CBSE Class 12 Biology Ecosystem Worksheet Set 03

Very Short Answer Questions 

Question: Define primary production.
Answer. It is defined as the amount of biomass or organic matter produced per unit area over a certain time period by plants during photosynthesis.

Question: Why is the rate of assimilation of energy at the herbivore level called secondary productivity?
Answer. It is because the biomass available to the consumer for consumption is a resultant of the primary productivity from plants.

Question: What is net primary productivity? 
Answer. The amount of energy or biomass remaining in a producer after meeting the cost of its respiration and is passed on to next trophic level is called the net primary productivity.

Question: Name the basic requirement of any ecosystem to function and sustain properly.
Answer. A constant input of solar energy is the ultimate source of all energy and requirement of any ecosystem to function and sustain properly.

Question: Differentiate between standing state and standing crop in an ecosystem.
Answer. In an ecosystem, standing crop is the mass of living material in each trophic level at a particular time. Whereas standing state refers to the amount of nutrients in the soil at any given time. 

Short Answer Questions

Question: What is an incomplete ecosystem? Explain with the help of a suitable example.
Answer. An ecosystem is a functional unit with biotic and abiotic factors interacting with one another resulting in a physical structure. Absence of any component will make an ecosystem incomplete as it will hinder the functioning of the ecosystem. Examples of such an ecosystem can be a fish tank or deep aphotic zone of the oceans where producers are absent.

Question: “It is possible that a species may occupy more than one trophic level in the same ecosystem at the same time.” Explain with the help of one example.
Answer. For example, sparrow is an omnivore. When it eats seeds, fruits or any other plant products, it occupies the primary trophic level. Whereas, when it eats worms and any other insect, it occupies the secondary trophic level. Thus, it occupies more than one trophic level in the same ecosystem.

Question: Primary productivity varies from ecosystem to ecosystem. Explain. 
Answer. Primary productivity varies from ecosystem to ecosystem because it depends on the plant species inhabiting the area and their photosynthetic activity. It also depends on various environmental factors, which vary in different ecosystems.

Question: Justify the importance of decomposers in an ecosystem. 
Answer. Decomposers which are heterotrophic organisms, mainly fungi and bacteria, break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients. They meet their energy and nutrient requirements by degrading dead organic matter or detritus. These are also known as saprotrophs. Decomposers secrete digestive enzymes that breakdown dead and waste materials into simple, inorganic materials, which are subsequently absorbed by them.

Question: How are productivity, gross productivity, net primary productivity and secondary productivity interrelated? 
Answer. Productivity is the rate of biomass production. GPP is rate of production of organic matter during photosynthesis.
GPP – R = NPP = 1
Where NPP is biomass available to consumers for secondary productivity. Secondary productivity is rate of formation of new organic matter by consumers. 

Question: Why is earthworm considered a farmer’s friend? Explain humification and mineralisation occurring in a decomposition cycle. 
Answer. Earthworms help in breakdown of complex organic matter as well as loosening of the soil. This helps in the proper growth of the crops. Therefore, they are considered farmer’s friend. 
Humification: The process of accumulation of a dark coloured amorphous substance, called humus, that is highly resistant to microbial action and undergoes decomposition at an extremely slow rate is called humification. Humus being colloidal is reservoir of nutrients.
Mineralisation: The process by which humus is further degraded by some microbes to release inorganic nutrients is called mineralisation. 

Long Answer Questions 

Question: Name the pioneer species on a bare rock. How do they help in establishing the next type of vegetation? Mention the type of climax community that will ultimately get established.
OR
Explain how does a primary succession start on a bare rock and reach a climax community.
Answer. Primary succession on rocks
- Lichens are the pioneer species on a bare area.
- The lichen secretes some acids to dissolve rock and help in weathering and soil formation.
- Later, some small bryophytes invade and hold the small amount of soil. 

Question: Explain xerarch succession highlighting the xeral communities.
Answer. The series of development stages of biotic succession in an arid area are is termed as xeroseres while biological succession on an arid area is called xerarch.
Primary succession on rocks
- Lichens are the pioneer species on a bare area.
- The lichen secretes some acids to dissolve rock and help in weathering and soil formation.
- Later, some small bryophytes invade and hold the small amount of soil.

Question: Describe the inter-relationship between productivity, gross primary productivity and net productivity. 
Answer. Productivity is the rate of biomass production per unit area over a period of time.
Gross primary productivity is the rate of production of organic matter during photosynthesis in an ecosystem.
Net productivity is the gross primary productivity minus respiration losses.

Question: Where and how does the primary succession occur? Explain.
Answer. Primary succession occurs on newly cooled lava or bare rocks or newly created pond or reservoir. (Any two)
Primary succession in water
- The pioneer species are phytoplanktons.
- The phytoplanktons are replaced by free-floating angiosperms.
Primary succession on rocks
- Lichens are the pioneer species on a bare area.
- The lichen secretes some acids to dissolve rock and help in weathering and soil formation. 

Question: Describe the effects of human activities in influencing natural ecosystem cycles with special reference to carbon cycle.
Answer. Human activities have significantly influenced the carbon cycle. Rapid deforestation and massive burning of fossil fuels for energy and transport have significantly increased the rate of release of carbon dioxide into the atmosphere. Carbon dioxide is a greenhouse gas which allows the solar radiations to enter but prevent the escape of heat radiations of longer wavelength. The absorbed radiations again come to earth’s surface and heat it up. Thereby increasing the average temperature of surface of the earth, i.e., global warming.

Question: (a) State any two differences between phosphorus and carbon cycles in nature.
(b) Write the importance of phosphorus in living organisms. 
Answer. (a)Differences between phosphorus and carbon cycles  
S.No. Phosphorus cycle                                                                                                              Carbon cycle
(i) It is a sedimentary cycle. It is a gaseous cycle.
(ii) Atmospheric inputs through rainfall are much smaller.                                               Atmospheric inputs through rainfall are more.
(iii) Gaseous exchange of phosphorus between organism and environment is nil.       Gaseous exchange of carbon between organism and environment is much more.

(b) Phosphorus is a major constituent of biological membranes, nucleic acids and cellular energy transfer system.