VBQs Class 12 Biology Ecosystem

VBQs For Class 12

Please refer to VBQs for Class 12 Biology Ecosystem. All value based questions for Biology Class 12 have been provided with solutions. We have provided below important values questions and answers. Students should learn these solved VBQs for Class 12 Biology as these will help them to gain more marks and help improve understanding of important topics.

Ecosystem VBQs Class 12 Biology with Answers

Ecosystem – Structure and Function

Very Short Answer Type Questions

Question. How is ‘stratification’ represented in a forest ecosystem? 
Answer. Vertical distribution of different species occupying different levels is called stratification. In a forest ecosystem, trees occupy top vertical strata or layer of a forest, shrubs occupy the second level and herbs and grasses occupy the bottom layers.

Long Answer Type Questions

Question. (a) Taking an example of a small pond, explain how the four components of an ecosystem function as a unit.
(b) Name the type of food chain that exists in a pond.
Answer.(a) The four components of a pond ecosystem which function as a unit are :
(i) Abiotic
(ii) Autotrophs
(iii) Heterotrophs
(iv) Decomposers
A small pond is a simple and fairly self-sustainable unit. Abiotic components include water, dissolved inorganic and organic substances and soil deposits at the bottom. Biotic components are producers, consumers and decomposers. Producers are autotrophs which include phytoplanktons, algae, submerged and floating plants. Consumers are differentiated into herbivores (zooplanktons, larvae, tadpole and some fish), primary carnivores (water scorpions, water beetle, dragon fly larvae, Hydra and some fish), secondary carnivores (large fish, water birds, etc.). Decomposers include fungi, bacteria and flagellates.
With the help of radiant energy of the sun, autotrophs convert the inorganic materials into organic matter. Autotrophs are consumed by heterotrophs which build up their own organic matter. Organic wastes and dead organisms are acted upon by decomposers. Minerals are released in this process. The minerals become available to autotrophs for reuse. There is cycling and recycling of matter. However, energy flow is unidirectional i.e., from autotrophs towards the higher trophic levels and there is dissipation of energy at each trophic level which is lost as heat to the environment.
(b) Grazing type of food chain exists in a pond.

Productivity

Very Short Answer Type Questions

Question. Write the equation that helps in deriving the net primary productivity of an ecosystem.
Answer. Net primary productivity is the weight of the organic matter stored by the producers which is available to heterotrophs for consumption. It is equal to the rate of organic matter produced during photosynthesis (GPP) minus respiratory losses (R).
NPP = GPP – R.

Question. All the primary productivity is not available to a herbivore. Give one reason.
Answer. All the primary productivity is not available to a herbivore because a considerable amount of gross primary productivity is used by plants in respiration. 

Question.What does ‘R’ represent in the given equation for productivity in an ecosystem?
GPP – R = NPP. 
Answer. In the given equation, NPP = GPP – R, R represents respiratory losses.

Question. How is net primary productivity different from gross primary productivity?
Answer.Differences between net primary productivity and gross primary productivity are as follows :

VBQs Class 12 Biology Ecosystem

Question. What is secondary productivity? 
Answer.Secondary productivity refers to the rate of formation of new organic matter by the consumers. 

Short Answer Type Questions

Question. What does secondary productivity in an ecosystem indicate? List any two factors by which productivity is limited in aquatic
ecosystems. 
Answer. The rate of synthesis of organic matter by the consumers is known as secondary productivity. Two main factors that limit productivity in an aquatic ecosystem are light and nutrient supply.

Question. How are productivity, gross productivity, net primary productivity and secondary productivity interrelated?
Answer. Productivity is the rate of biomass production per unit area in unit time at any trophic level. Gross productivity is the total organic matter synthesised by producers, in the process of photosynthesis per unit time and area. It is also called gross primary productivity (GPP). Net primary productivity is the weight of the organic matter stored by the producers in a unit area/volume per unit time. It is equal to the rate of organic matter synthesised during photosynthesis, i.e., gross primary productivity minus the rate of respiration and other losses, i.e., NPP = GPP – R Secondary productivity is the rate of formation of new organic matter by consumers.

Short Answer Type Questions

Question. (a) What is primary productivity? Why does it vary in different types of ecosystems?
(b) State the relation between gross and net primary productiviy. 
Answer. (a) Primary productivity is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.It is expressed in terms of gm–2 yr–1 or (kcal m–2 )yr–1.It varies in different types of ecosystems, because it depends upon plant species of the area, their photosynthetic capacity, availability of nutrients, solar radiations, precipitation, soil type and a number of other environmental factors.

