Chapter 5 Ecosystem

NIOS Class 12 Environmental Science Chapter 5

INTEXT QUESTIONS 5.1

1. List the abiotic components of an ecosystem.

Ans: The abiotic component can be grouped into the following three categories:-

• (i) Physical factors: Sunlight, temperature, rainfall, humidity and pressure. They sustain and limit the growth of organisms in an ecosystem.
• (ii) Inorganic substances: Carbon dioxide, nitrogen, oxygen, phosphorus, sulphur, water, rock, soil and other minerals.
• (iii) Organic compounds: Carbohydrates, proteins, lipids and humic substances. They are the building blocks of living systems and, therefore, link the biotic and abiotic components.

2. List the biotic components of the ecosystem.

Ans: Producers, consumers and decomposers.

3. What role do decomposers play in an ecosystem?

Ans: They help in decomposing dead organic material and dead plants and animals; therefore, they are important for the recycling of nutrients.

4. Mention two examples of (i) a natural ecosystem, (ii) man made ecosystem.

Ans: 

• (i) Pond, lake, forests, ocean (any two)
• (ii) Agriculture, aquaculture

INTEXT QUESTIONS 5.2

1. What are phytoplanktons?

Ans: Microscopic floating vegetation in an aquatic ecosystem.

2. Where will you search for the decomposers in a pond?

Ans: At the bottom of the pond in the sediments. 

3. How do nektons differ from zooplanktons?

Ans: Nektons are the animals that can swim and navigate at will. Zooplanktons are floating animals.

4. From where do the fish living at the bottom of the pond get their food?

Ans: Benthal animals like beetles, mites, mollusks and crustaceans are formed by this fission.

INTEXT QUESTIONS 5.3

1. Draw a simple food chain.

Ans: Grass → rat → snake → eagle → forest → deer → tiger

2. What is a food web?

Ans: Food web is the inter connected food chains of an area form a food web.

3. Give examples of an inverted pyramid

Ans: Pyramid of numbers in case of a tree or in a pond.

4. Which type of pyramid gives the true picture of the trophic structure of an ecosystem?

Ans: Pyramid of energy.

INTEXT QUESTIONS 5.4

1. What is the 10% rule of energy transfer in a food chain?

Ans: The 10% rule is, i.e. related to ecological efficiency and states that the amount of energy transferred at each tropic level is only 10% of the energy of the previous tropic level.

2. Give the formula of Lindman’s efficiency.

Ans: Ecological efficiency (also called Lindman’s efficiency) can be represented as

3. What is the significance of studying food chains?

Ans: Significance of studying food chains

• It helps in understanding the feeding relations and interactions among different organisms of an ecosystem.
• It explains the flow of energy and circulation of materials in ecosystems.
• It helps in understanding the concept of biomagnification in ecosystems.

INTEXT QUESTIONS 5.5

1. What is a sedimentary cycle?

Ans: Sedimentary cycle is a type of biogeochemical cycle where the main reservoir is the lithosphere.

2. Give an example of a gaseous cycle.

Ans: Nitrogen (N₂) and carbon

3. Why do forests act as a reservoir?

Ans: Forests act as reservoirs of CO_2, as carbon fixed by the trees remains stored in them for a long time due to their long life cycles. Forest trees have a long life, and therefore, the carbon fixed by them cycles very slowly.

4. Name a symbiotic nitrogen-fixing bacterium.

Ans: Rhizobium

5. What is precipitation?

Ans: Condensation of water vapours to form clouds.

TERMINAL EXERCISE

1. Define the following terms.

(i) Autotrophs

(ii) Heterotrophs

(iii) Primary carnivores

(iv) Saprotophs

(v) Omnivores

Ans: 

• (i) Autotrophs: Autotrophs are green plants that absorb water and nutrients from the soil, carbon dioxide from the air, and capture solar energy to manufacture food through photosynthesis. 
• (ii) Heterotrophs: Organisms that consume food synthesized by the Autotrophs are called heterotrophs.
• (iii) Primary carnivores are animals that feed on herbivores. 
• (iv) Saprotophs: Decomposers are called saprotrophs that feed on dead and decomposed organic matter of plants and animals. 
• (v) Omnivores: Omnivores are organisms that feed upon both plants and animals

2. Give reasons whether the following statements are true or false.

(i) Food chains are more stable than food webs.

