CBSE Class 12 Biology Chapter 14: Ecosystem NCERT Solutions

(a) Plants are called as autotrophs because they fix carbon dioxide through photosynthesis.

(b) In an ecosystem dominated by trees, the pyramid (of numbers) is typically of the inverted type. This is because a single large producer (like a tree) can support a large number of primary consumers.

(c) In aquatic ecosystems, the limiting factor for productivity is often light. As light penetration decreases with depth, photosynthesis is restricted in deeper waters.

(d) Common detritivores in our ecosystem are earthworms. They consume dead organic matter and help in its breakdown.

(e) A major reservoir of carbon on Earth is the oceans. They absorb and store vast amounts of carbon dioxide from the atmosphere.

The correct option is (d) Decomposers.

In most ecosystems, decomposers (like bacteria and fungi) are the most numerous organisms. They obtain energy by breaking down dead organic matter from all trophic levels. Producers typically have the largest biomass, but decomposers often have the largest population size due to their small size and rapid reproduction rates.

The correct option is (b) Zooplankton.

In a lake ecosystem, phytoplankton are the producers (first trophic level). Zooplankton feed on phytoplankton, making them primary consumers and thus occupying the second trophic level. Benthos are organisms living at the bottom, and fishes can occupy various trophic levels depending on their diet.

The correct option is (d) None of the above.

Producers, such as plants, are the primary producers as they synthesize their own food. Herbivores are primary consumers, and carnivores are secondary or tertiary consumers. There are no 'secondary producers' in the typical sense of organisms that produce food from inorganic sources; that role is exclusively held by primary producers.

The correct option is (b) 50%.

Approximately 50% of the total incident solar radiation is known as Photosynthetically Active Radiation (PAR). This is the range of wavelengths (400-700 nm) that plants and other photosynthetic organisms can use for photosynthesis.

Here are the distinctions between the given ecological terms:

(a) Grazing food chain and detritus food chain:

• Grazing food chain (GFC): Starts with producers (plants) which are eaten by herbivores (primary consumers), followed by carnivores. Energy flows from producers to consumers.
• Detritus food chain (DFC): Starts with dead organic matter (detritus) from dead plants and animals. Detritivores consume this detritus, and energy flows from dead organic matter to decomposers and then to detritivores and their predators.

(b) Production and decomposition:

• Production: The process by which organic compounds are synthesized from inorganic substances, primarily through photosynthesis by producers. It leads to an increase in biomass.
• Decomposition: The process by which complex organic matter is broken down into simpler inorganic substances by decomposers (bacteria, fungi). It releases nutrients and energy.

(c) Upright and inverted pyramid:

• Upright pyramid: In this pyramid, the number, biomass, or energy at each successive trophic level is greater than at the level below it. This is the most common type.
• Inverted pyramid: In this pyramid, the number, biomass, or energy at a higher trophic level is greater than at the level below it. For example, a pyramid of numbers can be inverted if a single large producer supports many consumers.

(d) Food chain and Food web:

• Food chain: A linear sequence showing the flow of energy from one organism to another through feeding relationships.
• Food web: A complex network of interconnected food chains, illustrating the multiple feeding relationships and energy pathways within an ecosystem.

(e) Litter and detritus:

• Litter: Freshly dead organic material, such as fallen leaves, twigs, and dead animals, that has not yet undergone significant decomposition.
• Detritus: Partially decomposed organic matter derived from litter, which is then consumed by detritivores and decomposers.

(f) Primary and secondary productivity:

• Primary productivity: The rate at which producers (plants) convert light energy into chemical energy in the form of organic compounds. It is the total biomass produced by producers per unit area per unit time.
• Secondary productivity: The rate at which consumers synthesize new organic matter from the food they consume. It represents the biomass produced by consumers at various trophic levels.