Decomposition

Short Answer Type Questions

Question. Describe how do oxygen and chemical composition of detritus control decomposition.
Answer. Decomposition of detritus is an oxygen requiring process, i.e., aerobic conditions are essential for the activity of decomposer organisms. Chemical composition of detritus effects its decomposition. Chitin and lignin decompose at a very slow rate. Cellulose decomposition also takes time whereas, detritus which is rich in nitrogen and water soluble substances (like sugars) decomposes rapidly.

Question. How does the dead organic matter get decomposed in nature? Explain.
Answer. In nature, dead organic matter gets decomposed by decomposer microorganisms like bacteria, fungi through the process of decomposition. Decomposition is the process of breaking down of complex organic matter into inorganic substances like carbon dioxide, water and nutrients.

Question. Differentiate between a detritivore and decomposer giving an examples. 
Answer. Differences between detritivores and decomposers are as follows :

VBQs Class 12 Biology Ecosystem
VBQs Class 12 Biology Ecosystem

Short Answer Type Questions

Question.Why is earthworm considered a farmer’s friend? Explain humification and mineralisation occurring in a decomposition cycle.
Answer. Earthworms are called farmer’s friend because they help in fragmentation of detritus, i.e., breakdown of complex organic matter and loosening of the soil. Humification and mineralisation occur during decomposition in the soil. Humification is the process of formation of highly resistant, dark coloured amorphous substance called humus from detritus or organic remains. Mineralisation is the release of inorganic substances, both non-mineral and minerals from organic matter.

Long Answer Type Questions

Question. Explain the conditions affecting the rate of decomposition. 
Answer. Factors affecting decomposition rate are as follows :
(i) Temperature : A soil temperature of 25°C and more hastens decomposition. A low temperature of less than 10°C reduces rate of decomposition.
(ii) Moisture : Moist environment favours decomposition.
(iii) pH : Neutral and slightly alkaline soils are rich in detritivores, like earthworms and decomposer microbes. Acidity decreases the number of detritivores and decomposers.
(iv) Chemical composition of detritus : In a particular climatic condition, decomposition rate is slower if detritus is rich in lignin and chitin, and quicker, if detritus is rich in nitrogen and watersoluble substances like sugars.
(v) Aerobiosis : Aerobic conditions are essential for activity of decomposer organisms because decomposition is oxygen requiring process.Anaerobiosis reduces decomposition and causes piling up of detritus.

Question. How is detritus decomposed step-by-step by different agents and made available as nutrients to the plants? Explain.
Answer. Detritus (dead remains of plants and animals) gets decomposed and is made available as nutrients to plants by the process of decomposition. The steps in the process of decomposition are fragmentation, leaching, cataboslim, humification and mineralisation.
(i) Fragmentation of detritus : Detritivores (e.g., earthworm) breakdown detritus into small particles.
(ii) Catabolism : Bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
(iii) Leaching : Water soluble inorganic nutrients go down into the deeper layers of soil and get precipitated as unavailable salts.
(iv) Humification : It is the formation of humus from detritus or organic remains. Humus is dark coloured amorphous substance rich in lignin and cellulose. It is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. It is colloidal in nature, a reservoir of nutrients and helpful in maintenance of soil moisture as well as aeration.
(v) Mineralisation : It is the release of inorganic substances by the degradation of humus with the help of microbes.

Energy Flow

Very Short Answer Type Questions

Question. What is a detritus food chain made up of? How do they meet their energy and nutritional requirements? 
Answer.Detritus food chain is made up of decomposers which are heterotrophic organisms (mainly fungi and bacteria). They meet their energy and nutrient requirements by degrading dead organic matter or detritus.

Question. What is the starting point of a detritus food chain?
Answer.Detritus food chain starts from dead organic matter.

Question.State what does standing crop of a trophic level represent.
Answer.Standing crop of a trophic level refers to mass of living material at a particular trophic level at a particular time. It is measured as biomass or the number in a unit area.

Question. “Man can be a primary as well as a secondary consumer.” Justify this statement.
Answer.Man is an omnivore i.e., he eats both plants and their products and animals. So, when he eats plants he is primary consumer and when he eats animals he is secondary consumer.

Question. Name the dominant producers in a deep aquatic ecosystem. What other name could you give to a primary consumer. 
Answer. The dominant producers in an aquatic ecosystem are phytoplanktons, such as diatoms, dinoflagellates and some microscopic algae. Primary consumers are also known as herbivores because they feed on plants or plant products, e.g., grasshopper, rabbit etc.