(ii) Pyramids of energy are never inverted where whereas the pyramid of biomass may be inverted.

(iv) Phytoplankton is the term applied to floating organisms in a pond.

(v) Aphotic is the upper zone of a pond.

Ans: 

• (i) False. Food webs are more stable than food chains as they are interconnected and provide alternate food sources for organisms. 
• (ii) True. The total amount of energy is highest in the lowest tropic level. On the other hand, in pond ecosystems, the total biomass of producers at any given time is less than the biomass of herbivores. 
• (iv) False. Phytoplanktons are floating microorganisms in a pond.
• (v) False. Aphotic is the lowest zone of a pond where no light is available. 

3. Give reasons for the following statements:

(i) We see more wall lizards near the tube light during summer.

(ii) Energy pyramids are never inverted.

(iii) We can not directly use atmospheric nitrogen.

(iv) There is a higher concentration of carbon dioxide in the aphotic zone.

(v) Food chains have a limited number of steps

Ans: 

• (i) Lizards prey on flying insects near the light for their food. 
• (ii) At each trophic a part of the available energy is lost in respiration or used up in metabolism. 2. A part of energy is lost at each transformation, i.e. when it moves from lower to higher trophic level as heat.
• (iii) We can not directly use atmospheric nitrogen because our body does not have the necessary enzyme to break the strong triple bond between the nitrogen atoms. 
• (iv) There is a higher concentration of carbon dioxide in the aphotic zone because there is no light available in this zone, so there is no photosynthesis. 
• (v) During the transfer of energy from one trophic level to the next, some energy is lost into the system as heat energy and is not available to the next trophic level. Therefore, the number of steps is limited in a chain to 4 or 5.

4. What is an ecosystem? Explain its structural components.

Ans: An ecosystem is a functional unit of nature encompassing complex interactions between its biotic (living) and abiotic (non-living) components. Components of the ecosystem are broadly grouped into abiotic and biotic components. 

• (a) Abiotic components (Nonliving)
  • (i) Physical factors: Sunlight, temperature, rainfall, humidity and pressure that sustain and limit the growth of organisms in an ecosystem.
  • (ii) Inorganic substances: Carbon dioxide, nitrogen, oxygen, phosphorus, sulphur, water, rock, soil and other minerals.
  • (iii) Organic compounds: Carbohydrates, proteins, lipids and humic substances. They are the building blocks of living systems and, therefore, link the biotic and abiotic components.
• (b) Biotic components (Living)
  • (i) Producers: The green plants that manufacture food for the entire ecosystem through the process of photosynthesis.
  • (ii) Consumers: They are called heterotrophs, and they consume food synthesized by the autotrophs. Based on food preferences, they can be grouped into three broad categories. Herbivores, carnivores and omnivores. 
  • (iii) Decomposers: Also called saprotrophs. These are mostly bacteria and fungi that feed on decomposed and dead organic matter of plants and animals by secreting enzymes outside their body on the decaying matter. 

5. Define decomposers and give their role in sustaining an ecosystem.

Ans: Decomposers are mostly bacteria and fungi that feed on decomposed and dead organic matter of plants and animals by secreting enzymes outside their body on the decaying matter. They play a very important role in the recycling of nutrients. They are also called saprotrophs, detritivores or detritus feeders.

6. Why are ecosystems dynamic in nature? Give the various functional components of an ecosystem.

Ans: Ecosystems are complex, dynamic systems. They perform certain functions. These are:-

• (i) Energy flows through the food chain
• (ii) Nutrient cycling (biogeochemical cycles)
• (iii) Ecological succession or ecosystem development
• (iv) Homeostasis (or cybernetic) or feedback control mechanisms

7. What is an ecological pyramid? Define and differentiate between the different pyramids of energy and the pyramid of numbers.

