CBSE Class 12 Biology Chapter 14: Ecosystem NCERT Solutions

(a) Plants are called as autotrophs because they are capable of fixing atmospheric carbon dioxide into organic compounds through photosynthesis.

(b) In an ecosystem where trees are the dominant producers, the pyramid of numbers is typically inverted. This is because a single large tree can support a large number of primary consumers (e.g., insects), and each of those consumers might support a smaller number of secondary consumers.

(c) In aquatic ecosystems, the availability of light is often the limiting factor for productivity. As light penetration decreases with depth, the rate of photosynthesis in submerged producers is reduced.

(d) Common detritivores found in our ecosystems include earthworms, millipedes, and various soil invertebrates that feed on dead organic matter.

(e) The major reservoir of carbon on Earth is the oceans. They absorb and store vast amounts of carbon dioxide from the atmosphere, playing a crucial role in the global carbon cycle.

The correct option is (d) Decomposers.

In any given food chain, decomposers, which include microorganisms like bacteria and fungi, typically form the largest population. They obtain their nutrients by breaking down the remains of dead organisms from all trophic levels (producers, consumers). This ensures the recycling of essential nutrients back into the ecosystem.

The correct option is (b) Zooplankton.

In a lake ecosystem, phytoplankton are the producers, forming the first trophic level. Zooplankton are microscopic animals that feed on phytoplankton; therefore, they are the primary consumers and occupy 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 and algae, are organisms that create their own food, typically through photosynthesis. They are also known as primary producers. There are no 'secondary producers' in the conventional sense within a food chain; herbivores are primary consumers, and carnivores are secondary or tertiary consumers.

The correct option is (b) 50%.

Photosynthetically Active Radiation (PAR) refers to the range of light wavelengths (typically 400-700 nm) that photosynthetic organisms can use for photosynthesis. Out of the total incident solar radiation reaching the Earth's surface, approximately 50% falls within this PAR range.

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 consumed by herbivores (primary consumers). Energy flows from producers to consumers.

- Detritus food chain (DFC): Starts with dead organic matter (detritus) from dead plants and animals. Detritivores (like bacteria, fungi, earthworms) consume this detritus, and energy flows from detritus to detritivores, and then to 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 in dead organisms and waste products is broken down into simpler inorganic substances by decomposers (bacteria, fungi). It releases nutrients back into the ecosystem.

(c) Upright and inverted pyramid:

- Upright pyramid: In this pyramid, the base (producers) is the largest, and each successive trophic level is smaller than the one below it. This is the most common type.

- Inverted pyramid: In this pyramid, a lower trophic level is larger than the one above it. For example, a pyramid of numbers can be inverted if a single large tree (producer) supports many insects (primary consumers).

(d) Food chain and Food web:

- Food chain: A linear sequence showing the flow of energy from one trophic level to the next, illustrating who eats whom.

- Food web: A complex network of interconnected food chains in an ecosystem, showing multiple feeding relationships and pathways for energy flow.

(e) Litter and detritus:

- Litter: Refers to the freshly fallen dead organic material on the ground, such as leaves, twigs, and dead flowers.

- Detritus: Refers to dead organic matter that has undergone some initial decomposition or fragmentation. It is the raw material consumed by detritivores.

(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 (biomass). It is the total amount of organic matter produced per unit area per unit time.

- Secondary productivity: The rate at which consumers synthesize new organic matter from the organic matter they ingest. It represents the biomass accumulated by consumers at various trophic levels.