Short Answer Type Questions

Question. Why are herbivores considered similar to predators in the ecological context? Explain.
Answer.In ecological context, herbivores are considered similar to predators because they feed on plants and their products for their food requirements just like predators feed on prey for their food needs.

Question. Name the type of food chains responsible for the flow of larger fraction of energy in an aquatic and a terrestial ecosystem respectively.Mention one difference between the two food chains. 
Answer.Grazing food chain and detritus food chain are the major conduits for flow of energy in aquatic and terrestrial ecosystems, respectively.
Differences between grazing food chain and detritus food chain are as follows:

VBQs Class 12 Biology Ecosystem

Question. State the difference between the rst trophic levels of detritus food chain and grazing food chain.
Answer. In a grazing food chain, producers constitute the first trophic level. Producers are autotrophic living green plants which can manufacture their own food through photosynthesis utilising inorganic raw materials and sunlight. In a detritus food chain detritivores and decomposers constitute the first trophic level. Decomposers are the saprophytes, chiefly bacteria and fungi that breakdown the complex compounds of dead organisms into simple substances.

Short Answer Type Questions

Question. “It is possible that a species may occupy more than one trophic level in an ecosystem at the same time”. Explain with the help of one example. 
Answer. In an ecosystem, a species may occupy more than one trophic level simultaneously. As the trophic level represents a functional level, not a species as such. A species may occupy more than one trophic level in the same ecosystem at the same time, for example, a sparrow is a primary consumer when it eats seeds, fruits, peas etc. and a secondary consumer when it eats insects and worms.

Question. Justify the importance of decomposers in an ecosystem. 
Answer. Decomposers are microorganisms that obtain nourishment from organic remains by secreting digestive enzymes over it. They help in converting complex organic substances into inorganic substances like carbon dioxide, water and nutrient. Sun is an endless source of enregy, but the chemical materials of the environment are not inexhaustible. The producers utilise inorganic substances (carbon dioxide, water etc.) and fix solar energy into the chemical energy of organic compounds. This stored energy is then passed to consumers by repeated eating and being eaten. Decomposers act on dead animals and plants (organic matter) and return the chemical nutrients to the environment. They also make space available for new producers. Without this, all life will ultimately cease to exist. Thus, the decomposers have a crucial role in the ecosystem. The decomposers are found in the soil and at the bottom of ponds, lakes and oceans.

Question. Discuss the relationship between detritus food chain and grazing food chain in a terrestrial ecosystem.
Answer. In a terrestrial ecosystem, producers constitute the first trophic level or base of a food chain. Producers are autotrophic organisms which are able to manufacture organic food from inorganic raw materials in the process of photosynthesis. Part or whole of the plants enters the food chain as food for consumers. Herbivores, the primary consumers feed on plants or plant products.
Primary carnivores prey upon herbivorous animals and are eaten up by secondary carnivores. The top carnivores are the last order consumers or carnivores which are not preyed upon by other animals because of their size and ferociousness.
Decomposers may act on any trophic level. They feed on dead organic remains of plants and animal and return the biological nutrients stored in them to the soil for reuse by autotrophs i.e., they again enter the food chain. Hence, both detritus and grazing food chain are interrelated.

Question. Construct labelled grazing and detritus food chain with minimum 3 trophic levels each.
Answer. Grazing food chain

VBQs Class 12 Biology Ecosystem

Ecological Pyramids

Short Answer Type Questions

Question. Explain with the help of two examples, how the pyramid of number and the pyramid of biomass can look inverted.
Answer. Pyramid of number is inverted in parasitic food chain. Here, a single large sized producer like tree provides nourishment to several herbivores (e.g., fruit-eating birds) and each herbivore in turn may support a still larger population of parasities. Thus, from the producer towards consumers, the number of organisms gradually shows an increase, making the pyramid inverted in shape.

VBQs Class 12 Biology Ecosystem

Pyramid of biomass is inverted in shape in aquatic habitats where the biomass of a trophic level depends upon reproductive potential and longevity of its members. In a pond, the producers are small organisms, so their biomass is least and this value gradually shows an increase towards the apex of the pyramid, thus making the pyramid inverted in shape.

VBQs Class 12 Biology Ecosystem

Question. Why the pyramid of energy is always upright? Explain. 
Answer. The pyramid of energy is always upright in shape as there is always a gradual decrease in the energy content at successive trophic levels from producers to various consumers. This is because some energy is used at each trophic level for various metabolic activities and some energy is lost as heat, so only 10% of the energy is available to the next trophic level (Lindeman’s 10% law).

