CBSE Class 12 Biology Ecosystem Assignment Set 03

Question. Some amount Carbon dioxide that is removed from carbon circulation is stored in
(a) Organic material in living organism
(b) Plants
(c) Microbes
(d) Sediments

Answer: D

Question. Rocks contain phosphorus in the form of
(a) Phosphite
(b) Phosphoric acid
(c) Phosphates
(d) None of the options

Answer: C

Question. What percentage of global carbon is found dissolved in oceans
(a) 49%
(b) 85%
(c) 25%
(d) 71%

Answer: D

Question. According to estimate what quantity of carbon dioxide is fixed in biosphere by photosynthesis
(a) 4*10¹³kg
(b) 4*10¹⁶kg
(c) 4*10¹⁶kg
(d) 4*10¹⁶tonnes

Answer: A

Question. Product of ecosystem processes are known are
(a) Ecosystem results
(b) Ecosystem services
(c) Ecosystem effect
(d) None of the options

Answer: B

Question. Researchers have put an average price tag of_____ nature’s life supporting services
(a) USD 33×10³ billion
(b) USD 33 billion
(c) USD 33×10³ million
(d) none of the options

Answer: A

Very Short Answer Type Questions

Question. Why green plants are not found beyond a certain depth in the ocean? 
Answer: Beyond a certain depth in the ocean, sunlight is not able to penetrate. Due to which green plants cannot photosynthesise and thus, do not survive.

Question. What is detritus?
Answer: Dead organic matter or remains of plant such as leaves, bark, flower and dead remain of animals, including faecal matter constitute detritus.

Question. “Man can be a primary as well as a secondary consumer.” Justify this statement.
Answer: Man has a varied diet. When on vegetarian diet, they are primary consumers and when on nonvegetarian diet, they are secondary consumers.

Question. Define mineralisation.
Answer: It is the process in which the humus is degraded by certain microbes and thus inorganic nutrients are released in the soil.

Question. Mention the role of pioneer species in primary succession on rocks. 
Answer: The pioneer species invade a bare area and pave way for other species.

Question. Name the pioneer species:
(i) on a bare rock
(ii) in a water body
Answer: (i) Lichens
(ii) Phytoplanktons

Question. Why is an earthworm called a detritivore? 
Answer: This is because earthworm breaks down detritus into smaller particles.

Question. Under what conditions would a particular stage in the process of succession revert back to an earlier stage? 
Answer: Natural or human induced disturbances like fire, deforestation, etc.

Short Answer Type Questions

Question. What would happen to the successive trophic levels in the pyramid of energy, if the rate of reproduction of phytoplanktons was slowed down? Suggest two factors which could cause such a reduction in phytoplankton reproduction.
Answer: If the rate of reproduction of phytoplanktons slows down then the net primary productivity decrease. As a result, flow of energy will also decrease in the successive trophic level.
The following two factors cause reduction in phytoplankton reproduction:
(i) Less water availability
(ii) Less nutrient availability.

Question. Name the two type of nutrient cycles existing in nature. Where are their reservoirs present ? State the functions of the reservoirs.
Answer: The nutrients are never lost from an ecosystem but are recycled time to time and again indefinitely.
Nutrient cycles are of two types :
(i) Gaseous cycles : The reservoir for gaseous cycles exists in the atmosphere e.g. carbon cycle, nitrogen cycle.
(ii) Sedimentary cycles : The reservoir for sedimentary cycles exists in the earth’s crust e.g phosphorus cycle, sulphur cycle.
The function of the reservoir is to meet with the deficit, which occurs due to imbalance in the rate of influx and efflux. 

Question. What could be the reason for the faster rate of decomposition in the tropics?
Answer: The rate of decomposition is regulated by climatic factors like temperature and soil moisture as they have an effect on the activities of soil microbes. The tropics with its hot and humid climatic condition provides an environment which is ideal for the microbes to speed up the process of decomposition.

Question. Name the pioneer and the climax species in a water body. Mention the changes observed in the biomass and the biodiversity of the successive seral communities developing in the water body.
Answer: Pioneer species — Phytoplanktons
Climax species — Forest or trees
Biomass will be gradually increased and phytoplanktons are replaced by free-floating angiosperms then by rooted hydrophytes followed by different seral communities thus, biodiversity also increases.

