CBSE Class 12 Biology Ecosystem Worksheet Set A

Very Short Answer

Question. What are pioneer species?
Answer. The species that invade a bare area is known as pioneer species.

Question. What is humus?
Answer. Humification leads to accumulation of a dark coloured amorphous substance is called humus.

Question. What are the two components occur during the decomposition in the soil?
Answer. Humification and mineralisation occur during decomposition in the soil.

Question. Define mineralisation?
Answer. The humus is degraded by some microbes and release of inorganic nutrients occur by the process is called mineralisation.

Question. What is stratification?
Answer. Vertical distribution of the different species occupying different levels is called stratification.

             

Short Answer

Question. What do you understand by nutrient cycling?
Answer. A nutrient cycle refers to the movement and exchange of organic and inorganic matter back into the production of living matter. The process is regulated by the food web pathways previously presented, which decompose organic matter into inorganic nutrients. Nutrient cycles occur within ecosystems.

Question. Define ecosystem?
Answer. An ecosystem is a large community of living organisms (plants, animals and microbes) in a particular area. The living and physical components are linked together through nutrient cycles and energy flows. Ecosystems are of any size, but usually they are in particular places.

Question. Distinguish between primary productivity and secondary productivity?
Answer. 
1. Primary productivity is the rate of synthesis of the biomass or energy fixation by the plants whereas secondary productivity is the rate of the synthesis of biomass by consumers i.e. herbivores and carnivores.
2. Primary productivity is comparatively quite high whereas secondary productivity is small and decrease with rise of the tropic level.

Question. What do you mean by ecosystem services?
Answer. Ecosystem services are the many and varied benefits to humans gifted by the natural environment and from healthy ecosystems. Such ecosystems include, for example, agro ecosystems, forest ecosystems, grassland ecosystems and aquatic ecosystems. These ecosystems, functioning in healthy relationship, offer such things like natural pollination of crops, clean air, and extreme weather mitigation, human mental and physical wellbeing.

Question. How litter is different from detritus?
Answer. 
1. Litter contains all kinds of wastes generated above the ground whereas
detritus is composed of the remains of dead plants and animals.
2. Litter contains both biodegradable and non-biodegradable matter
whereas detritus contains only biodegradable matter.

             

Long Answer

Question. State the difference between carbon cycle and phosphorous cycle?
Answer. 
1. The series of processes by which compounds of carbon are interconverted in the ecosystem are referred to as the carbon cycle whereas the processes by which the compounds of phosphorous are interconverted in the ecosystem are referred as the phosphorous cycle.
2. Carbon cycle has a rapid process whereas phosphorous cycle has a slow process.
3. Carbon cycle interacts with atmosphere whereas phosphorous cycle does not interact with atmosphere.
4. Carbon cycle is a type of gaseous cycling whereas phosphorous cycle is a type of sedimentary cycle.
5. Carbon cycle releases carbon di oxide to the atmosphere by respiration whereas in phosphorous cycle there is no release of gaseous components to the atmosphere.

Question. Write short note on ecological pyramid?
Answer. An ecological pyramid is a graphical representation of the relationship between different organisms in an ecosystem. Each of the bars that make up the pyramid represents a different trophic level, and their order, which is based on who eats whom, represents the flow of energy. The three types of ecological pyramids include: pyramid of numbers, pyramid of biomass and pyramid of energy. Energy pyramid, sometimes called trophic pyramid or ecological pyramid, is useful in quantifying the energy transfer from one organism to another along the food chain. Energy decreases as one move through the trophic levels from the bottom to the top of the pyramid. Thus, the energy pyramid is always upward. The ecological pyramids do not take into account the same species belonging to more than one trophic level. It assumes a simple food chain that almost never exists in nature. It does not explain food webs. The bottom of an ecological pyramid is the broadest and is occupied the producers, which form the first trophic level. Producers are at the lowest level. Just as in a food chain, the producers are consumed by the primary consumers, in an ecological pyramid; the next level is occupied by the primary consumers. The next level of the pyramid is occupied by the secondary consumers and the last, by the tertiary consumers. An ecological pyramid not only shows us the feeding patterns of organisms in different ecosystems, but can also give us an insight into how inefficient energy transfer is, and show the influence that a change in numbers at one trophic level can have on the trophic levels above and below it .