Question. Explain ‘standing crop’ in an ecosystem. Draw a pyramid of biomass when a small standing crop of phytoplanktons supports and large standing crop of zooplanktons in the sea.
Answer.Standing crop refers to mass of living material at a particular trophic level at a particular time. It is measured as biomass or the number in a unit area. Pyramid of biomass in sea is inverted as shown below:

VBQs Class 12 Biology Ecosystem

Question. Construct a pyramid of numbers considering a big banyan tree supporting a population of insects, small birds and their predators.
Answer.In the given case, pyramid of numbers will be spindle shaped as shown here.

VBQs Class 12 Biology Ecosystem

Short Answer Type Questions

Question. Differentiate between two different types of pyramids of biomass with the help of one example of each. 
Answer.Pyramid of biomass is a graphic representation of biomass present sequence-wise per unit area of different trophic levels with producers at the base and top carnivores at the tip. Pyramid of biomass may be upright or inverted.

VBQs Class 12 Biology Ecosystem

Question. Construct an ideal pyramid of energy when 1,000,000 joules of sunlight is available. Label all its trophic levels. 
Answer.An ideal pyramid of energy with primary producers storing only 1% of solar energy as NPP is shown below:

VBQs Class 12 Biology Ecosystem

Question. “In a food chain, a trophic level represents a functional level, not a species”. Explain.
Answer.In a food chain, each trophic level represents a functional level not a species because:
(i) At a particular time, a trophic level is never occupied by a single species.
(ii) At a particular time, all organisms of a trophic level are considered together for calculating biomass, number and energy not a single species.
(iii) A particular species may occupy more than one trophic level, e.g., omnivorous species may occupy more than one trophic level in the same ecosystem at the same time.
(iv) If we consider few species then no generalisation can be made for studying ecosystem.

Question. Construct a pyramid of biomass starting with phytoplanktons. Label 3 trophic levels. Is the pyramid upright or inverted. Why? 
Answer.Pyramid of biomass starting with phytoplanktons will be for an aquatic ecosystem. It can be drawn as follows:

VBQs Class 12 Biology Ecosystem

The pyramid of biomass in aquatic ecosystem is generally inverted because the biomass of fishes far exceeds that of phytoplanktons. 

Long Answer Type Questions

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) Ecosystem requires a constant input of energy. Energy flow in the ecosystem is very important as it is the basis of life. Food provides both matter and energy. Flow of energy determines the diversity of organisms. It also determines the developmental and functional status of the ecosystem. Energy flow in an ecosystem is always unidirectional or one way, i.e., solar radiations → producers → herbivores → carnivores. It cannot pass in the reverse direction as there is always decrease in the content and flow of energy with the rise in trophic level. A part of energy captured by producers (gross primary productivity) is used for maintenance (lost in respiration) and as food to herbivores. Only 10% of the gross productivity of producers is entrapped by herbivores for their body building. Herbivores are eaten by primary carnivores. Herbivores not preyed by carnivores die a natural death and energy trapped in their body is transferred to decomposers. Only 10% of the herbivores productivity is utilised for raising productivity of primary carnivores.The rest is consumed in ingestion, respiration, maintenance of body heat and other activities. Higher carnivores similarly are able to retain only 10% of energy present in primary carnivores.

VBQs Class 12 Biology Ecosystem

Therefore, the pyramid of energy is always upright with base representing the producers and apex occupied by top carnivores. Each bar of the pyramid represents one trophic level.
(b) The limitations of ecological pyramids are :
(i) Ecological pyramids assume that food chains are simple. Simple food chains do not occur in nature. Instead, food webs are present.
(ii) A single species may operate at two or more trophic levels. Ecological pyramids have no method of accomodating such cases.
(iii) Ecological pyramids have no place for detrivores and decomposers though they play a vital role in ecosystem.

Question. “It is often said that the pyramid of energy is always upright. On the other hand, the pyramid of biomass can be both upright and inverted.” Explain with the help of examples and sketches. 
Answer.Pyramid of energy is a graphic representation of amount of energy trapped per unit time and area at different trophic levels of a food chain with producers forming the base and top carnivores the tip. The energy content is expressed as kcal/m2/yr. Pyramid of energy is always upright, because there is always a gradual decrease in the energy content at successive trophic levels from the producers to various consumers. As the energy passes onto higher trophic levels, its amount decreases because of its dissipation as heat, its use in overcoming entropy as well as for performing various body activities. An ideal pyramid of energy is as follows:

