NCERT Class 12 Biology Chapter 14 Ecosystem

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NCERT Class 12 Biology Chapter 14 Ecosystem

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Ecosystem

Chapter: 14

BIOLOGY

UNIT – V ECOLOGY AND ENVIRONMENT

TEXT BOOK QUESTIONS ANSWERS

Q. 1. Fill in the blanks:

(a) Plants are called as ____________ because they fix carbon dioxide.

Ans: Autotrophs.

(b) In an ecosystem dominated by trees, the pyramid (of numbers) is ____________ type.

Ans: Inverted type.

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

Ans: Sunlight.

(d) Common detritivores in our ecosystem are ____________. 

Ans: Fungi and bacteria.

(e) The major reservoir of carbon on earth is ____________.

Ans: Oceans.

Q. 2. Which one of the following has the largest population in a food chain?

(a) Producers.

(b) Primary consumers.

(c) Secondary consumers.

(d) Decomposers.

Ans: (a) Producers.

Q. 3. The second trophic level in a lake is

(a) Phytoplankton.

(b) Zooplankton.

(c) Benthos.

(d) Fishes.

Ans: (b) Zooplankton.

Q. 4. Secondary producers are

(a) Herbivores.

(b) Producers.

(c) Carnivores.

(d) None of the above. 

Ans: (d) None of the above.

(Plants are the only producers and called primary producers. There are no other producers in a food chain).

Q. 5. 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%.

Q. 6. Distinguish between:

(a) Grazing food chain and detritus food chain.

Ans: Grazing food chain and detritus food chain:

GRAZING FOOD CHAIN (GFC): 

(i) It consists of producers and consumers.

(ii) E.g. of food chain on land: 

(a) Grass (Producer) → Goat (Primary consumer) → Man (Secondary consumer) 

(iii) E.g of food chain in aquatic ecosystem: 

(a) Phytoplankton → Zooplankton → Small →

Crustaceans → Insects → Fishes

(iv) GFC is the major conduit for energy flow in an aquatic ecosystem.

DETRITUS FOOD CHAIN (DFC): 

(i) DFC starts with dead organic matter or detritus.

(ii) It is made up of decomposers or saprotrophs mainly fungi and bacteria.

(iii) Decomposers secrete digestive enzymes that breakdown dead and waste materials into simple inorganic materials which are subsequently absorbed by them.

(iv) Larger fraction of energy flows through detritus food chain in a terrestrial ecosystem.

(v) e.g. detritus → earthworm → frog → Snake → Peacock.

(b) Production and decomposition.

Ans: Production and decomposition 

(i) Production: Productivity is rate of biomass production per unit area in unit time. 

(ii) Decomposition: is break down of complex organic matter into inorganic substances like carbon dioxide, water and nutrients with the help of decomposers.

(c) Upright and inverted pyramid.

Ans: Upright and inverted pyramid:

(a) PYRAMID OF NUMBER:

(i) Grass Land: Upright shape.

(ii) Forest (Tree): Inverted shape.

(iii) Aquatic (Pond): Upright shape.

(b) PYRAMID OF BIOMASS:

(i) Grass Land: Upright shape.

(ii) Forest (Tree): Upright shape.

(iii) Aquatic (Pond): Inverted shape.

(c) PYRAMID OF ENERGY: 

(i) All ecosystems have upright shape.

(d) Food chain and Food web.

Ans: Food chain and Food web

Food chain is sequence of organism in an ecosystem according to feeding habits. 

Food web is network of food chains interconnected at various trophic levels to form a number of feeding alternatives among the different organisms of a biotic community.

(e) Litter and detritus.

Ans: Litter and detritus:

Litter is all kinds of wastes generated above the ground.

Detritus: Dead plant and animal remains including fecal matter comprise detritus and it is raw material for decomposition.

(f) Primary and secondary productivity.

Ans: Primary and secondary productivity: 

Primary productivity is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis

Secondary productivity is defined as the rate of formation of new organic matter by consumers.