Question. (a) Describe primary succession that occurs on bare rock.
(b) Differentiate between xerarch and hydrarch successions.
Answer: (a) Refer to Basic Concepts Poiiin). 14
(b) (i) Hydrarch succession: The plant succession which takes place in wet area or water, leading to successional series, progress from hydric to the mesic conditions.
(ii) Xerarch succession: The plant succession which takes place in a dry area, leading to successional series from xeric to mesic conditions.

Question. Explain the function of ‘reservoir’ in a nutrient cycle. List the two types of nutrient cycles in nature.
Answer: The function of the reservoir is to meet the deficit of nutrients which occurs due to imbalance in the rate of influx and efflux.
The two types of nutrient cycles are:
(i) Gaseous, and (ii) Sedimentary

Question. Define decomposition and describe the process and products of decomposition.
Answer: Steps in Decomposition
(i) Fragmentation: The process of breaking down of detritus into smaller particles is called fragmentation, e.g., as done by earthworm (= farmer’s friend).
(ii) Leaching: The process by which water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts is called leaching.
(iii) Catabolism: The enzymatic process by which bacterial and fungal enzymes degrade detritus to simpler inorganic substances is called catabolism.
Decomposition produces a dark coloured nutrient rich substance called humus which on degradation releases CO₂, water and other nutrients in the soil.

Question. (a) Name the type of detritus that decomposes faster. List any two factors that enhance the rate of decomposition.
(b) Write the different steps taken in humification and mineralisation during the process of decomposition. 
Answer: (a) O Detritus rich in nitrogen decomposes faster. These are water-soluble substances like sugar.
O Factors enhancing rate of decomposition: Warm temperature, moist environment, availability of oxygen.
(b) Humification: Accumulation of dark coloured amorphous substance called humus which is resistant to micorbial action and undergoes decomposition at a very slow rate.
Mineralisation: Humus is further degraded by microbes releasing inorganic nutrients.

Question. What are the shortcomings of ecological pyramids in the study of ecosystem?
Answer: The ecological pyramid assumes a simple food chain and does not accommodate food webs.
Thereby, it does not take into account the fact that species may belong to two or more trophic levels at a time. Also saprophytes despite their vital role in ecosystem are given no place in the ecological pyramids.

Question. How does primary succession start in water and lead to the climax community? Explain.
Answer: Primary succession in water
• The pioneer species are phytoplanktons.
• The phytoplanktons are replaced by free-floating angiosperms.

Question. (a) Colonisation of a rocky terrain is a natural process. Mention the group of organisms which invade this area first. Give an example.
(b) Over the years, it has been observed that some of the lakes are disappearing due to urbanisation. In absence of human interference, depict by making a flow chart, how do the successional series progress from hydric to mesic condition.
(c) Identify the climax community of hydrarch and xerarch succession.
Answer: (a) Pioneer species invade the area first. For example, lichen
(b) Phytoplankton – (hydr→ic) Submerged plant stage → Submerged free floating plant stage → Reed swamp stage → Marsh – meadow stage → Scrub stage → Forest stage (Mesic condition)
(c) Forest is the climax community for both successions.

Question. What are the limitations of ecological pyramids?
Answer: Limitations of ecological pyramids:
(i) It never takes into account the same species belonging to
(ii) It assumes a simple food chain, which never exists in nature. It does not accommodate a food web.
(iii) In spite of the vital role played by saprophytes/decomposers, they are not given any position in ecological pyramids.

Long Answer Type Questions

Question. (i) Healthy ecosystems are the base of wide range of (ecosystem) services. Justify.
(ii) Explain the difference and the similarities between hydrarch and xerarch successions of plants.
Answer: (i) Healthy ecosystems are the base for a wide range of economic, environmental and aesthetic goods and services. The products of ecosystem processes are named as ecosystem services. For examples, healthy forest ecosystem purifies air and water, minimize droughts and floods, cycle nutrients, generates fertile soils, provides wildlife habitat, maintains biodiversity, pollinate crops, provide storage site for carbon and also provide aesthetic, cultural and spiritual values. Though value of such services of biodiversity is difficult to determine, it seems reasonable to think that biodiversity should carry a healthy price tag.
(ii) Based on the nature of the habitat – whether it is water (or very wet areas) or it is on very dry areas – succession of plants is called hydrarch or xerarch, respectively.
Differences : Hydrarch succession takes placein wetter areas and the successional series progress from hydric to mesic conditions. Asagainst this, xerarch succession takes place indry areas and the series progress from xeric tomesic conditions.
Similarity : Both hydrarch and xerarch successions lead to medium water conditions (mesic), neither too dry (xeric) nor too wet (hydric). 