VBQs Class 12 Biology Ecosystem

Pyramid of biomass is a graphic representation of biomass present per unit area at different trophic levels with producers at the base and top carnivores kept at the tip. Pyramid of biomass is upright for terrestrial habitats. However, inverted pyramids are obtained in aquatic habitats. It is because the biomass of a trophic level depends upon reproductive potential and longevity of its members. Thus, the biomass of phytoplankton is smaller than that of zooplankton and that of the latter less than of primary carnivores. (a) Inverted pyramid of biomass in an aquatic ecosystem. (b) Upright pyramid of biomass in terrestrial ecosystem and are given as follows:

VBQs Class 12 Biology Ecosystem

Question. (a) Explain the significance of ecological pyramid with the help of an example.
(b) Why are the pyramids referred to as ‘upright’ or ‘inverted’? Explain. 
Answer.
(a) An ecological pyramid is a graphic representation of an ecological parameter present in various trophic levels of a food chain with producers forming the base and top carnivores the tip. It expresses the relationship between the organisms at different trophic levels with reference to their number, energy and biomass. For example, pyramid of numbers is a graphic representation of the number of individuals per unit area of various trophic levels stepwise with producers at the base and top carnivores at the tip.
Pyramid of biomass shows the total amount of living matter (fresh weight or dry weight) present in each level and pyramid of energy is a graphic representation of amount of energy trapped per unit time and area in different trophic levels of a food chain with producers forming the base and top carnivores the tip.
(b) Usually producers exceed higher trophic levels in biomass, number and energy. Hence a structure with broad base, gradually tapering body and narrow apex is obtained which resembles an upright pyramid.
In some cases e.g., for biomass in pond ecosystem, for number of individuals in a parasitic food chain the relationship changes. Here producers show the least biomass or number and the top consumers show the highest biomass or number. As a result a structure resembling an ‘inverted’ pyramid is obtained.

Question. (a) Draw a ‘pyramid of numbers’ of a situation where a large population of insects feed upon a very big tree. The insects in turn, are eaten by small birds which in turn are fed upon by big birds.
(b) Differentiate giving reason, between the pyramid of biomass of the above situation and the pyramid of numbers that you have drawn.
Answer. (a) In the given case, pyramid of numbers will be spindle shaped as shown here.

VBQs Class 12 Biology Ecosystem

(b) Pyramid of biomass in the above situation will be upright as tree has the maximum biomass which reduces with successive trophic levels resulting in an upright pyramid whereas the pyramid of number in above case is spindle-shaped where producers which are mainly large sized trees are lesser in number and form the base of the pyramid. The insects are more in number than producers. Then there is gradual decrease in the number of successive carnivores, thus making the pyramid upright again. This results in spindle shaped pyramid as shown here:

VBQs Class 12 Biology Ecosystem

Ecological Succession

Short Answer Type Questions

Question. List the features that make a stable biological community.
Answer. The main features of stable biological community are :
– The community is in equilibrium with the environment.
– The community consists of numerous large and small sized organisms.
– The organisms of the stable community have long life span.

Question. Explain the response of all communities of environment over time. 
Answer. The composition and structure of all communities in an environment change constantly in response to the changing environmental conditions. This change is orderly and sequential, parallel with the changes in the physical environment. These changes lead finally to a community that is in near equilibrium with the environment and is called as climax community.

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. In a hydrosere (succession in a water body) pioneer species include phytoplanktons and climax species include large trees. There is a gradual increase observed in the biomass and the level of biodiversity in the successive seral communities. Sequence of communities during hydrosere is shown below:

VBQs Class 12 Biology Ecosystem

Short Answer Type Questions

Question. Explain succession of plants in xerophytic habitat until it reaches climax community.
Answer. Xerarch succession occurs on bare rocks. The habitat lacks soil, has intense light, fluctuations of temperatures and winds. The species that invade a bare area are called pioneer species. In primary succession on rocks lichens are usually the pioneer species which are able to secrete acids to dissolve rocks, helping in weathering and soil formation. These pave way for some small plants like bryophytes, which are able to take hold in the small amount of soil.They are, with time, succeeded by herbs stage, then shrub stage and then bigger plants, and ultimately a stable climax community, i.e., forest is formed. The climax community remains stable as long as the environment remains unchanged. With time, the xerophytic habitat gets converted into a mesophytic one.
Lichens (Pioneer community)
  ↓
Bryophytes
  ↓
Herbaceous plants
  ↓
Shrubs
  ↓
Trees (Climax community)

Question. Define climax community. How does a sere differ from a seral community.
Answer. Climax community is the stable, self perpetuating and final biotic community that develops at the end of biotic succession and is in perfect harmony with the physical environment. Seral communities, also termed transitional communities are the various biotic communities that develop during biotic succession. On the other hand, the name sere is given to the entire sequence of developmental stages of biotic succession from pioneer to a climax community.

