NCERT Solutions for Class 12 Biology Chapter 12 Ecosystem

Q1. Fill in the blanks.

(a) Plants are called as_________because they fix carbon dioxide.

Answer:

Plants are called autotrophs because they fix carbon dioxide.

Q1. Fill in the blanks.

(b) In an ecosystem dominated by trees, the pyramid (of numbers) is_________type.

Answer:

In an ecosystem dominated by trees, the pyramid (of numbers) is inverted.

Q1. Fill in the blanks.

(c) In aquatic ecosystems, the limiting factor for productivity is_________.

Answer:

In aquatic ecosystems, the limiting factor for productivity is light.

Q1 . Fill in the blanks.

(d) Common detritivores in our ecosystem are_________.

Answer:

Common detritivores in our ecosystem are earthworms

Q1. Fill in the blanks.

(e) The major reservoir of carbon on Earth is_________.

Answer:

The major reservoir of carbon on Earth is the ocean.

Q2 . Which one of the following has the largest population in a food chain?

(a) Producers

(b) Primary consumers

(c) Secondary consumers

(d) Decomposers

Answer:

(d) Decomposers

Decomposers are microorganisms, including fungi and bacteria. These break down the remains of dead plants and animals. Decomposers form the largest community of organisms in a food chain.

Q3. The second trophic level in a lake is

(a) Phytoplankton

(b) Zooplankton

(c) Benthos

(d) Fishes

Answer:

(b) Zooplankton

In a lake, the first trophic level is formed by phytoplankton, while the second trophic level is formed by zooplankton.

Q4. Secondary producers are:

(a) Herbivores

(b) Producers

(c) Carnivores

(d) None of the above

Answer:

(d) None of the above

In a food chain, plants are the only producers, there are no other producers. Herbivores and carnivores are primary and secondary consumers, respectively.

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%

Answer:

(b) 50%

Out of the total incident solar radiation, 50 % is formed by photosynthetically active radiation.

Q6. Distinguish between

(a) Grazing food chain and detritus food chain

Answer:

Q6. Distinguish between

(b) Production and decomposition

Answer:

Q6. Distinguish between

(c) Upright and inverted pyramid

Answer:

Q6. Distinguish between

(d) Food chain and Food web

Answer:

Q6. Distinguish between.

(e) Litter and detritus

Answer:

Q6. Distinguish between.
(f) Primary and secondary productivity

Answer:

Q7. Describe the components of an ecosystem.

Answer:

An ecosystem refers to the functional unit of nature in which living organisms interact among themselves and also with their surrounding physical environment to perform nutrient cycling, energy flow, decomposition and productivity. There are many types of ecosystems such as pond ecosystems, forest ecosystems etc.

Components of the ecosystem

The components of the ecosystem can be divided into abiotic components and biotic components

1. Abiotic components- These include the non-living components of nature such as light, temperature, water, soil, air, inorganic nutrients etc.

2. Biotic components- The biotic components of an ecosystem refer to the living organisms present in that ecosystem. The biotic components are divided into producers, consumers and decomposers. Producers are plants, consumers include animals, which can be herbivores or carnivores, and decomposers include microorganisms like fungi and bacteria.

Q8. Define ecological pyramids and describe with examples, the pyramids of number and biomass.

Answer:

Ecological pyramid- The graphical representation of an ecological parameter such as number, biomass or energy, sequence-wise in various trophic levels of a food chain in which producers are at the base, herbivores in the middle and carnivores at the top level. Ecological pyramids can be upright, inverted, or spindle-shaped. The three common types of ecological pyramids include the pyramid of numbers, the pyramid of biomass and the pyramid of energy.

1. Pyramids of numbers- It is the number of individuals per unit area at various trophic levels. It is generally upright, however, the pyramid of numbers in the case of a big tree is generally inverted because several insects that feed on the tree generally exceed in number.

2. Pyramids of biomass- These represent the biomass in various trophic levels. A pyramid of biomass is upright except in an aquatic food chain. A pyramid of biomass in the sea is generally inverted because the biomass of fish is generally greater than that of phytoplankton.

3. Pyramids of energy- It is the graphic representation of the amount of energy trapped at different trophic levels per unit area. The pyramid of energy is always upright.

Q9. What is primary productivity? Give a brief description of factors that affect primary productivity.

Answer:

Primary productivity refers to the amount of organic matter or biomass produced by producers per unit area over some time. The primary productivity of an ecosystem depends upon several factors such as light, temperature, water, precipitation, availability of nutrients etc.

Q10. Define decomposition and describe the processes and products of decomposition.

Answer:

Decomposition is the process of breaking down complex organic matter of detritus into inorganic substances such as carbon dioxide, water and nutrients. Dead remains of plants and animals constitute detritus. The process of decomposition involves steps like fragmentation, leaching, catabolism, humification and mineralisation.

1. Fragmentation of Detritus- In this step, the detritus is broken down into small fragments by earthworms

2. Leaching- In leaching, the water-soluble nutrients seep down into the soil and become unavailable salts.

3. Catabolism- The small fragments are decomposed by decomposers like fungi and bacteria with the help of the action of enzymes.

4. Humification- In this step, humus is formed. Humus is a dark-coloured, amorphous solid substance that acts as a nutrient reserve.

5. Mineralization- In this step by the action of microbes, the inorganic nutrients are released from the humus.

Q11. Give an account of energy flow in an ecosystem

Answer:

Energy Flow in an Ecosystem:

All living organisms are dependent on their food producers, directly or indirectly. There is a unidirectional flow of energy from the sun to producers and then to consumers. Photosynthetically active radiation (PAR) is responsible for the synthesis of food by plants. Animals obtain their food from plants, so they are called consumers. The process of eating and being eaten is called a food chain, in which energy flows from producers to consumers. For example, in the Grazing food chain, the grass is eaten by goats, which are further eaten by man. Similarly, in the detritus food chain, the sequence begins with dead organic matter. It is made up of decomposers, which are heterotrophic organisms (fungi and bacteria). These are also known as saprotrophs. Decomposers secrete digestive enzymes that break down dead and waste materials into simple, inorganic materials, which are subsequently absorbed by them. The natural interconnection of the food chain forms the food web. Thus, in an ecosystem, energy flow occurs through food chains and food webs.