Chapter 14: Ecosystems and Energy Flow [ Exercise,Solutions,Notes ]

Chapter 14: Ecosystems and Energy Flow [ Exercise,Solutions,Notes ]

Chapter 14: Ecosystems and Energy Flow [ Exercise,Solutions,Notes ]

Multiple choice question

Exercise | Q 1.1 | Page 320
Which one of the following has the largest population in a food chain?
  • Producers
  • Primary consumers
  • Secondary consumers
  • Decomposers

Exercise | Q 1.2 | Page 320
The second trophic level in a lake is ________________.
  • Phytoplankton
  • Zooplankton
  • Benthos
  • Fishes
The second trophic level in a lake is Zooplankton.

Exercise | Q 1.3 | Page 320
Secondary consumers are __________.
  • Herbivores
  • Producers
  • Carnivores
  • Autotrophs
Secondary consumers are Carnivores.

Exercise | Q 1.4 | Page 320
What is the % of photosynthetically active radiation in the incident solar radiation?
  • 100%
  • 50 %
  • 1-5%
  • 2-10%
50 %

Exercise | Q 1.5 | Page 320
Give the term used to express a community in its final stage of succession?
  • End community
  • Final community
  • Climax community
  • Dark community
Climax community

Exercise | Q 1.6 | Page 320
After landslide which of the following type of succession occurs?
  • Primary
  • Secondary
  • Tertiary
  • Climax
Explanation: Landslides are considered to be an example of primary succession because the initial disturbance usually removes most of the original soil content and habitat; however, landslides can often retain remnants of the original soil.
Succession is generally regarded as primary when it occurs on a landslide where rock and subsoil, virtually free of organic matter are exposed, despite the fact that the landslide was formed the substratum just beneath a developed soil carrying vegetation.

Exercise | Q 1.7 | Page 320
Which of the following is most often a limiting factor of the primary productivity in any ecosystem.
  • Carbon
  • Nitrogen
  • Phosphorus
  • Sulphur

Very short answer question

Exercise | Q 2.1 | Page 320
Give an example of an ecosystem that shows an inverted pyramid of numbers.
A tree ecosystem is an example of an inverted pyramid of numbers.

Exercise | Q 2.2 | Page 320
Give an example of an ecosystem that shows an inverted pyramid of biomass.
The oceanic ecosystem is an example of an inverted pyramid of biomass.

Exercise | Q 2.3 | Page 320
Which mineral acts as a limiting factor for productivity in an aquatic ecosystem.
Phosphorus is the mineral that acts as a limiting factor for productivity in an aquatic ecosystem.

Exercise | Q 2.4 | Page 320
Name the reservoir and sink of carbon in the carbon cycle.
i. Reservoir of carbon in the carbon cycle in the atmosphere and ocean.
ii. Carbon which is present in rock and fossil fuels (like oil, coal, and natural gas) are the sink of the carbon cycle

Exercise | Q 3.1 | Page 320
Distinguish between the upright and inverted pyramid of biomass.
 Upright Pyramid of BiomassInverted Pyramid of Biomass


It is the type of ecological pyramid where the producers have maximum biomass and occupy a broad base and the consumers decrease in terms of biomass.It is the type of ecological pyramid where the producers have less biomass and form a narrow base, while the consumers are more in terms of biomass.

Upright pyramid of biomass in the grassland ecosystem.

The inverted pyramid of biomass in the oceanic ecosystem.

Exercise | Q 3.2 | Page 320
Distinguish between Food chain and food web
 Food ChainFood Web
i.The food chain is a definite sequence of interactions between producers, consumers, and decomposers (saprophytes).The Food web is a network of food chains that are interconnected at various levels forming an intricate web instead of a linear chain.

If any of the intermediate organisms are removed from the chain it affects the whole food chain.

In the food web, there is more than one alternative of food to most of the organisms; hence the removal of an organism does not affect the food web directly.

Exercise | Q 4.1 | Page 320
Describe with examples of pyramids of number, and biomass.
An ecological pyramid is a graphical representation of various environmental parameters such as the number of individuals present at each trophic level, the amount of energy, or the biomass present at each trophic level. Ecological pyramids represent producers at the base, while the apex represents the top-level consumers present in the ecosystem.

Exercise | Q 4.1 | Page 320
Define ecological pyramids.
Pyramid of number: The number of individuals at each trophic level is shown in the pyramid. The pyramid of numbers (for example of a grassland) is upright. In this, there is a decrease in the number of organisms starting from primary producers (plants) to top consumers (carnivores).

