CBSE Class 12 Biology Ecosystem Worksheet Set B

Question. Trophic levels are formed by : 
(a) plants
(b) animals
(c) organisms linked in food chain
(d) carnivores
Answer. C

Question. Desert can be converted into green land by planting
(a) oxylophytes
(b) psammophytes
(c) halophytes
(d) trees
Answer. B

Question. A scorpion stalks, kills, and then eats a spider.
Based on its behavior, which ecological terms describe the scorpion?
(a) producer, herbivore, decomposer
(b) producer, carnivore, heterotroph
(c) predator, carnivore, consumer
(d) predator, autotroph, herbivore
Answer. A

Question. In the vast marine ecosystem, certain sea develop red colouration. This red colour is due to the presence of large population of which one of the following organisms? 
(a) Trichodesmium erythrium
(b) Physarium
(c) Dinoflagellates
(d) Diatoms and members of red algae
Answer. D

Question. The food chain in which microbes breakdown energy rich compounds synthesized by producers is called 
(a) ecosystem
(b) parasitic food chain
(c) detritus level chain
(d) predator food chain
Answer. C

Question. Mr. X is eating curd/yoghurt. For this food intake in a food chain Mr. X should be considered as occupying 
(a) first trophic level
(b) second trophic level
(c) third trophic level
(d) fourth trophic level
Answer. C

Question. Given below is one of the types of ecological pyramids. This type represents

(a) pyramid of numbers in a grassland
(b) pyramid of biomass in a fallow land
(c) pyramid of biomass in a lake
(d) energy pyramid in a spring
Answer. C

Question. Whale is 
(a) Primary producer
(b) Carnivorous, secondary consumer
(c) A decomposer
(d) Herbivorous
Answer. D

Question. Which one of the following is not a function of an ecosystem? 
(a) Energy flow
(b) Decomposition
(c) Productivity
(d) Stratification
Answer. D

Question. Food chain starts with 
(a) autotrophs
(b) herbivores
(c) carnivores
(d) decomposers
Answer. A

ASSERTION REASON QUESTIONS

Directions : These questions consist of two statements, each printed as Assertion and Reason.While answering these questions, you are required to choose any one of the following five responses.
(a) If both Assertion and Reason are correct and the Reason is a correct explanation of the Assertion.
(b) If both Assertion and Reason are correct but Reason is not a correct explanation of the Assertion.
(c) If the Assertion is correct but Reason is incorrect.
(d) If both the Assertion and Reason are incorrect.
(e) If the Assertion is incorrect but the Reason is correct.

Question. Assertion: Insectivorous habitat of plants is to cope up O2 deficiency.
Reason: Insectivorous plants are partly autotrophic and partly heterotrophic.
Answer. E

Very Short Answer Questions

Question. How is ‘stratification’ represented in a forest ecosystem?
Answer. Stratification in the vertical distribution of species at different levels. Trees occupy top vertical strata, shrubs the second layer and herbs/ grasses occupy the bottom layers.

Question. Give an account of energy flow in an ecosystem.
Answer. Energy Flow
• The sun is the only source of energy for all ecosystems on earth.
• Out of the total incident solar radiation, only 50 per cent of it is photosynthetically active radiation (PAR).
• Plants capture only 2–10 per cent of the PAR and this small amount of energy sustains the entire living world. So, there is unidirectional flow of energy from the sun to producers and then to consumers.

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

Question. What is a detritus food chain made up of? How do they meet their energy and nutritional requirements?
Answer. Dead plant and animal remains and their faecal matter constitute Detritus. DFC includes decomposers which obtain energy by decomposing the dead materials.

Question. State what does ‘standing crop’ of a trophic level represent.
Answer. Standing crop represents the mass of living material (biomass) at a particular time.

Question. Arrange the following as you observe in vertical stratification of a forest—Grass, Shrubby plants, Teak, Amaranthus. 
Answer. Grass, Amaranthus, Shrubby plants, Teak.