Question. Describe the components of an ecosystem.
Answer: An ecosystem consists of two types of components, i.e., biotic or living and abiotic or non-living.
There are three main types of biotic components on the basis of mode of obtaining their food—producers, consumers and decomposers.
(i) Producers (autotrophs): They are photosynthetic or autotrophic plants that synthesise their own organic food from inorganic raw materials with the help of solar radiations. Common producers are algae, plants and photosynthetic bacteria. Phytoplanktons are the producers of aquatic ecosystems.
(ii) Consumers (heterotrophs): They are animals which feed on other organisms or producers for obtaining their nourishment. Common consumers are deer, goat, etc.
(iii) Decomposers: They are saprotrophs which obtain nourishment from organic remains. They release digestive enzymes to digest the organic matter. Common decomposers are detritivores, e.g., earthworm. Abiotic component of ecosystem consists of non-living substances and factors which are as follows:
(a) Temperature (b) Light
(c) Wind (d) Humidity
(e) Precipitation (f) Water, etc.

Question. State the function of a reservoir in a nutrient cycle.
Explain the simplified model of carbon cycle in nature. 
Answer: Function : To meet the deficit which occurs due to imbalance in the rate of influx and efflux. 
Carbon cycle :

Detailed Answer :
Function of reservoir : To make up for the imbalance caused (deficit) due to loss of balance in the efflux and influx of minerals. 
Carbon Cycle : The carbon cycle is a gaseous cycle with its source and sink as the atmosphere and the oceans.
The carbon is present in atmosphere as carbon dioxide which is released by :
(i) Respiration by plants, animals and other living beings.
(ii) Burning of fossil fuels.
The fixation of carbon dioxide is carried out by plants (photosynthesis) so deforestation hampers cycle.
Carbon is also present in nature as calcareous sediments, on oceans (usually) and land (nutrients). 

Question. (i) Name the specific cellular components
where phosphorus is in abundance in living organisms.
(ii) Name the natural reservoir of phosphorus.
(iii) Explain the phosphorus cycle.
Answer: (i) Phosphorus in living systems occur in biological membranes / nucleic acids / cellular energy transfer system / bones, teeth / in animal shells.
(ii) The natural reservoir of phosphorus is rock which contains it in the form of phosphates. 
(iii) Phosphorus cycle : The phosphates dissolved in soil solution are absorbed by the roots of higher plants. The dead organic remains of animals and plants are decomposed by microorganism like bacteria to release phosphorus again in the soil from where it is again absorbed by plants.
For diagram:

Question. Define ecological succession. Give three differences between seral stages and climax community during succession.
Answer: The sequential, gradual and predictable changes in the species composition in an area are called succession or ecological succession.
Table 14.13: Differences between seral stages and climax community

Question. (a) With suitable examples, explain the energy flow through different trophic levels. What does each bar in this pyramid represent?
(b) Write any two limitations of ecological pyramids. 
Answer: (a) In an ideal energy pyramid, the primary producers use only 1% of the energy in the sunlight available to them. The subsequent trophic levels pass on 10% of the energy received from previous trophic level to the next trophic level.

Each bar or level in the pyramid represents the amount of energy transferred to the next trophic level.
(b) (i) It does not take into account the same species belonging to two trophic levels.
(ii) It assumes simple food chain and not food web.
(iii) Saprophytes are not considered. (Any two)

Question. (a) Draw a simplified model of phosphorus cycling in a terrestrial ecosystem.
(b) Write the importance of such cycles in ecosystems. 
Answer: (a)

(b) Such cycles recycle nutrients again and again and maintain the balance in ecosystem.

Question. Describe the process of decomposition of detritus under the following heads: Fragmentation; leaching; catabolism; humification and mineralisation. 
Answer: The process of breaking down complex organic matter into inorganic substances like—, water
and nutrients is called decomposition. The raw material for decomposition is called detritus.
They are dead remains of plants and animals.
Steps in decomposition:
(a) Fragmentation: The process of breaking down of detritus into smaller particles is called fragmentation, e.g., as done by earthworm.
(b) Leaching: The process by which water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
(c) Catabolism: The enzymatic process by which degraded detritus is converted into simple inorganic substances is called catabolism.
(d) 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.
(e) Mineralisation: The process by which humus is further degraded by some microbes and release inorganic nutrients is called mineralisation.