Question. What is ecological succession? Where and why would the rate of succession be faster, in newly created pond or a forest destroyed by a forest fire? 
Answer. Ecololgical succession refers to natural development of series of biotic communities at the same site, one after the other till a climax community develops. Succession will be faster in a forest destroyed by fire because soil, organic matter and vegetative propagules will be present there.

Question. Differentiate between primary and secondary succession. Provide one example of each.
Answer. Differences between primary succession and secondary succession are as follows

VBQs Class 12 Biology Ecosystem
VBQs Class 12 Biology Ecosystem

Question. Explain the differences and the similarities between hydrach and xerach successsion of plants. 
Answer.Differences between hydrarch and xerarch succession are as follows :

VBQs Class 12 Biology Ecosystem

Similarities between hydrarch and xerarch succession are as follows:
(i) Both start with pioneer communities and end in a climax one.
(ii) Both end in a mesic environment.
(iii) Both lead from low to high diversity of life.
(iv) Both result in gradual increase of biomass.
(v) Both involve considerable niche specialisation.

Long Answer Type Questions

Question. Explain how xerarch succession progresses from xeric to mesic condition and forms stable climax community. You may use a flow chart.
Answer.Xerarch succession is the sequence of successional stages that occur on bare rocks. The various stages and their component plant species appearing on a rock are as follows :

VBQs Class 12 Biology Ecosystem

Question. (a) Explain how a hydrarch succession progresses from hydric to mesic condition and forms a stable climax community.
(b) Why is the rate of secondary succession faster than that of primary succession?
Answer. (a) Hydrarch succession refers to series of biotic communities that develop one after the other in a newly formed pond or lake. Various successive stages of hydrosere are :
(i) Plankton stage : It is the pioneer stage of hydrosere. The autotrophic organisms called phytoplanktons, (e.g., diatoms), zooplanktons (which feed on phytoplankton) and decomposers constitute this stage. Death and decomposition of plankton produce organic matter which mixes up with clay and settles at the bottom to form soft mud favourable for growth of next seral stage.
(ii) Submerged stage : The bottom lined by soft mud having organic mater is favourable for growth of submerged plants like Hydrilla, Potamogeton and Najas. They are rooted in the mud and form dense growth. On decaying, the older plants and buried parts of other plants form humus which enriches the newly built up bottom and makes it favourable for growth of next stage.
(iii) Floating stage : Floating leaved anchored plants (e.g., Nymphaea, Nelumbo, etc.) appear when water becomes shallow. The plants make the water rich in mineral and organic matter. It becomes suitable for growth of free floating plants like Lemna, Spirodela etc. Rapid growth of floating stage further builds up bottom so that water becomes shallow.
(iv) Reed swamp stage : Amphibious plants e.g., Phragmites, Typha etc. grow when the water body becomes shallow. Their tangled growth accumulates slit.
(v) Sedge or marsh meadow stage : Reed swamp stage are invaded by Carex (Sedge), grasses like Themeda and Dichanthium and herbs like Campanula etc. The plants transpire rapidly and add abundant humus. Therefore, soil is build up for the next stage.
(vi) Woodland stage : Sedge meadow stage is invaded by some rhizome bearing plants which can tolerate bright sunlight as well as water logged conditions, e.g., Cornus (Bogwood) etc. They invite invasion by trees capable of bearing bright sunlight and water logging, e.g., Populus (Cottonwood), Alnus (Alder). They built up more soil.
(vii) Climax forest : New trees invade the area. These trees grow to greater heights with time a hydrophytic habitat gets converted to mesophytic one and forms a stable community.
(b) Rate of secondary succession is faster as a secondarily barred area has a built in soil organic matter. It is biologically fertile so succession is completed quickly. Underground parts, like vegetative propagules and seeds etc., of plants and invaders quickly give rise to a new community as soon as conditions become favourable. It has few seral changes and quickly gives rise to climax community.

Question. “All successions proceed to a similar climax community–the mesic.” Explain. 
Answer. Based on the nature of the habitat, succession of plants is of two types-hydrarch and xerarch. Hydrosere or hydrarch starts in regions where water is in plenty and progresses from hydric to mesic conditions (adequate moisture conditions). On the other hand, xerosere or xerarch starts in regions where moisture is present in minimal or negligible amounts such as dry deserts, rocks etc. and it progresses from xeric to mesic conditions. Hence, both hydrarch and xerach successions lead to medium water conditions.