Question. List any two ways of measuring the standing crop of a trophic level.
Answer. Standing crop is measured as the biomass or the number of plant in a unit area.

Question. What is meant by humification?
Answer. The process of formation of humus from detritus or dead organic matter is called humification.

Question. Why is a food web formed in nature?
Answer. Many organisms occupy positions in different food chains and several food chains become interconnected to form a food web.

Short Answer Questions

Question. Differentiate between two different types of pyramids of biomass with the help of one example of each.
Answer. Table 14.8: Differences between upright and inverted pyramids of biomass

Question. State the difference between the first trophic levels of detritus food chain and grazing food chain.
Answer. Table 14.9: Differences between first trophic level of detritus food chain and grazing food chain

Question. “In a food-chain, a trophic level represents a functional level, not a species.” Explain.
Answer. A given species may occupy more than one trophic level in the same ecosystem (in different food chains) at the given time. If the function of the mode of nutrition of species changes, its position shall change in the trophic levels. The same species can be at primary consumer level in one food chain and at secondary consumer level in another food chain in the same ecosystem at the given time.  

Answer. The given ecological pyramid is the inverted pyramid.
Inverted pyramid of biomass in a lake: Phytoplankton → Zooplankton → fishes.
Inverted pyramid of number: Tree → insects → birds.

Question. Apart from plants and animals, microbes form a permanent biotic component in an ecosystem.
While plants have been referred to as autotrophs and animals as heterotrophs, what are microbes referred to as? How do these microbes fulfil their energy requirements?
Answer. Microbes are referred to as heterotrophs and saprotrophs. They fulfil their energy requirement by feeding on dead remains of plants and animals through the process of decomposition.

Question. “Decomposition is an oxygen requiring process” Comment.
Answer. Detritus is rich in nitrogen and sugars. For oxidation of nitrogen and sugars oxygen is required by a class of aerobic microbes.

Long Answer Questions

Question. (a) Differentiate between primary and secondary ecological successions.
(b) Explain the different steps of xerarch succession occurring in nature. 
Answer. (a) Primary succession: It begins in areas where no living organisms ever existed. Therefore, the establishment of a biotic community is very slow, e.g., newly cooled lava, bare rock, newly created pond or reservoir.
Secondary succession: It begins in areas where natural biotic communities have been destroyed, e.g., abandoned farm lands, buried or cut forests. Since soil is available, it is a faster process.
(b) Xerarch Succession in Nature:
OU Lichens are the pioneer species on a bare area.
OU The lichen secretes some acids to dissolve rocks and help in weathering and soil formation.
OU Later, some small bryophytes invade and hold the small amount of soil.
OU The bryophytes are succeeded by herbs, shrubs and ultimately big trees.
OU At last, a stable climax forest is formed.
OU The xerophytic habitat gets converted into a mesophytic one.

Question. (a) Explain primary productivity and the factors that influence it.
(b) Describe how do oxygen and chemical composition of detritus control decomposition.
Answer. (a) Refer to Basic Concept Points 5 and 6.
(b) Decomposition of detritus is slow if it contains lignin, chitin, tannins and cellulose, whereas it is quicker if detritus is made up of nitrogenous compounds and water-soluble substances like sugars. This is because the latter are easy to degrade.
Oxygen is required for the activity of decomposers and detritivores. Therefore, a reduced oxygen amount will slow down the process of decomposition.

Question. (a) What is a trophic level in an ecosystem? What is ‘standing crop’ with reference to it?
(b) Explain the role of the ‘first trophic level’ in an ecosystem.
(c) How is the detritus food chain connected with the grazing food chain in a natural ecosystem?
Answer. (a) The specific places occupied by an organism in the food chain is called trophic level.
Each trophic level has a certain mass of living material at a particular time which is called as the standing crop.
(b) The first trophic level is comprised by the producers which trap solar energy to convert it into chemical bond energy of food. They serve as food for subsequent trophic levels.
(c) The organisms of the detritus food chain (DFC) are the prey to the grazing food chain (GFC) organisms. The dead remains of GFC are decomposed into simple inorganic materials. These materials are then absorbed by DFC organisms.

