NCERT Solutions for Class 12th: Ch 14 Ecosystem Biology
Page No: 256
Exercises
(a) Plants are called as_________because they fix carbon dioxide.
(b) In an ecosystem dominated by trees, the pyramid (of numbers) is_________type.
(c) In aquatic ecosystems, the limiting factor for the productivity is_________.
(d) Common detritivores in our ecosystem are_________.
(e) The major reservoir of carbon on earth is_________.
(a) Autotrophs
(b) inverted
(c)Light
(d)Earthworm
(e)Ocean
Page No: 257
2. Which one of the following has the largest population in a food chain?
(a) Producers
(b) Primary consumers
(c) Secondary consumers
(d) Decomposers
Answer
(d) Decomposers
Decomposers include micro-organisms such as bacteria and fungi. They form the largest population in a food chain and obtain nutrients by breaking down the remains of dead plants and animals
3. The second trophic level in a lake is
(a) Phytoplankton
(b) Zooplankton
(c) Benthos
(d) Fishes
Answer
(b) Zooplankton
Zooplankton are primary consumers in aquatic food chains that feed upon phytoplankton. Therefore, they are present at the second trophic level in a lake.
4. Secondary producers are
(a) Herbivores
(b) Producers
(c) Carnivores
(d) None of the above
Answer
(d) None of the above
Plants are the only producers. Thus, they are called primary producers. There are no other producers in a food chain.
5. What is the percentage of photosynthetically active radiation (PAR) in the incident solar radiation?
(a) 100%
(b) 50 %
(c) 1-5%
(d) 2-10%
Answer
(b) 50%
Out of total incident solar radiation, about fifty percent of it forms photosynthetically active radiation (PAR).
6. Distinguish between
(a) Grazing food chain and detritus food chain
(b) Production and decomposition
(c) Upright and inverted pyramid
(d) Food chain and Food web
(e) Litter and detritus
(f) Primary and secondary productivity
Answer
(a) Grazing food chain and detritus food chain
| Grazing food chain | Detritus food chain |
| In this food chain, energy is derived from the Sun. | In this food chain, energy comes from organic matter (or detritus) generated in trophic levels of the grazing food chain. |
| It begins with producers, present at the first trophic level. | It begins with detritus such as dead bodies of animals or fallen leaves, which are then eaten by decomposers or detritivores. |
| This food chain is usually large. | It is usually smaller as compared to the grazing food chain. |
(b) Production and decomposition
| Production | Decomposition |
| It is the rate of producing organic matter (food) by producers. | It is the process of breaking down of complex organic matter or biomass from the body of dead plants and animals with the help of decomposers into organic raw materials. |
| It depends on the photosynthetic capacity of the producers. | It occurs with the help of decomposers. |
| Sunlight is required by plants for primary production. | Sunlight is not required for decomposition by decomposers. |
(c) Upright and inverted pyramid
| Upright pyramid | Inverted pyramid |
| The pyramid of energy is always upright. | The pyramid of biomass and the pyramid of numbers can be inverted. |
| The number and biomass of organisms in the producer level of an ecosystem is the highest, which keeps on decreasing at each trophic level in a food chain. | The number and biomass of organisms in the producer level of an ecosystem is the lowest, which keeps on increasing at each tropic level. Light is not required for decomposition by decomposers |
(d) Food chain and Food web
| Food chain | Food web |
| Food chain is single pathway of energy transfer from upper to lower. | Food web is made of several interconnecting pathways. |
| One individual occupies one trophic level at a time. | One individual occupies many trophic level at a time. |
| It decreases stability of the ecological system and less adaptive. | It increases stability of the ecological system and more adaptive. |
(e) Litter and detritus
| Litter | Detritus |
| Litter contains all kinds of waste materials above the surface of the earth. | Detritus contains dead animals and plants below and above the surface of the earth. |
| It contains both biodegradable as well as non-biodegradable wastes. | It contains only biodegradable wastes. |
(f) Primary and secondary productivity
| Primary productivity | Secondary productivity |
| Rate of amount of production of organic matter by producers over a period of time. | Rate of amount of production of organic matter by consumers over a period of time. |
| It is due to photosynthesis. | It is due to herbivory and predation. |
7. Describe the components of an ecosystem.
Answer
An ecosystem is defined as an interacting unit that includes both the biological community as well as the non-living components of an area. The living and the non-living components of an ecosystem interact amongst themselves and function as a unit, which gets evident during the processes of nutrient cycling, energy flow, decomposition, and productivity. There are many ecosystems such as ponds, forests, grasslands, etc.
The two components of an ecosystem are:
(a) Biotic component: It is the living component of an ecosystem that includes biotic factors such as producers, consumers, decomposers, etc. Producers include plants and algae which contain chlorophyll pigment, which helps them carry out the process of photosynthesis in the presence of light. Thus, they are also called converters or transducers. Consumers or heterotrophs are organisms that are directly (primary consumers) or indirectly (secondary and tertiary consumers) dependent on producers for their food. Decomposers include micro-organisms such as bacteria and fungi. They form the largest population in a food chain and obtain nutrients by breaking down the remains of dead plants and animals.
(b) Abiotic component: They are the non-living component of an ecosystem such as light, temperature, water, soil, air, inorganic nutrients, etc.