Q. 7. Describe the components of an ecosystem.

Ans: Ecosystem has two components: 

(i) Biotic component is the living component of an ecosystem that includes biotic factors-producers, consumers, decomposers. 

(ii) Abiotic component: They are the non-living component of an ecosystem-light temperature, water, soil, air, inorganic nutrients.

Q. 8. Define ecological pyramids and describe with examples, pyramids of number and biomass.

Ans: Ecological pyramids are graphical representations that express relationship between organisms at different trophic level of a food chain in terms of number, biomass or energy. The base of each pyramid represents the producers or the first trophic level while the apex represents tertiary or top level consumer.

Pyramid of numbers: It is graphical representation of number of individuals per unit area of various trophic levels with producers at the base and top carnivores at the tip e.g. grassland ecosystem has upright pyramid of numbers. 

Pyramid of biomass: It is graphical representation of biomass per unit area of various trophic levels with producers at the dbase and top carnivores at the tip e.g. grasslands and forest ecosystems has upright Pyramid of biomass).

Q. 9. What is primary productivity? Give brief description of factors that y affect primary productivity.

Ans: Primary productivity is the amount Et of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight (g/m²) or energy (kcal/m²).

Factors affecting primary productivity are:

(i) Plant species inhabiting a particular area and their photosynthetic capacity. 

(ii) Environmental factors like availability of nutrients, soil, solar radiations.

Q. 10. Define decomposition and describe the processes and products of decomposition.

Ans: Decomposition is break down of complex organic matter into inorganic substances like carbon dioxide, water and nutrients with the help of decomposers.

Dead plant and animal remains including fecal matter comprise detritus and it is raw material for decomposition.

Process of decomposition includes fragmentation, leaching, catabolism, humification and mineralisation.

Q. 11. Give an account of energy flow in an ecosystem.

Ans: Solar energy is the only source of energy for all ecosystems on earth except deep sea hydro-thermal ecosystem. Approximately 50% of incident solar radiation is photo synthetically active radiation (PAR). Plants capture only 2-10% of the PAR and this small amount of energy sustains the entire living world. There is unidirectional flow of energy from the sun to producers and then to consumers. 10% law (Lindeman rule): Only 10% of the energy is transferred to each trophic level from the lower trophic level.

Q. 12. Write important features of a sedimentary cycle in an ecosystem.

Ans: Sedimentary cycle e.g. sulphurs and phosphorus cycle. The reservoir for gaseous nutrient cycles is atmosphere and for the sedimentary cycle is Earth’s crust. (Lithosphere) Features of Phosphorus cycle:

(i) Phosphorus is component of biological membranes, nucleic acids and cellular energy transfer systems.

(ii) Animals require phosphorous to make shells, bones and teeth.

(iii) Natural reservoir of phosphorus is rock which contains phosphorus phosphates.

(iv) Weathering of rocks provide phosphates which dissolve in soil solution and are absorbed by plants.

(v) Phosphorous reaches animals from. plants through the food chain.

(vi) The organic waste products and the dead organisms are decomposed by bacteria. releasing phosphorus.

Q. 13. Outline salient features of carbon cycling in an ecosystem.

Ans: (i) Carbon constitutes 49 % dry weight of organisms and is next only to water. 

(ii) The oceanic reservoir regulates the amount of carbon dioxide in atmosphere

(iii) Fossil fuel also represents a reservoir of carbon. 

(iv) Carbon cycling occurs [i] through atmosphere, ocean and [ii] through living & dead organisms.

(v) Carbon is fixed in the biosphere through photosynthesis 

(vi) Carbon returns to the atmosphere as CO2 through:

(a) Respiratory activities of the producers and consumers.

(b) Decomposition of organic waste and dead organic matter of land or oceans by decomposers.

(c) Weathering of carbonate rocks. 

(d) Burning of wood, forest fire and combustion of organic matter, fossil fuel, volcanic activity.

(vii) Some amount of the fixed carbon is removed from circulation by formation of organic and calcareous sediments.

(viii) Human activities like rapid deforestation, massive burning of fossil fuel for energy and transport has significantly Increased the rate of release of carbon dioxide into the atmosphere.