Ans: Ecological pyramids are the graphic representations of trophic levels in an ecosystem. They are pyramidal in shape, and they are of three types: The producers make the base of the pyramid, and the subsequent tiers of the pyramid represent herbivore, carnivore and top carnivore levels.

• Pyramid of energy: This pyramid represents the total amount of energy at each trophic level. Energy is expressed in terms of rate, such as kcal/unit area /unit time or cal/unit area/unit time. Eg, in a lake, autotroph energy is 20810 kcal/m/year. Energy pyramids are never inverted.
• Pyramid of numbers: This represents the number of organisms at each trophic level. For example, in a grassland, the number of grasses is more than the number of herbivores that feed on them, and the number of herbivores is more than the number of carnivores. In some instances, the pyramid of numbers may be inverted, i.e herbivores are more than primary producers, as you may observe that many caterpillars and insects feed on a single tree.

8. List the various steps of the nitrogen cycle in a sequence.

Ans: 

• (a) Nitrogen fixation: This process involves the conversion of gaseous nitrogen into Ammonia, a form in which it can be used by plants. Atmospheric nitrogen can be fixed by the following three methods:-
  • (i) Atmospheric fixation: Lightning, combustion and volcanic activity help in the fixation of nitrogen.
  • (ii) Industrial fixation: At high temperature (400 °C) and high pressure (200 atm.), molecular nitrogen is broken into atomic nitrogen, which then combines with hydrogen to form ammonia.
  • (iii) Bacterial fixation: There are two types of bacteria-
    • (i) Symbiotic bacteria, e.g. Rhizobium in the root nodules of leguminous plants.
    • (ii) Freeliving or symbiotic, e.g. 1. Nostoc 2. Azotobacter 3. Cyanobacteria can combine atmospheric or dissolved nitrogen with hydrogen to form ammonia.
• (b) Nitrification: It is a process by which ammonia is converted into nitrates or nitrites by Nitrosomonas and Nitrococcus bacteria, respectively. Another soil bacterium, Nitrobacter, can convert nitrate into nitrite.
• (c) Assimilation: In this process, nitrogen fixed by plants is converted into organic molecules such as proteins, DNA, RNA, etc. These molecules make up the plant and animal tissue.
• (d) Ammonification: Living organisms produce nitrogenous waste products such as urea and uric acid. These waste products, as well as dead remains of organisms, are converted back into inorganic ammonia by the bacteria. This process is called ammonification. Ammonifying bacteria help in this process.
• (e) Denitrification: Conversion of nitrates back into gaseous nitrogen is called denitrification. Denitrifying bacteria live deep in soil near the water table, as they like to live in an oxygen-free medium. Denitrification is the reverse of nitrogen fixation.

9. The following organisms were identified in a pond ecosystem-Spirogyra, Euglena, Hydra, Daphnia, arthropod larvae, bass and sunfish. Make a food web and identify the trophic level of each one of them.

Ans: 

Producers (Level 1):

• Spirogyra: A type of green algae that photosynthesizes to produce food 
• Euglena: A single-celled organism that can also photosynthesize 

Primary Consumers (Level 2):

• Daphnia: Small crustaceans that feed on algae and other phytoplankton 
• Hydra: A predatory cnidarian that eats small invertebrates like Daphnia 

Secondary Consumers (Level 3): 

• Arthropod Larvae: Larvae of insects that prey on smaller invertebrates

Tertiary Consumers (Level 4): 

• Bass: A predatory fish that consumes other fish and smaller organisms
• Sunfish: Another predatory fish that also feeds on smaller fish and insects

Additional Study Materials

• Chapter 1 Origin of Earth and Evolution of the Environment
• Chapter 2 Environment and Human Society
• Chapter 3 Degradation of Natural Environment
• Chapter 4 Principles of Ecology