Question. State the difference between food web and food chain?
Answer. 
1. Food chain refers to a feeding hierarchy in which organisms in an ecosystem are grouped into trophic levels and are shown in a succession to represent a linear flow of food energy and the feeding relationships between them whereas food web refers to a graphical model showing the interconnecting food chains in an ecological community.
2. A food chain is a single unit whereas a food web is a collection of several interconnected food chains.
3. Food chain may consist of 4-6 trophic levels whereas food web consists of numerous trophic levels.
4. A food chain is a single, linear pathway of energy flow whereas food web consists of a number of interconnected pathways through which the energy flows within an ecosystem.
5. The food chain increases the instability of an ecosystem whereas a food web increases the stability of an ecosystem.

Question. Write short note on energy flow in ecosystem?
Answer. Energy flow, also known as calorific flow, it refers to the flow of energy through a food chain, and is the focus of study in ecological energetics. In an ecosystem ecologists seek to the quantify the relative importance of different component species and feeding relationships. The flow of energy in the ecosystem is unidirectional because the energy lost as heat from the living organisms of a food chain cannot be reused by plants in photosynthesis. During the transfer of energy through successive trophic levels in an ecosystem, there is a loss of energy all along the path. Ecosystems maintain themselves by cycling energy and nutrients obtained from external sources. The low rate of energy transfer between trophic levels makes decomposers generally more important than producers in terms of energy flow. Energy is needed at all levels of the food chain, as are nutrients. However, when the energy passes on to organism after organism from the initial plants, it is also used up and exhausted, and ultimately, nothing remains that can be recycled to form more energy. Sun is the biggest and ultimate source of energy to Earth. Sunlight energy is trapped autotrophs, which use it to produce food for them. The trapped energy is passed to herbivores animals, which eat the producers. They in turn are eaten by carnivores’ animals, which are themselves eaten by tertiary consumers. This pathway is called a food chain.

Question. Explain about decomposition in ecosystem?
Answer. Decomposition is the process by which dead organic substances are broken down into simpler organic or inorganic matter such as carbon dioxide, water and mineral salt. The process is a part of the nutrient cycle and is essential for recycling the finite matter that occupies physical space in the biosphere. Decomposition of organic matter in soils is an important process in any ecosystem. Decomposer microorganisms feed on the organic matter and break it down into its simplest components, i.e. any excess nutrients are released and are available for plants to use to grow. During the process of the decomposition, the decomposers provide food for themselves by extracting chemicals from the dead bodies or organic wastes; using these to produce energy. The decomposers will then produce waste of their own. Decomposition helps us solve complex problems and manage large projects. This approach has many advantages. It makes the process a manageable and achievable one large problem is daunting, but a set of smaller, related tasks are much easier to take on. Oxygen is needed for many decomposers to respire, to enable them to grow and multiply. This is why we often seal food in bags or cling film before putting it in the fridge. As the volume of available oxygen increases, the rate of decomposition also increases. Some decomposers can survive without oxygen.

Very Short Answer Questions

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 is utilised by the plant by respiration.

Question. Name an omnivore which occurs in both grazing food chain and the decomposer food chain.
Answer. Sparrow/crow

Question. Climax stage is achieved quickly in secondary succession as compared to primary succession. Why? 
Answer. The rate of succession is much faster in secondary succession as the substratum (soil) is already present as compared to primary succession where the process starts from a bare area (rock).

Question. Outline salient features of carbon cycling in an ecosystem.
Answer. Carbon Cycle –
• Carbon constitutes 49 per cent of dry weight of an organism.
• 71 per cent of the carbon is found dissolved in oceans which is responsible for its regulation in atmosphere.
• The carbon cycle occurs through atmosphere, oceans and through living and dead organisms.

Question. Standing crop and biomass are related to each other, how?
Answer. The standing crop is measured as the mass of living organisms or the number of plants in a unit area. The biomass of a species is expressed in terms of fresh or dry weight.

