NCERT Solutions: Ecosystem

Q1: Fill in the blanks.
(a) Plants are called as_________ because they fix carbon dioxide.
(b) In an ecosystem dominated by trees, the pyramid (of numbers) is _________ type.
(c) In aquatic ecosystems, the limiting factor for the productivity is _________.
(d) Common detritivores in our ecosystem are_________.
(e) The major reservoir of carbon on earth is_________.
Ans:
(a) Plants are called as 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 productivity is light.
(d) Common detritivores in our ecosystem are earthworms.
(e) A major reservoir of carbon on Earth is oceans.

Q2: Which one of the following has the largest population in a food chain?
(a) Producers
(b) Primary consumers
(c) Secondary consumers
(d) Decomposers
Ans: (d) Decomposers
Explanation: Producers form the base of the food chain and are usually the most numerous. They capture solar energy and convert it into food, supporting all higher trophic levels. As energy is lost at each trophic level (10% law), the number of organisms decreases progressively from producers to top consumers. Hence, producers have the largest population in a typical food chain.

Food Chain Decomposers

Q3: The second tropic level in a lake is
(a) Phytoplankton
(b) Zooplankton
(c) Benthos
(d) Fishes
Ans: (b) Zooplankton
Explanation: In aquatic food chains, phytoplankton (microscopic plants) form the first trophic level as producers. Zooplankton feed on phytoplankton and so occupy the second trophic level as primary consumers. Benthos are organisms living on or in the bottom sediments and fishes are higher consumers in many lake food chains.

Zooplankton

Q4: Secondary producers are
(a) Herbivores
(b) Producers
(c) Carnivores
(d) None of the above

Ans: (d) None of the above

Explanation: Producers are organisms (mainly green plants and algae) that convert solar energy into organic matter and are called primary producers. The term "secondary producers" is not used in standard ecological terminology; herbivores are primary consumers and carnivores are secondary or tertiary consumers depending on their position. Therefore, none of the given options correctly names a distinct group called secondary producers.

Q5: What is the percentage of photosynthetically active radiation (PAR), in the incident solar radiation.
(a) 100%
(b) 50 %
(c) 1-5%
(d) 2-10%

Ans: (b) 50%

Explanation: About half of the incident solar radiation reaching the Earth's surface falls within the visible wavelengths that can be used by photosynthetic pigments; this portion is referred to as photosynthetically active radiation (PAR). The remainder of the incident radiation is in wavelengths that are not efficiently used in photosynthesis or is reflected/absorbed by the atmosphere.

Q6: Distinguish between

(a) Grazing food chain and detritus food chain
(b) Production and decomposition
(c) Upright and inverted pyramid
(d) Food chain and Food web
(e) Litter and detritus
(f) Primary and secondary productivity

Ans: (a) 

(b) 

(c) 

(d)

(e) 

(f) 

Q7: Describe the components of an ecosystem.
Ans: An ecosystem is an interacting unit that includes both the biological community and the non-living components of an area. The living and non-living components interact and function together, which is seen in processes such as nutrient cycling, energy flow, decomposition and productivity. Examples of ecosystems include ponds, forests and grasslands.

Components of an Ecosystem

The two components of an ecosystem are:

• Biotic component: The living part of an ecosystem, comprising producers, consumers and decomposers.
  Producers: Green plants and algae that contain chlorophyll and make organic food from inorganic substances using light energy by photosynthesis; they are also called converters or transducers.
  Consumers: Heterotrophic organisms that obtain food directly or indirectly from producers. Primary consumers (herbivores) feed on producers; secondary and tertiary consumers (carnivores and omnivores) feed on other consumers.
  Decomposers: Micro-organisms such as bacteria and fungi that break down dead plants and animals into simpler inorganic substances and play a key role in nutrient recycling.
• Abiotic component: The non-living physical and chemical factors such as light, temperature, water, soil, air and inorganic nutrients that influence the living organisms and ecosystem processes. These factors determine the distribution, growth and productivity of organisms within the ecosystem.