Pyramid of biomass : The pyramid of biomass is a graphic representation of the amount of biomass per unit area sequence-wise in rising trophic levels with producers at the base and top carnivores at the apex. Pyramids of the biomass of a tree or. grassland ecosystem is upright and the pyramid of a pond ecosystem is inverted.

Exercise | Q 4.2 | Page 320
What is primary productivity?
Primary productivity:
Primary productivity is the rate of generation of biomass in an ecosystem that is expressed in units of mass per unit surface (or volume) per unit time i.e. g/m2/day.
The mass unit may relate to dry matter or to the mass of carbon generated.

Exercise | Q 4.2 | Page 320
Give a brief description of the factors that affect primary productivity.
Factors that affect primary productivity are as follows:
  • It depends on the plant species inhabiting a particular area.
  • It depends upon environmental factors such as light, temperature, water, precipitation, etc.
  • It depends upon the availability of nutrients.
  • It also depends upon the photosynthetic capacity of plants. The greater the photosynthetic activity, the higher will be the primary productivity.
Exercise | Q 4.3 | Page 320
Define decomposition.
i. Decomposition is the process of the breakdown of complex organic matter into inorganic substances like carbon dioxide, water, and nutrients by decomposers.
ii. Raw materials for decomposition are dead remains of plants and animals, fecal matter, detritus.
iii. This process requires oxygen. Temperature and soil moisture are important factors that indirectly help soil microbes for decomposition.
iv. Warm and moist environment favors decomposition whereas, low temperature and anaerobic conditions inhibit the process.

Exercise | Q 4.3 | Page 320
Describe the processes and products of decomposition.
The steps of decomposition are fragmentation, leaching, catabolism, humification, and mineralization.
a. Fragmentation: Detritivores like earthworms break down detritus into smaller fragments or particles.
b. Leaching: In this process, water-soluble inorganic nutrients percolate into the soil horizon and get precipitated as unavailable salts.
c. Catabolism: The bacterial and fungal enzymes degrade detritus into simpler inorganic substances. All of the above steps occur simultaneously.
d. Humification: It leads to the accumulation of a particularly decomposed, dark-coloured, amorphous, a colloidal organic substance called humus. Humus serves as a reservoir of nutrients. It is resistant to microbial action and undergoes decomposition at an extremely slow rate. Humus changes the soil texture and increases the capacity of water holding in the soil.
e. Mineralization: Some microorganisms degrade humus and release inorganic nutrients by the process of mineralization.

Exercise | Q 4.4 | Page 320
Write important features of a sedimentary cycle in an ecosystem.
  1. Sedimentary cycles have their reservoirs in the Earth’s crust or rocks. Nutrient elements are found in the sediments of the Earth. Elements such as sulphur, phosphorus, potassium, and calcium have sedimentary cycles.
  2. Features of a sedimentary cycle in an ecosystem are as follows:
  3. Earth’s crust is the main reservoir of phosphorus and other minerals, such as calcium and potassium that undergo sedimentary cycles.
  4. The rate of release of minerals that take part in the sedimentary cycle is regulated by various environmental factors temperature, moisture, and nature of the soil.
  5. Sedimentary cycles are slower than the gaseous cycles therefore they take more time to complete. They take a long time to complete their circulation and are considered as less perfect cycles. This is because, during recycling, nutrient elements may get locked in the reservoir pool, thereby taking a very long time to come out and continue circulation. Thus, it usually goes out of circulation for a long time.
  6. Sedimentary cycles are considered as less perfect cycles as, during recycling, nutrient elements may get locked in the reservoir pool, thereby taking a very long time to come out and continue circulation