Question. Write the equation that helps in deriving the net primary productivity of an ecosystem.
Answer. GPP – R = NPP

Question. Why is the pyramid of biomass inverted in a pond ecosystem?
Answer. The pyramid of biomass is inverted in a pond ecosystem because the biomass of fish (top consumer) is far larger than the producers (phytoplanktons).

Question. Write a difference between net primary productivity and gross productivity.
Answer. Gross productivity (GPP) is the rate of production of organic matter during photosynthesis.
Net primary productivity (NPP) is the available biomass for the consumption by heterotrophs.
GPP − R = NPP

Short Answer Questions

Question. What is primary productivity? Give brief description of factors that affect primary productivity.
Answer. Primary productivity is the rate of synthesis of biomass by producers, per unit time, per unit area through the process of photosynthesis.
For factors affecting primary productivity, refer to Basic Concepts Point 6.

Question. Differentiate between a detritivore and a decomposer giving an example of each.
Answer. Table 14.10: Differences between detritivore and decomposer (Table 513)

Question. Define ecological pyramids and describe with examples, pyramids of number and biomass.
Answer. Ecological Pyramid
QU The relationship between producers and consumers at different trophic levels in an ecosystem can be graphically represented in the form of a pyramid called ecological pyramid.
QU Structure: The base always represents the producers or the first trophic level and the apex represents top level consumer or the last trophic level.

Question. Sometimes due to biotic/abiotic factor the climax remain in a particular seral stage (preclimax)without reaching the climax. Do you agree with this statement. If yes, give a suitable example. 
Answer. It is true that any change in the abiotic/biotic factor will arrest a particular seral stage leading to a pre-climax condition before the climax stage is achieved. This can happen in cases of forest fires, landslide, change in soil characteristics, increase in herbivore population leading to over-grazing.

Question. Construct a grazing food chain and detritus food chain using the following, with 5 links each:
Earthworm, bird, snake, vulture, grass, grasshopper, frog, decaying plant matter.
Answer. Grazing food chain:
Grass → Grasshopper → Frog → Snake → Vulture
OR
Grass → Grasshopper → Bird → Snake → Vulture

Detritus food chain:
Decaying plant matter Earthworm Bird Snake Vulture

Question. Explain with the help of two examples, how the pyramid of number and the pyramid ofbio mass can look inverted.
Answer. The pyramid of biomass in sea is generally inverted because the biomass of fishes far exceeds that of phytoplankton and the number of big fishes eating the small fishes is also greater than the small ones. Also in pyramid of number, the number of insects feeding on a big tree is far greater than the number of trees. Now the number of small birds depending on the insects and the number of larger birds eating the smaller ones also increases in the order.

Question. Write important features of a sedimentary cycle in an ecosystem.
Answer. Sedimentary cycle is circulation of non-gaseous biogeochemical nutrients between abiotic and biotic components of ecosystem with reservoir pool being lithosphere or sediments of earth. 
Important features of a sedimentary cycle in an ecosystem are:
(i) Input: Weathering of rocks, industrial processing and mining adds the nutrient to the cycling pool.
(ii) Internal cycling: Nutrients contained in the cycling pool are picked up by producers and the process is called uptake. Organic matter with nutrients is then passed to the next trophic levels.
Wastes and dead remains of organisms give rise to detritus which undergoes decomposition.
(iii) Output: It is the loss of nutrients from cycling pool. It occurs through soil erosion, run-off water, etc.

Long Answer Questions

Question. (a) What is meant by ecological succession? Explain how it occurs.
(b) What properties distinguish a pioneer community from a climax community?
Answer. (a) Ecological Succession
• The sequential, gradual and predictable changes in the species composition in an area are called succession or ecological succession.
• It mainly focuses on changes in vegetation that in turn affect types of animals.
• The entire sequence of communities that successively changes in a given area are called sere(s).
• The individual transitional communities are termed as seral stages or seral communities.
• The community that is in near equilibrium with the environment is called a climax community.
(b) Table 14.12: Differences between pioneer community and climax community 

S.No.	Pioneer community	Climax community
(i)	The species which invade a bare area or land to initiate succession is called pioneer community.	The last or final stage in a succession constitute the climax community.
(ii)	The pioneer species have high reproductive rate.	The climax species have low reproductive rate.
(iii)	The pioneer species have short life span.	The climax species have long life span.
(iv)	They are replaceable.	They are stable and not replaced.