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 biomass: The relationship between producers and consumers in an ecosystem can be represented in the form of a pyramid in terms of biomass called pyramid of biomass. It can be (a) Upright, e.g., in case of grassland ecosystem; or
(b) Inverted, e.g., in case of pond ecosystem as biomass of fishes for exceeds that of phytoplanktons.

Pyramid of energy: The relationship between producers and consumers in an ecosystem can be represented in the form of a pyramid, in terms of flow of energy called pyramid of energy. It is always upright because energy is always lost as heat at each step and as it follow 10% law.

Question. Name the kind of organisms which constitute the pioneer community of xerarch and hydrarch succession, respectively.
Answer. Xerarch succession—Lichens and Hydrarch succession—Phytoplanktons

Question. Citing lake as an example of a simple aquatic ecosystem, interpret how various functions of this ecosystem are carried out. Make a food chain that is functional in this ecosystem.
Answer. (i) Productivity: Conversion of inorganic into organic material with the help of solar energy by the autotrophs.
(ii) Energy flow: Unidirectional movement of energy towards higher trophic level (and its dissipation and loss as heat to the environment).
(iii) Decomposition: Fragmentation, leaching, catabolism, humification, mineralization by bacteria, fungi and flagellates (abundant at the bottom of lake).
(iv) Nutrient cycling: Decomposition of dead matter to release the nutrients back to be re-used by the autotrophs.
Food chain in aquatic ecosystem (lake):
Phytoplanktons → Zooplanktons → Small fish → Big fish (Any other appropriate example).

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.
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.

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. 

Question. It is stated that the pyramid of energy is always upright. Justify. 
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. “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. 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.

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 phyto- planktons will be for an aquatic ecosystem. 
The pyramid of biomass in aquatic ecosystem is generally inverted because the biomass of fishes far exceeds that of phytoplanktons.

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. 
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.

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.
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. (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. 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.  

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. 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. Explain how does a primary succession start on a bare rock and reach a 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. Explain the differences and the similarities between hydrach and xerach successsion of plants. 
Answer. Differences between hydrarch and xerarch succession are as follows :

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.

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

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. (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. 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 :
Pioneer stage : The first vegetation to occupy the rocky habitat is of crustose lichens, e.g., species of Rhizocarpon, Rhinodina and Lecanora. They produce some acids which bring about weathering of rocks which invites foliose lichen. The CO₂ released during the respiration of these lichens react with water and forms acid. Thus, the process of breaking up of rock becomes more rapid.
             ↓
Moss stage : The bryophytes and particularly the mosses develop and spread very rapidly on the rocks. They accumulate more soil and organic matter which makes the surface suitable for next stage.
             ↓
Herb stage : This stage is constituted by shallow rooted grasses as Eleusine, Poa, Aristida, etc. These accumulate more soil and more minerals.
             ↓
Shrub stage : Due to much accumulation of soil, the habitat becomes suitable for shrub vegetation. This habitat includes the species of Zizyphus, Caparis, etc. These, in turn, are finally replaced by trees, the climax community.
             ↓
Forest stage : Some xerophytic tree species invade the area. Further weathering of rocks and increasing humus content of the soil favour the arrival of more trees, and vegetation finally becomes mesophytic. Thus, there develops finally a forest 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. 

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 effiux. Two types of nutrient cycles are gaseous and sedimentary.

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

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 effiux.

Question. Carbon cycle in nature is a biogeochemical event. Explain.
Outline salient features of carbon cycling in an ecosystem. 
Answer. 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 self- regulated 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.

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.

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.  State the difference between the first 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.

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. “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. 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. 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.

Question. Describe the process of decomposition of detritus under the following heads: Fragmentation, leaching; catabolism; humification and mineralisation. 
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.