Exercise | Q 4.5 | Page 320
Describe the carbon cycle and add a note on the impact of human activities on the carbon cycle.
i. Reservoir of carbon:
  1. All life forms on earth are carbon-based because carbon is the main component of all the organic compounds of protoplasm. It constitutes 49% of the dry weight of organisms.
  2. 71% of carbon is found dissolved in oceans. The oceanic reservoir regulates the amount of carbon dioxide in the atmosphere.
  3. Carbon present in the rock and fossil fuels like oil, coal, and natural gas has been away from the rest of the carbon cycle for a long time. These long-term storage places are known as the sink.
  4. The element carbon is a part of seawater, the atmosphere, rocks such as limestone and coal, soils, as well as all living things.
ii. Cyclic pathway of carbon:
  1. Carbon as CO2 moves from the atmosphere to plants during the process of photosynthesis to produce food.
  2. Carbon moves from plants to animals, through food chains.
  3. At the time of exhalation, the CO2 gas is released into the atmosphere. Thus, carbon moves from living things to the atmosphere.
  4. Decomposers also contribute substantially to CO2 in the atmosphere, by their processing of waste materials and dead organic matter of land and oceans.
  5. When fossil fuels burn to power factories, power plants, motor vehicles, most of the carbon quickly enters the atmosphere as carbon dioxide gas.
  6. Most of the remainder is dissolved in seawater and deposited as calcium or magnesium carbonate compounds that make up shells of marine animals.
  7. The additional sources for releasing CO2 in the atmosphere are the burning of wood, forest fire and combustion of organic matter, fossil fuel, and volcanic activity.
  8. The ocean absorbs some carbon in the form of CO2 from the atmosphere. This carbon gets dissolved in the ocean water. Some amount of the carbon which is fixed is lost to sediments and removed from circulation.
iii. The impact of human activities on the carbon cycle
  1. The carbon cycle is significantly influenced by human activities.
  2. Rapid deforestation and the massive burning of fossil fuels for energy and transport have significantly increased the rate of release of carbon dioxide into the atmosphere.

Ecosystem and energy flow solutions and notes

An ecosystem is a self regulatory and self sustaining structural and functional unit of nature (biosphere). It contains both biotic and abiotic components. Biotic components interact with each other and also with the surrounding environment. Tansley (1935) coined the term ecosystem. 

Ecosystems vary greatly in size from a small pond to large oceans or small farmland to village. Entire biosphere can be considered as one global ecosystem, made up of many local ecosystems. Since the earth ecosystem is too big and too complex to be studied, it is divided into two basic categories, viz. terrestrial and aquatic. 

Forest, grassland and desert are the types of terrestrial ecosystems while lakes, wetlands, rivers and estuaries are the types of aquatic ecosystems. The ecosystems can also be classified as Natural ecosystems and Artificial ecosystems. Natural ecosystems do not require any human inputs, in other words they are self-sustainable. Artificial ecosystems e.g. a farm land, a fish tank or even a large pond used for rearing fish, require constant input in terms of energy or materials

In this chapter, we will study and analyse the structure of the ecosystem, in order to appreciate the input (productivity), transfer of energy (food chain/web, nutrient cycling) and the output (degradation and energy loss). We will also look at the relationships, chains and webs that are created because of the energy flows within the system.

Ecosystem : Structure and Function : We have already studied the various biotic and abiotic components of the environment. We know that all these biotic and abiotic components influence each other. Let us now look at these components with an integrated approach and see how the flow of energy takes place in ecosystem. Interaction of biotic and abiotic components, results in a physical structure that is characteristic for each type of ecosystem. Identification and enumeration of plant and animal species of an ecosystem gives its species composition. 

Biotic and abiotic components differ as the locations vary in space and time. The variation due to space results in spatial pattern. There are two types of spatial patterns. viz. Stratification and Zonation. Vertical distribution of different species of plants and animals occupying different levels, is known as stratification. 

For example, trees occupy top vertical strata or layer of a forest, shrubs the second and herbs and grasses occupy the bottom layer. Similar stratification is also observed in the open seas as epipelagic, mesopelagic, bathy-pelagic and benthic zones Horizontal distribution of plants and animals on land or in water, is called zonation. Zonation is observed in aquatic (wetlands) as well as terrestrial ecosystems, but it is easily seen at the junction of the two. Edges of a large lake or beach show pronounced zonation in the form of Inter-tidal, Littoral, Sub-littoral zones.

The biotic and abiotic components of an ecosystem are all linked together to function as an ‘ecosystem unit’ through various processes like, Productivity, Decomposition, Nutrient cycling and Energy flow. In fact, these are functional aspects of ecosystem. Any ecosystem must perform these four processes for its sustainance (to be self– sustaining). 

The ecosystem understudy may be as small as a pond or entire biosphere as a whole. The process of productivity involves conversion of inorganic chemicals into organic material with the help of the radiant energy of the sun by the autotrophs and consumption of the autotrophs by heterotrophs. The Decomposition is the break down of dead organic material and mineralization of the dead matter. The nutrient cycling is the storage

 and transport of nutrients. (minerals released in decomposition process are used again by autotrophs). The energy flow is unidirectional flow of energy from producers to consumers and finally dissipation and loss as heat. Example- Think of a small pond ecosystem. It is fairly a self-sustainable unit that explains the complex interactions which exist in any aquatic ecosystem. 