Question. Differentiate between primary and secondary succession. Provide one example of each.
Answer. Table 14.11: Differences between primary and secondary succession

S.No.	Primary Succession	Secondary Succession
(i)	It begins with areas where no living organisms ever existed.	It begins in areas where natural biotic communities have been destroyed.
(ii)	Establishment of a biotic community is very slow.	Establishment of a biotic community is faster.
(iii)	Example: Newly cooled-lava/barerocks/newly created ponds or reservoir.	Example: Abandoned farm lands/burnt or cut forests/lands that have been flooded.

Question. Carbon cycle in nature is a biogeochemical event. Explain.
OR
Explain the carbon cycle with the help of a simplified model. 
Answer. Carbon is returned to atmosphere as CO2 by animals and plants through respiration and the activities of decomposers.
• Some amount of fixed carbon is lost as sediments and removed from circulation.
• Burning of wood, forest fire, volcanic activity and combustion of organic matter and fossil fuels are some additional sources for releasing CO2 in the atmosphere.
• Human activities like deforestation and vehicular burning of fossil fuels have caused an increase in the amount of CO2 in atmosphere.

Question. Fill in the missing stages in the given primary hydrarch succession.
Phytoplankton → (a) → (b) → (c) → Submerged free-floating → (d) → Forest plant stage
What is common between hydrarch and xerarch succession?
Answer. (a) Reed-swamp stage
(b) Submerged plant stage
(c) Marsh-meadow stage
(d) Scrub stage
Both the hydrarch and xerarch lead to mesic conditions of forest.

Question. Describe the advantages for keeping the ecosystems healthy. 
Answer. By keeping the ecosystem healthy we can take advantage of the ecosystem services which are the products of ecosystems.
Following are the economic and environmental goods that we obtain from the ecosystem. They 
(i) Purify air and water
(ii) Mitigate drought and floods
(iii) Cycle nutrients
(iv) Generate fertile soil
(v) Provide wildlife habitat
(vi) Maintain biodiversity
(vii) Pollinate crops
(viii) Provide storage site for carbon
(ix) Provide aesthetic, cultural and spiritual value
(x) Provide stable food chain
(xi) Provide economically useful forest products
(xii) Provide sustainable biological legacy to future generations.

Question. State the function of a reservoir in a nutrient cycle. Explain the simplified model of carbon cycle in nature. 
Answer. The function of a reservoir is to meet the deficit of nutrient which occurs due to imbalance in the rate of influx and efflux.
For Carbon Cycle,

Question. Why is the length of a food chain in an ecosystem generally limited to 3–4 trophic levels? Explain with an example.
Answer. The amount of energy flow decreases with successive trophic levels as only 10% of energy is transferred from one trophic level to the next successive level. The energy is lost in the form of respiration and other vital activities to maintain life. If more trophic levels are present, the residual energy will be limited and decreased to such an extent that it cannot further support any trophic level by the flow of energy. So, the food chain is generally limited to 3–4 trophic levels only.
For, e.g., 

Question. What will happen to an ecosystem if
(a) All producers are removed;
(b) All organisms of herbivore level are eliminated; and
(c) All top carnivore population is removed.
Answer. (a) Reduction in primary productivity. No biomass available for consumption by higher trophic levels/heterotrophs and hence heterotrophs also die of starvation.
(b) Increase in primary productivity and biomass of producers. Carnivore population will subsequently dwindle due to food shortage.
(c) Increase in number of herbivores which leads to over-grazing by herbivores, finally resulting in desertification.