8. Define ecological pyramids and describe with examples, pyramids of number and biomass.
Answer
An ecological pyramid is a graphical representation of various ecological 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. There are three types of pyramids:
(a) Pyramid of numbers
(b) Pyramid of energy
(c) Pyramid of biomass
Pyramid of numbers: It is a graphical representation of the number of individuals present at each trophic level in a food chain of an ecosystem. The pyramid of numbers can be upright or inverted depending on the number of producers. For example, in a grassland ecosystem, the pyramid of numbers is upright. In this type of a food chain, the number of producers (plants) is followed by the number of herbivores (mice), which in turn is followed by the number of secondary consumers (snakes) and tertiary carnivores (eagles). Hence, the number of individuals at the producer level will be the maximum, while the number of individuals present at top carnivores will be on the other hand, in a parasitic food chain, the pyramid of numbers is inverted. In this type of a food chain, a single tree (producer) provides food to several fruit eating birds, which in turn support several insect species.
Pyramid of energy: An energy pyramid is a graphical model of energy flow in a community. The different levels represent different groups of organisms that might compose a food chain. From the bottom-up, they are as follows: Producers bring energy from nonliving sources into the community.
Pyramid of biomass: A pyramid of biomass is a graphical representation of the total amount of living matter present at each trophic level of an ecosystem. It can be upright or inverted. It is upright in grasslands and forest ecosystems as the amount of biomass present at the producer level is higher than at the top carnivore level. The pyramid of biomass is inverted in a pond ecosystem as the biomass of fishes far exceeds the biomass of zooplankton (upon which they feed).east.
9. What is primary productivity? Give brief description of factors that affect primary productivity.
Answer
It is defined as the amount of organic matter or biomass produced by producers per unit area over a period of time.
Primary productivity of an ecosystem depends on the variety of environmental factors such as light, temperature, water, precipitation, etc. It also depends on the availability of nutrients and the availability of plants to carry out photosynthesis.
10. Define decomposition and describe the processes and products of decomposition.
Answer
Decomposition is the process that involves the breakdown of complex organic matter or biomass from the body of dead plants and animals with the help of decomposers into inorganic raw materials such as carbon dioxide, water, and other nutrients. The various processes involved in decomposition are as follows:
(i) Fragmentation: It is the first step in the process of decomposition. It involves the breakdown of detritus into smaller pieces by the action of detritivores such as earthworms.
(ii) Leaching: It is a process where the water soluble nutrients go down into the soil layers and get locked as unavailable salts.
(iii) Catabolism: It is a process in which bacteria and fungi degrade detritus through various enzymes into smaller pieces.
(iv) Humification: The next step is humification which leads to the formation of a dark-coloured colloidal substance called humus, which acts as reservoir of nutrients for plants.
(v) Mineralization: The humus is further degraded by the action of microbes, which finally leads to the release of inorganic nutrients into the soil. This process of releasing inorganic nutrients from the humus is known as mineralization.
Decomposition produces a dark coloured, nutrient-rich substance called humus. Humus finally degrades and releases inorganic raw materials such as CO2, water, and other nutrient in the soil.
11. Give an account of energy flow in an ecosystem.
Answer
Energy
enters an ecosystem from the Sun. Solar radiations pass through the atmosphere
and are absorbed by the Earth's surface. These radiations help plants in
carrying out the process of photosynthesis. Also, they help maintain the
Earth's temperature for the survival of living organisms. Some solar radiations
are reflected by the Earth's surface. Only 2-10 percent of solar energy is
captured by green plants (producers) during photosynthesis to be converted into
food. The rate at which the biomass is produced by plants during photosynthesis
is termed as 'gross primary productivity'. When these green plants are consumed
by herbivores, only 10% of the stored energy from producers is transferred to
herbivores. The remaining 90 % of this energy is used by plants for various
processes such as respiration, growth, and reproduction. Similarly, only 10% of
the energy of herbivores is transferred to carnivores. This is known as ten
percent law of energy flow.
12. Write important features of a sedimentary cycle in an ecosystem.
Answer
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.
Sedimentary cycles are very slow. 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.
13. Outline salient features of carbon cycling i n an ecosystem.
Answer
The carbon cycle is an important gaseous cycle which has its reservoir pool in the atmosphere. All living organisms contain carbon as a major body constituent. Carbon is a fundamental element found in all living forms. All biomolecules such as carbohydrates, lipids, and proteins required for life processes are made of carbon. Carbon is incorporated into living forms through a fundamental process called 'photosynthesis'. Photosynthesis uses sunlight and atmospheric carbon dioxide to produce a carbon compound called 'glucose'. This glucose molecule is utilized by other living organisms. Thus, atmospheric carbon is incorporated in living forms. Now, it is necessary to recycle this absorbed carbon dioxide back into the atmosphere to complete the cycle. There are various processes by which carbon is recycled back into the atmosphere in the form of carbon dioxide gas. The process of respiration breaks down glucose molecules to produce carbon dioxide gas. The process of decomposition also releases carbon dioxide from dead bodies of plants and animals into the atmosphere. Combustion of fuels, industrialization, deforestation, volcanic eruptions, and forest fires act as other major sources of carbon dioxide.
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