means to decompose. They secrete digestive enzymes outside their body to break down dead and waste organic materials into simple, inorganic materials. These simpler substances are subsequently absorbed by them.

• In an aquatic ecosystem, the major conduit for energy flow is the Grazing Food Chain (GFC). In a terrestrial ecosystem, a much larger fraction of energy flows through the detritus food chain in comparison with the GFC.

• The Detritus food chain can be connected with the grazing food chain because some of the organisms of DFC are prey to the GFC animals. Also in a natural ecosystem, some animals that exist are omnivores such as cockroaches, crows, etc.

• Food Web: A system of food chains in which the interlocking and interdependency are found.  

• Trophic Level: The specific place in the food chain occupied by an organism based on the source of their nutrition or food is called the trophic level.

• First Trophic Level: Producers occupy the first trophic level.

• Second Trophic Level: The primary consumers.

• Third Trophic Level: The secondary consumers.

• Fourth Trophic Level: The tertiary consumers.

• Fifth Trophic Level: The quaternary consumers.

• A food chain can have a maximum of five trophic levels as beyond that the amount of energy is insufficient to sustain another consumer. 

• At successive trophic levels, the amount of energy gets decreases. 

• When an organism dies it is converted into detritus which serves as a source of energy for the decomposers. 

• Standing Crop: At a particular given time the total mass at a trophic level is standing crop. It is measured as the mass of living organisms (biomass) or the number in a unit area. 

• Biomass can also be expressed as fresh or dry weight. However, the use of the dry weight is more accurate. 

• 10 Percent Law: This law states that only ${ 10 }$% of the energy of a trophic level is transferred to the next level. 

12.5: Ecological Pyramids: 

• An ecological pyramid is a graphical representation of the relationship between different organisms in an ecosystem. 

• Each bar of the pyramid represents a specific trophic level along with their order. The order depends on who is been eaten by whom. 

• This represents the flow of energy. 

• The base of each pyramid is the representation of the producers or the first trophic level whereas the apex is the representation of the tertiary or top-level consumers. 

• The three commonly studied ecological pyramids are: 

(a) Pyramid of number 

(b) Pyramid of biomass

(c) Pyramid of energy 

• All species at that trophic level should be included in any estimations of energy content, biomass, or population. At any trophic level, it is impossible to generalise based on merely a few individuals.

• An organism can be a part of more than one food chain and therefore belong to different trophic levels. 

• The pyramids of biomass, energy, and number are upright in most ecosystems. That is, the producers are more in number than the herbivores, the herbivores are more than the carnivores, etc. 

• Pyramids of number and biomass can be inverted in some cases but the pyramid of energy is always upright. This is due to the fact that when energy goes from one level to another, some of it is lost as heat.

• Only food chains, not food webs, can be accommodated in ecological pyramids. They are also incapable of depicting saprotrophs.

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Ecosystem Class 12 Notes Biology - Basic Subjective Questions

Section−A (1 Mark Questions)

1. Give examples of natural ecosystems and man-made or artificial ecosystems.

Ans. Forests, deserts, grasslands, lakes, rivers, seas, etc. are examples of natural ecosystems. Crop fields and an aquarium may also be considered man-made ecosystems.

2. What does the base tier of the ecological pyramid represent? (Exemplar)

Ans. The base of each ecological pyramid represents the producers or the first trophic level.

3. How much time is taken by a primary succession to complete? 

Ans. Primary succession is a very slow process, taking maybe thousands of years for the climax to be reached.

4. Why is the rate of assimilation of energy at the herbivore level called secondary productivity? (Exemplar)

Ans. The rate of assimilation of energy at the herbivore level is called secondary productivity because the biomass available to the organisms of the next trophic level (consumer) for further consumption is a result of the primary productivity which is formed by autotrophs (plants).

5. Define ecological succession.

Ans. The gradual and fairly predictable change in the species composition of a given area is called ecological succession. 

Section−B (2 Mark Questions)

6. State the importance of phosphorus in living organisms.

Ans. 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.

7. Is the pyramid of biomass in the sea inverted or upright? Give a reason.

Ans. The pyramid of biomass in the sea is generally inverted. This is because the biomass of fish far exceeds that of phytoplankton. Phytoplanktons are microscopic organisms with a short lifespan but very high reproductive potential. Although their number far exceeds the number of fishes present in the sea, their biomass is much lesser than the biomass of fishes (much larger) at any given time.

8. Define pioneer species. Name the pioneer species responsible for primary succession on a rock.

Ans. The species that invade a bare area are called pioneer species. In primary succession on rocks, these are usually lichens which can secrete acids to dissolve rock, helping in weathering and soil formation.

9. Define stratification. Give an example to support your answer.

Ans. The vertical distribution of different species occupying different levels is called stratification. For example, trees occupy the top vertical strata or layer of a forest, shrubs the second, and herbs and grasses occupy the bottom layers.

10. Define GPP. How is it related to NPP?

Ans. The Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. A considerable amount of GPP is utilised by plants in respiration. Gross primary productivity minus respiration losses (R), is the net primary productivity (NPP) i.e., GPP – R = NPP. Net primary productivity is the available biomass for the consumption of heterotrophs (herbivores and decomposers).

11. Why are the number of trophic levels restricted in a grazing food chain? Is there any such limitation in a detritus food chain?

Ans. In any kind of food chain, grazing or detritus, the number of trophic levels is limited to 4-5 as the transfer of energy follows the 10% law i.e., only 10% of the energy is transferred to each trophic level from the lower trophic level. Moreover, the detritus food chain starts with decomposing organic matter which is already low in energy content. Hence, the trophic levels are usually even lower than 4-5.

5 Important Topics of Biology Class 12 Chapter 12 You Shouldn’t Miss!

Some Most frequently asked Important Questions during Exams

1. What are the components of the Ecosystem?

2. What is the difference between the biotic and abiotic components of the ecosystem?

3. How do the decomposers get their food?

4. What is a food chain? How many types of food chains are there?

5. What do you mean by a food web? Explain with the help of a diagram.

6. What are the features of a nitrogen cycle in an ecosystem?

7. What is the impact of human activities on the carbon cycle and what are its consequences?

8. What are trophic levels? Name and explain the two fundamental trophic levels.

9. Explain in detail the ecological pyramid.

10. Describe the energy flow in an ecosystem.