Question. Distinguish between
(a) Grazing food chain and detritus food chain
(b) Production and decomposition
(c) Litter and detritus
(d) Upright and inverted pyramid
(e) Food chain and food web
(f) Primary and secondary productivity
Answer. (a) Grazing food chain and detritus food chain:

S.No.	Grazing food chain (GFC)	Detritus food chain (DFC)
(i)	It starts with green plants called producers as first trophic level.	It begins with dead organic matter and decomposers called saprophytes as first trophic level. Decomposers secrete digestive enzymes that breakdown dead and waste into simple, inorganic materials which are absorbed by them.
(ii)	A much less fraction of energy flows through this type of food chain.	A much large fraction of energy flows through this type of food chain.
(iii)	Energy for food chain comes from sun.	Energy for the food chain comes from organic remain or detritus.

(b) Production and decomposition

S.No.	Production	Decomposition
(i)	It is the process of formation of fresh biomass from inorganic matter by producers (plants) using sunlight.	It is the process of degradation of waste biomass into its constituents by decomposers.
(ii)	It traps energy.	It releases energy.
(iii)	It needs sunlight.	It does not require sunlight.
(iv)	It is an anabolic process.	It is a catabolic process.

(c) Litter and detritus

S.No.	Litter	Detritus
(i)	It is made of dried fallen plant matter.	It is freshly deposited organic matter, i.e., remains of plants and animals.
(ii)	It is found above the ground.	It is found both above and below the ground.

(d) Upright and inverted pyramid

S.No.	Upright pyramid	Inverted pyramid
(i)	The base bar comprises of producers in large number.	The base bar comprises producers in smallest number.
(ii)	The number of consumers decrease and become least in top consumer level.	The number of consumers increase and become largest in top consumer level.
(iii)	Pyramid of energy is always upright.	Pyramid of number and biomass may be inverted.

(e) Food chain and food web

S.No.	Food chain	Food web
(i)	The transfer of energy from producers to top consumers through a series of organisms is called food chain.	A number of food chains are inter-connected with each other, forming a web-like pattern is called food web.
(ii)	One organism holds only one position.	One organism can hold more than one position.
(iii)	The flow of energy can be easily calculated.	The flow of energy is very difficult to calculate.
(iv)	It is always straight and proceed in a progressive straight line.	Instead of straight line it is a series of branching lines.

(f) Primary and secondary productivity

S.No.	Primary productivity	Secondary productivity
(i)	It is the rate at which organic matter is built up by producers.	It is the rate at which organic matter is built up by consumers.
(ii)	It is the result of synthesis of fresh organic matter from inorganic materials.	It is the result of synthesis of organic matter from plant organic matter.

 

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.

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.

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. 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. How is the net primary productivity of an ecosystem derived? 
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. How is net primary productivity different from gross primary productivity? 
Answer. Differences between net primary productivity and gross primary productivity are as follows :

Net primary productivity	Gross primary productivity
It is the amount of organic matter stored by producers per unit time per unit area.	It is the amount of organic matter synthesised by producers per unit time per unit area.
Net primary productivity is equal to organic matter synthesised by photosynthesis minus utilisation in respiration and other losses.	Gross primary productivity is equal to rate of increase in body weight of producers plus loss suffered through respiration and damages.
It depends upon gross primary productivity as well as amount of consumption of photosynthates.	It depends upon photosynthetic eciency of producers, availability of solar energy as well as organic nutrients.

 

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

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 is 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. 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.

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. (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.
(b) 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.

Question. Explain primary productivity and the factors that influence it. 
Answer. 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.

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. 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. 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.

Question. How does a detritivore differ from a decomposer? Explain with an example each.
Answer. 

Detritivores	Decomposers
They are animals which feed on detritus.	They are microorganisms which obtain nourishment from organic remains.
Detritivores ingest the organic matter.	They decompose the organic matter by secreting digestive enzymes over it.
Ecologically they cause pulverisation or fragmentation of detritus. e.g., Earthworm.	Ecologically they cause humication and mineralisation of organic matter. e.g., Pseudomonas.

 

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 decom- position.
(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 water- soluble 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. “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. 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. 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. 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.

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

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:

Grazing food chain	Detritus food chain
Thfood chain begins with producers as the first trophic level.	The food chain begins with detritivores and decomposers as their first trophic level.
Energy for the food chain comes from sun.	Energy for the food chain comes from organic remains or detritus.

 

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. 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)

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