NCERT Solutions
Class 12 • Biology • Chapter 12 • Ecosystem- (a) Plants are called as Producers (or Autotrophs) because they fix carbon dioxide.
- (b) In an ecosystem dominated by trees, the pyramid (of numbers) is Inverted type.
- (c) In aquatic ecosystems, the limiting factor for the productivity is Light.
- (d) Common detritivores in our ecosystem are Earthworms.
- (e) The major reservoir of carbon on earth is Oceans (Dissolved Carbon).
(d) Decomposers
Explanation: Decomposers (bacteria and fungi) are microscopic and present in billions in soil and organic matter, far outnumbering producers or consumers in a food chain.
(b) Zooplankton
Explanation:
1. Phytoplankton (Producers) – 1st Trophic Level.
2. Zooplankton (Primary Consumers) – 2nd Trophic Level.
3. Small Fishes (Secondary Consumers) – 3rd Trophic Level.
(d) None of the above
Explanation: In ecological terms, producers are always autotrophs (Plants). Herbivores and Carnivores are consumers. While herbivores generate “secondary productivity” (biomass), they are not called “Secondary Producers”. They are Primary Consumers.
(b) 50%
Explanation: Of the incident solar radiation, less than 50% is Photosynthetically Active Radiation (PAR). Plants capture only 2-10% of this PAR.
| (a) Grazing Food Chain (GFC) | Detritus Food Chain (DFC) |
|---|---|
| Begins with living green plants (Producers). | Begins with dead organic matter (Detritus). |
| Energy source: Sun. | Energy source: Organic detritus. |
| Ex: Grass \(\to\) Goat \(\to\) Man. | Ex: Dead leaves \(\to\) Earthworm \(\to\) Bird. |
| (b) Production | Decomposition |
|---|---|
| Process of synthesis of organic matter (biomass) from inorganic substances. | Process of breaking down complex organic matter into simpler inorganic substances. |
| Requires energy (Sunlight). | Releases energy. |
| (c) Upright Pyramid | Inverted Pyramid |
|---|---|
| Base (Producers) is broader than top. Trophic levels decrease in value upwards. | Base (Producers) is narrower than top. Trophic levels increase in value upwards. |
| Ex: Energy Pyramid (Always), Biomass (Terrestrial). | Ex: Biomass (Aquatic), Number (Tree ecosystem). |
| (d) Food Chain | Food Web |
|---|---|
| A single linear sequence of organisms where nutrients and energy are transferred. | A complex network of interconnected food chains. |
| Provides limited stability to ecosystem. | Provides high stability due to multiple pathways. |
| (e) Litter | Detritus |
|---|---|
| Freshly fallen plant material (leaves, twigs) on the soil surface. | Dead organic matter (including litter and animal remains) undergoing decomposition. |
| (f) Primary Productivity | Secondary Productivity |
|---|---|
| Rate of production of biomass by Producers (Plants). | Rate of formation of new organic matter by Consumers. |
- Physical factors: Temperature, Light, Humidity, Water.
- Inorganic substances: C, H, O, N, P, K in soil/water.
- Edaphic factors: Soil structure and composition.
- Producers (Autotrophs): Green plants, algae, photosynthetic bacteria.
- Consumers (Heterotrophs):
- Macro-consumers: Herbivores, Carnivores.
- Micro-consumers (Decomposers): Bacteria, Fungi (Saprophytes).
Ecological Pyramids: Graphic representations of the trophic structure and function of an ecosystem, starting with producers at the base and top carnivores at the apex.
Represents the number of individuals at each trophic level.
- Upright: Grassland ecosystem (Many grasses \(\to\) Fewer herbivores \(\to\) Few carnivores).
- Inverted: Tree ecosystem (One tree \(\to\) Many birds \(\to\) Many parasites).
Represents the total dry weight of living matter at each trophic level.
- Upright: Forest ecosystem (High biomass of trees \(\to\) Low biomass of carnivores).
- Inverted: Aquatic ecosystem (Low biomass of phytoplankton \(\to\) High biomass of zooplankton/fishes).
Primary Productivity: The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.
- Plant Species: Photosynthetic capacity varies among species.
- Environmental Factors: Availability of sunlight, temperature, and moisture (e.g., tropical forests have higher productivity than deserts).
- Nutrients: Availability of nutrients (N, P, K) in the soil.
- Photosynthetic Capacity: Efficiency of chlorophyll and leaf area index.
Decomposition: The process of breaking down complex organic matter (detritus) into inorganic substances like \(CO_2\), water, and nutrients by decomposers.
- Fragmentation: Detritivores (e.g., earthworms) break down detritus into smaller particles.
- Leaching: Water-soluble inorganic nutrients go down into the soil horizon and precipitate as unavailable salts.
- Catabolism: Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
- Humification: Formation of dark-colored amorphous substance called Humus (resistant to microbial action, reservoir of nutrients).
- Mineralisation: Humus is further degraded by microbes to release inorganic nutrients.
Energy flow in an ecosystem is Unidirectional and follows the Laws of Thermodynamics.
- Source: Sun is the ultimate source. Plants capture only 2-10% of PAR.
- Flow: Energy flows from Producers \(\to\) Herbivores \(\to\) Carnivores.
- 10% Law (Lindeman’s Law): Only 10% of energy is transferred from one trophic level to the next. The remaining 90% is lost as heat (respiration/metabolism).
- Loss: Due to this loss, food chains are usually limited to 3-4 trophic levels.