Q8: Define ecological pyramids and describe with examples, pyramids of number and biomass.
Ans: An ecological pyramid is a graphical representation showing the relative amounts of various ecological parameters - number of individuals, biomass or energy - at successive trophic levels of a food chain. Producers form the base and top carnivores form the apex.
There are three types of pyramids:

1. Pyramid of numbers
2. Pyramid of energy
3. Pyramid of biomass

Pyramid of numbers: This shows the number of individuals at each trophic level of a food chain. It may be upright or inverted.
Example (upright): Grassland - many plants (producers) support fewer herbivores (mice), which in turn support still fewer secondary consumers (snakes) and even fewer tertiary consumers (eagles).
Example (inverted): A single tree (producer) may support many birds and many insects; here the producer count is small while the consumer counts are large, producing an inverted pyramid of numbers.

Pyramid of numbers

Pyramid of biomass: This represents the total biomass (living matter) at each trophic level. It is upright in grasslands and forests where plant biomass is large compared to herbivores and carnivores. In some aquatic systems the pyramid of biomass is inverted because the biomass of consumers (for example fishes) can exceed the standing biomass of their prey (for example zooplankton) due to rapid turnover of the latter.

Q9: What is primary productivity? Give brief description of factors that affect primary productivity.
Ans: Primary productivity is the rate at which producers (mainly green plants and algae) synthesise organic matter (biomass) from inorganic substances per unit area over a period of time.
Factors affecting primary productivity include:

• Light intensity and duration: Photosynthesis requires light; seasonal and daily changes affect productivity.
• Temperature: Influences enzymatic activities and metabolic rates of producers.
• Water availability: Essential for photosynthesis and plant growth; drought reduces productivity.
• Nutrient availability: Essential minerals (nitrogen, phosphorus, potassium, etc.) limit growth when scarce.
• CO₂ concentration: Carbon dioxide is a substrate for photosynthesis; its availability can influence rates.
• Edaphic factors: Soil characteristics such as texture, pH and aeration affect root growth and nutrient uptake.
• Biological factors: Species composition, plant density and herbivory can alter productivity.

Primary Productivity

Q10: Define decomposition and describe the processes and products of decomposition.
Ans: Decomposition is the breakdown of complex organic matter from dead plants and animals into simpler inorganic substances such as carbon dioxide, water and mineral nutrients by the action of decomposers and detritivores.

Decomposition Process

The various processes involved in decomposition are as follows:

1. Fragmentation: Detritivores such as earthworms and insects break down large dead material (detritus) into smaller fragments, increasing the surface area available for microbial action.
2. Leaching: Water-soluble substances are washed out of dead material into the soil where they may become part of the inorganic nutrient pool or get immobilised as salts.
3. Catabolism: Micro-organisms (bacteria and fungi) produce enzymes that chemically break down complex organic molecules into smaller compounds.
4. Humification: Partially decomposed organic matter is transformed into humus, a dark-coloured, colloidal substance that improves soil structure and acts as a reservoir of nutrients.
5. Mineralization: Humus and remaining organic compounds are further broken down by microbes, releasing inorganic nutrients (mineral salts), CO₂ and water back into the environment for uptake by plants.

Products of decomposition include humus, CO₂, water and mineral nutrients such as nitrates, phosphates and sulfates which are returned to the soil and atmosphere and become available for primary producers.

Q11: Give an account of energy flow in an ecosystem.
Ans: Energy enters an ecosystem as solar radiation. Plants (producers) capture part of this solar energy by photosynthesis and convert it into chemical energy stored as organic matter. Only a fraction of the incident solar radiation is usable for photosynthesis (PAR), and plants capture a small portion of that; overall only a few percent of incoming solar energy is fixed as biomass.
Gross primary productivity (GPP) is the total rate at which producers convert solar energy into chemical energy. Producers use some of this energy for their own respiration; the remaining energy, after respiratory losses, is called net primary productivity (NPP) and is available to consumers.
Energy transfer between trophic levels is inefficient. Typically only about 10% of the energy stored at one trophic level is incorporated into the biomass of the next higher level; most energy is lost as heat, used in metabolic processes or left in uneaten parts. This progressive loss explains why food chains are normally short and why energy pyramids are always upright.

Flow of energy