A pond is a shallow water body in which all the above four aforesaid basic processes of an ecosystem are observed. The abiotic component is water with all the dissolved inorganic and organic substances and also the rich soil deposit at the bottom of the pond. The solar input, the cycle of temperature, day-length and other climatic conditions regulate the rate of function of the entire pond. 

The producers include the phytoplankton, algae and other aquatic plants. The consumers are represented by the zooplankton, aquatic insects and fish. The decomposers are the fungi, bacteria located at the bottom of the pond.

a. Productivity : A constant input of solar energy is the basic requirement for any ecosystem to function and sustain. Productivity refers to the rate of generation of biomass in an ecosystem. It is expressed in units of mass per unit surface (or volume) per unit time, for instance grams per square metre per day (g/ m2 / day). The mass unit may relate to dry matter or to the mass of carbon generated. 

It can be divided into gross primary productivity (GPP) and net primary productivity (NPP). Gross primary productivity of an ecosystem is the rate of production of organic matter during photosynthesis. Plants themselves use a considerable proportion of this GPP for their respiration. Hence, gross primary productivity minus respiratory losses (R) constitute the net primary productivity (NPP).

Net primary productivity is the available biomass for the consumption, to heterotrophs (herbivores, carnivores and decomposers). The annual net primary productivity of the whole biosphere is approximately 170 billion tons (dry weight) of organic matter. Of this, the productivity of the oceans is only 55 billion tons. Rest of course, is from land based ecosystems Primary productivity (GPP) depends on the plant species inhabiting a particular area.

 It also depends on a variety of environmental factors, availability of nutrients and photosynthetic capacity of plants. Therefore, it varies in different types of ecosystems. Secondary productivity is defined as the rate of formation of new organic matter by consumers. Alternatively, it is the rate of assimilation of food energy at the level of consumers. It is the amount of energy available to consumer for transfer to the next trophic level.

b. Decomposition : Decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients, and the process is called decomposition. Dead remains of plants and animals, including fecal matter, constitute detritus, which is the raw material for decomposition. The important steps in the process of decomposition are fragmentation, leaching, catabolism, humification and mineralization. Detritivores (e.g. earthworm) break down detritus into smaller fragments or particles. This process is called fragmentation.

By the process of leaching, water soluble inorganic nutrients go down (percolate) into the soil horizon and get precipitated as unavailable salts. Bacterial and fungal enzymes degrade detritus into simpler inorganic substances. This process is called as catabolism. It is important to note that all the above steps in decomposition operate simultaneously on the detritus.

Humification and mineralization occur during decomposition in the soil. Humification leads to accumulation of partially decomposed, a dark coloured, amorphous, colloidal organic substance called humus that is resistant to microbial action and undergoes decomposition at an extremely slow rate. Humus formation changes soil texture and increases water holding capacity of soil. Being colloidal in nature humus serves as a reservoir of nutrients. 

The humus is further degraded by some microbes and release of inorganic nutrients occurs by the process known as mineralisation. Decomposition as a process requires oxygen. Temperature and soil moisture are the most important factors that regulate decomposition indirectly to help soil microbes. Warm and moist environment favours decomposition whereas low temperature and anaerobic conditions inhibit decomposition.
 14.2 Energy Flow : Sun is the only source of energy for all ecosystems on the earth except for the deep-sea ecosystems. Of the total incident solar radiation, less than 50 % of it is photosynthetically active radiation (PAR). Plants and photosynthetic bacteria (autotrophs) fix energy to prepare food from simple inorganic materials. Plants capture only 2-10 % of the PAR and this small amount of energy sustains the entire living world. 

Therefore, it is very important to know how the solar energy captured by plants flows through different organisms of an ecosystem. Directly or indirectly, all organisms are dependent for their food on producers. Hence there is unidirectional flow of energy from sun to producers and then to consumers. The direction can not be reversed. Energy can be used only once in the ecosystem.

 The autotrophs need a constant supply of energy to synthesize the molecules they require. The autotrophs are called producers. In a terrestrial ecosystem, major producers are herbaceous and woody plants. Likewise, producers in an aquatic ecosystem are phytoplankton and algae.

called consumers (heterotrophs). If they feed directly on the plants, they are called primary consumers, and if the animals eat other animals which eat plants, they are called secondary consumers. Likewise, you could have tertiary consumers too. 

