NCERT Questions for Class 12 Biology Chapter 12 – Ecosystem

Important Questions with Solutions of Class 12 Biology Chapter 12 – Ecosystem

Short Answer Questions

1) What are the characteristic pioneer species of Xerarch and Hydrarch successions?

Ans – Lichens are the pioneer species of Xerarch succession while tiny phytoplankton is the pioneer species of Hydrarch succession.

2) What percentage of photosynthetically active light is absorbed by plants?

Ans – 2-10% of the photosynthetic radiation is absorbed by the plants.

3) Determine the trophic levels of secondary and tertiary consumers.

Ans – Secondary consumers belong to the third trophic level, and tertiary consumers belong to the fourth trophic level.

4) What is the approximate value of the net primary productivity of the biosphere?

Ans – The approximate value of the net primary productivity of the biosphere is 170 billion tonnes.

5) Identify the inverted ecological pyramid present in the tree environment.

Ans – The pyramid of numbers is an ecological pyramid that is inverted in a forest habitat.

6) Gaseous nutrient cycles and sedimentary nutrient cycles have their respective reservoirs. Enumerate. What does a reservoir require?

Ans – The atmosphere acts as a reservoir for the gaseous nutrient cycle, whereas the Earth’s crust serves as a reservoir for the sedimentary nutrient cycle. The reservoir is required to overcome the deficiency caused by the imbalance between inflow and outflow.

7) Why secondary succession takes place faster as compared to the primary succession?

Ans – Secondary succession is the re-establishment of a community in a habitat that previously supported mature communities, comprising organic as well as inorganic environmental components. Secondary succession occurs faster than primary succession because, here, appropriate soil is already present in such bare places which can support proper growth.

3 mark questions

1) What is ecological succession? Discuss the different types of stages of ecological succession.

Ans – Ecological succession refers to a community-controlled process in which the structure and composition of the community go through successive and orderly changes and ultimately form a climax community.

The stages of Succession:

Invasion is the introduction of propagating structures, seeds, spores, or bulbils into an unoccupied area that is undergoing either primary or secondary succession. Those with favorable conditions germinate in a new region while others develop to become adult plants. The newcomers from external regions are called pioneers.

Establishment: This involves the processes of acclimatization among migrants after they move to different regions. There are three simple functions in this process, which include germination, growth, and reproduction.

Aggregation: Aggregation is the collection of individuals belonging to different species within a particular area.

Competition: Those species that have the same nutritional requirements are competitive while species with varying requirements are complementary species.

Reaction: – It is the changes caused by colonizers in the environment. The effect of vegetation on the place is called response until and unless a stable community has formed in that area.

2) Describe the components of an ecosystem.

Ans – An ecosystem is a functional unit of nature comprising biotic and abiotic elements, wherein living organisms interact with one another and their physical environment. The ecosystem comprises two components:

a) Abiotic Components:

• Inorganic substances such as phosphorus, sulphur, carbon, nitrogen, and hydrogen.
• Organic compounds such as carbohydrates, proteins, and lipids.
• Climatic conditions such as illumination, humidity, precipitation, and temperature.

b) Biotic Components:

• Producers: Organisms that synthesise food for themselves and all living entities from inorganic raw materials and solar energy are termed producers.
• Consumers: – These are live heterotrophic organisms within the environment that ingest the food produced by autotrophs. They are generally categorised as:
  • Primary consumers are directly reliant on producers, known as herbivores. Examples: – Rat, deer, cow, goat.
  • Secondary Consumers: Organisms that consume main consumers are referred to as carnivores. Example: – Foxes, felines, lions.
  • Tertiary Consumers: These are apex carnivores that hunt other carnivores and herbivores. Illustration: – Raven, individual.
• Decomposers: Organisms that decompose the remains of plants, animals, and associated waste products are referred to as decomposers. For instance: – Bacteria, Fungi, etc.

3) What is decomposition? Describe the many stages involved in decomposition.

Ans – Decomposers (bacteria and fungi) break down complex organic materials into simpler inorganic substances such as CO₂, H₂O, and minerals. This process is known as decomposition. Detritus includes dead plant remains such as leaves, bark, and flowers, as well as the remains of dead animals and faeces. The key steps involved in decomposition are:

1. Fragmentation: – Detritivores are organisms that reduce debris to smaller particles.
2. Leaching occurs when water-soluble inorganic nutrients penetrate the soil horizon and precipitate as inaccessible salts.
3. Catabolism: Enzymes from bacteria and fungi breakdown debris into simple inorganic compounds.
4. Humification: – Humification causes the accumulation of humus, a dark-coloured amorphous substance. This is very resistant to microbial action and decomposes at a significantly slower rate.
5. Mineralisation: – Some microorganisms further breakdown the humus, releasing inorganic nutrients through the mineralisation process.

5 Marks Questions

1) Describe and discuss the phosphorus cycle in an ecosystem with a diagram.

Ans –

• The primary repository of phosphorus is sedimentary rocks, which are accessible to the biogeochemical cycle in limited quantities due to weathering processes.
• This phosphorus undergoes weathering and is then transferred to the soil by wind and water, where it exists as inorganic dissolved phosphates.
• The fundamental phosphorus cycle commences with dissolved phosphates that are assimilated by plants to synthesise their tissues.
• Subsequently, animals consume plants. Decomposing bacteria degrade the tissues of deceased animals, hence returning phosphates to the soil.
• The water-soluble phosphates are discharged into the ocean’s depths via runoff.
• The primary mechanism for reintroducing phosphorus to terrestrial environments is the uplift of sea sediments. A portion of phosphorus is returned to absorb inorganic phosphate; at death, the majority of the ingested phosphate is recycled into the surrounding matter. This type of cycling is referred to as the biological cycle or metabolic cycle.

2) Explain the carbon cycle within the ecosystem.

Ans – Carbon is obtained from the atmosphere and water. Carbon mostly exists in the atmosphere as carbon dioxide. It is crucial for carbohydrate synthesis during photosynthesis. It is integrated into the tissues of green plants, such as carbohydrates, proteins, and lipids.

Marine organisms such as protozoans, corals, and molluscs utilise dissolved CO₂ in seawater for their survival. In these organisms, CO₂ is transformed into calcium carbonate, which is utilised for shell formation.

CO₂ + H₂O → H₂CO₃ → CaCO₃ + H⁺

Upon the demise of marine organisms, the CaCO₃ contained in their shells is either deposited as sedimentary rock or dissolved in water, resulting in the emission of CO₂. A specific quantity of carbon is deposited as coal carbon. Coal is released into the atmosphere as CO₂ via combustion and weathering processes. Consequently, carbon from the atmospheric reservoir transfers to green plants, subsequently to animals, and ultimately to bacteria, fungi, and other organisms. They release it back into the atmosphere via breakdown.