Nutrient Cycling

Short Answer Type Questions

Question. Name the two types of nutrient cycle existing in nature. Where are their reservoirs present? State the functions of reservoirs.
Answer. Two types of nutrient cycle existing in nature are gaseous and sedimentary. Atmosphere and lithosphere are reservoirs for gaseous and sedimentary cycle respectively. The function of reservoir is to meet deficiency of nutrient which occurs due to difference in rate of influx and efflux.

Question. Explain the function of ‘reservoir’ in a nutrient cycle. List the two types of nutrient cycles in nature. 
Answer. The function of reservoir is to meet the deficiency of nutrient which occurs due to imbalance in the rate of influx and eflux. Two types of nutrient cycles are gaseous and sedimentary.

Question. Global carbon isfixed in the biosphere through photosynthesis. Explain any two ways by which carbon is returned to the atmosphere.
Answer. The two ways by which carbon is returned to the atmosphere are :
(i) By respiration of all living organisms: During respiration, all living organisms release carbon dioxide which returns to atmosphere thereby replenishing its amount in the atmosphere.
(ii) By burning of fossil fuels: Fossil fuels like coal, petroleum and natural gas are rich source of carbon.On burning, they release carbon in the form of carbon dioxide back to the atmosphere.

Question. How does phosphorus cycle differ from carbon cycle? 
Answer. Differences between carbon cycle and phosphorus cycle are as follows:

VBQs Class 12 Biology Ecosystem

Short Answer Type Questions

Question. Write the importance of phosphorus in living organisms.
Answer. Phosphorus is an important component of nucleic acids, biomembranes as phospholipids, cellular transfer system as ATP, body structure as shells, bones and teeth. It takes part in metabolic reactions involved in release of energy from food and utilisation of this energy in various functions of the body.

Long Answer Type Questions

Question. Explain the carbon cycle with the help of a simplified model.
Answer.A simplified model of carbon cycle is as follows:

VBQs Class 12 Biology Ecosystem

Carbon forms the backbone for complex organic molecules of protoplasm like carbohydrates, lipid, proteins etc. Main sources of carbon are air, water, fossil fuel and rocks. Carbon dioxide is added to the cycle pool of atmosphere by respiration, decomposition of organic waste, burning of fossil fuels, volanic eruption, etc. Carbon fixed by producers enters the food chain and hence passes to herbivores, carnivores, decomposers, etc. Natural exchange between lithosphere and hydrosphere or atmosphere is a very slow process. Major exchange in carbon cycle is between organisms (absorption by producers, released by all in respiration) and the atmosphere or hydrosphere. This cycling is a selfregulated feed back system but has recently been upset due to rapid deforestation and increasing combustion of fossil fuels.

Question. What does the term ‘standing state of soil’ signify? How are the nutrients recycled in the ecosystem? Write a cyclic account of carbon movement in nature.
Answer. Standing state signifies the amount of biogenetic nutrients present at any time in the growth medium (soil/water) of ecosystem. It tends to vary from season to season and ecosystem to ecosystem. Nutrients are exchanged through various components of ecosystem (living and non living) so that they can be used again and again.
Carbon forms the backbone for complex organic molecules of protoplasm like carbohydrates, lipid, proteins etc. Main sources of carbon are air, water, fossil fuel and rocks. Carbon dioxide is added to the cycle pool of atmosphere by respiration, decomposition of organic waste, burning of fossil fuels, volanic eruption, etc. Carbon fixed by producers enters the food chain and hence passes to herbivores, carnivores, decomposers, etc. Natural exchange between lithosphere and hydrosphere or atmosphere is a very slow process. Major exchange in carbon cycle is between organisms (absorption by producers, released by all in respiration) and the atmosphere or hydrosphere. This cycling is a selfregulated feed back system but has recently been upset due to rapid deforestation and increasing combustion of fossil fuels.

Question. Describe the role of living organism in cycle. 
Answer. Phosphate present in the soil may occur in the insoluble form. It is dissolved by chemicals secreted by microorganisms and plant roots. The dissolved phosphate is absorbed by the plants and changed to organic form. Phosphate fertilisers are added to the soil to increase its availability. Soil obtained from lake of ocean bed and guano (excreta of marine birds) are also good source of phosphorus. From plants, phosphorus travels to animals along through the food chain. Animal excretions and dead bodies of organisms are acted upon by decomposers. Phosphorus is released in the process. The same becomes available for re-utilisation by plants.