The primary consumers are also known as herbivores. Some common herbivores are insects (grasshopper, aphids), birds (parrot) and some mammals (sheep, cattles, goat, donkey) in terrestrial ecosystem and molluscs in aquatic ecosystem. The consumers that feed on these herbivores are carnivores, (secondary consumers). Those animals that depend on the primary carnivores for food are called secondary carnivores.

You have studied several food chains and food webs that exist in nature. Food chains are always straight and usually have four or five trophic levels. There are three types of food chains viz. grazing, detritus and parasitic. Starting from the plants (or producers) food chains and food webs are formed such that an animal feeds on a plant or on another animal and in turn is food for another. The energy trapped by the producer, is either passed on to a consumer or remains trapped till the producer organism dies. Death of organism is the beginning of the detritus food chain/web.

The detritus food chain (DFC) begins with dead organic matter. It is composed of decomposers which are heterotrophic organisms, mainly fungi and bacteria. They meet their energy and nutrient requirements by degrading the detritus. These are known as saprotrophs. Decomposers secrete enzymes that breakdown dead organic materials into simple, inorganic materials, which are absorbed by them. Detritus food chain may be connected with the grazing food chain at some levels. In a natural ecosystem, some animals like cockroaches, crows, bears, man, etc. are omnivores. Omnivores eat producers as well as consumers. These natural interconnection of food chains make it a food web.

ade depend on the condition of the soil, availability of water, the environment as also the seeds or other propagules present. Since soil is already there, the rate of succession is much faster and hence, climax is also reached more quickly. Figure 14.14 shows the sequence of stages 1 to 8 in a forest ecosystem after fire.
It is important to understand that succession, particularly primary succession, is a very slow process, maybe requiring thousands of years for the climax to be reached. Another important fact to understand is that all the successions whether taking place in water or on land, proceed to a similar climax community – the mesic.

Ecosystem Services : Healthy ecosystems are the base for a wide range of economic, environmental and aesthetic goods and services. The products of ecosystem processes are named as ecosystem services, for example, healthy forest ecosystems purify air and water, mitigate droughts and floods. The Millennium Ecosystem Assessment report 2005 defines Ecosystem services as benefits people obtain from ecosystems and identifies four categories of ecosystem services as follows.

• Supporting services include services such as nutrient cycling, primary production, soil formation, habitat provision and pollination maintaining balance of ecosystem.

• Provisioning services include food (including seafood), raw materials (including timber, skins, fuel wood), genetic resources (including crop improvement genes, and health care), water, medicinal resources (including test and assay organisms) and ornamental resources (furs, feathers, ivory, orchids, butterflies, etc.)

• Regulating services include Carbon sequestration, Predation regulates prey populations, Waste decomposition and detoxification, Purification of water and air, and pest control.

• Cultural services include cultural, spiritual and historical, recreational experiences, science and education, and Therapeutics (including animal assisted therapy) Following are the main ecological services : Fixation of atmospheric CO2 and release of O2 are the most important services provided by an ecosystem. Photosynthetic activity of photoautotrophs sequesters carbon (in CO2 form) from the atmosphere and releases O2 as a 

byproduct. O2 not only purifies air but is also used for respiration by all aerobes. Pollination of plants brought about by wind, water or other biotic agencies, is also an important ecosystem service, without which there would be no crops and no fruits. Though the value of all such services of biodiversity is difficult to determine, it seems reasonable to think that biodiversity should carry a hefty price tag. 


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Balbharati solutions for Biology 12th Standard HSC for Maharashtra State Board
Chapter 1: Reproduction in Lower and Higher Plants
Chapter 2: Reproduction in Lower and Higher Animals
Chapter 3: Inheritance and Variation
Chapter 4: Molecular Basis of Inheritance
Chapter 5: Origin and Evolution of Life
Chapter 6: Plant Water Relation
Chapter 7: Plant Growth and Mineral Nutrition
Chapter 8: Respiration and Circulation
Chapter 9: Control and Co-ordination
Chapter 10: Human Health and Diseases
Chapter 11: Enhancement of Food Production
Chapter 12: Biotechnology
Chapter 13: Organisms and Populations
Chapter 14: Ecosystems and Energy Flow
Chapter 15: Biodiversity, Conservation and Environmental Issues

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