Question. (a) Draw a simplified model of phosphorus cycling in terrestrial ecosystems.
(b) Write importance of such cycles in ecosystems.
Answer.(a) A simplified model of phosphorous cycle is as follows:

VBQs Class 12 Biology Ecosystem

(b) Nutrient cycling helps to maintain stability and enables proper functioning of an ecosystem. Such cycles help in circulation of nutrients between abiotic and biotic components of an ecosystem and thereby help in maintaining ecosystem balance. Nutrient input in an ecosystem occurs through rain and dust fall, nitrogen fixation and weathering of rocks. These nutrient are taken up by plants and are passed on to different trophic levels of a food chain operating in an ecosystem. These nutrients are again made available to the lithosphere by death of living beings, followed by decomposition of the dead organic matter by microbes. The amount of nutrient uptake is equal to amount of nutrients recycled in a balanced ecosystem. Hence, nutrient cycles ensure proper and continuous supply of nutrients in an ecosystem.

Question.

VBQs Class 12 Biology Ecosystem

(a) (i) Name the biogeochemical (nutrient) cycle shown above.
(ii) Name an activity of the living organisms not depicted in the cycle by which this nutrient is returned to the atmosphere.
(b) How would the flow of the nutrient in the cycle be affected due to large scale deforestation? Explain giving reasons.
(c) Describe the effect of an increased level of this nutrient in the atmosphere on our environment. 
Answer.(a) (i) The biogeochemical cycle shown in the figure is carbon cycle.
(ii) Decomposition by microorganisms is not depicted in the given cycle.
(b) Due to large scale deforestation, the level of carbon dioxide in the environment will be disturbed because plants are the major consumers which utilise carbon dioxide for photosynthesis. This would disturb the balance of oxygen and carbon dioxide in the atmosphere.
(c) Increase in the level of carbon dioxide in atmosphere will result in greenhouse effect. This will lead to global warming (rise in earth’s mean annual temperature). Rise in temperature will result in climatic changes called El Nino effects which may create severe environmental issues and disturb the ecological balance.

Question. Explain with the help of a flow chart recycling of phosphorus in nature. How is phosphorus cycle different from carbon cycle in nature?
Answer. Differences between carbon cycle and phosphorus cycle are as follows:

VBQs Class 12 Biology Ecosystem

Ecosystem Services

Short Answer Type Questions

Question. Biodiversity must be conserved as it plays an important role in many ecosystem services that nature provides. Explain any two services of the ecosystem.
Answer.For best services the ecosystem must be healthy.There are several advantages for keeping ecosystem healthy. Ecosystem services provided by nature are as follows:
(i) Soil formation and soil protection are the major ecosystem services accounting for nearly 50% of their total worth. Plant cover protects the soil from drastic changes in temperature.There is little wind or water erosion as soil particles are not exposed to them. The soil remains spongy and fertile. There are no landslides and no floods.
(ii) Plant litter and humus prevent run off water, hold water like sponge and allow percolation of water. A lot of water is held in the soil which slowly passes towards perched water table. It comes out as springs. They are source of perennial freshwater which is quite pure.
(iii) Plant cover of natural ecosystems absorb polluting gases, cause settling of suspended particulate matter, removes CO2 and releases O2.Hence, purified air becomes available.
(iv) They protect the land from floods, remove sediments and other pollutants and recharge ground water.
(v) There is increase in atmospheric humidity, good rainfall and moderating effect on climate.
(vi) Producers of the ecosystem release lot of oxygen during photosynthesis. Release of oxygen by the producers helps in replenishing this gas being consumed in respiration and combustion.
(vii) There is nutrients cycling and the same are repeatedly circulated and recirculated. It keeps the fertility of soil intact.
(viii) Producers of the ecosystem pick up CO2 from the atmosphere and convert it into organic compounds in the process of photosynthesis. This sustains ecosystem and also provides food to the heterotrophs of the ecosystem.
(ix) Bees and other insects of natural ecosystems visit nearby farmlands and pollinate the crop plants.Elimination of pollinator will eliminate the plant species due to non-reproduction.
(x) A large number of tribals live in forests.
(xi) They provide grazing areas for numerous cattle.
(xii) Natural ecosystems are a source of spiritual, cultural and aesthetic values.

Question. Healthy ecosystems are the base of wide range of (ecosystem) services. Justify. 
Answer.Healthy ecosystems are the base of a wide range of economic, environmental and aesthetic goods and services. Some of these are as follows:
(i) Purification of air and water
(ii) Nutrient cycling
(iii) Provide habitat to wildlife
(iv) Maintenance of biodiversity
(v) Mitigation of droughts and floods.
(vi) Forests provide storage site for carbon etc.