NCERT Solutions Class 12 Biology Chapter 14

Q.1 Fill in the blanks:
(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 .

Ans-

(a) producers

(b) upright

(c) light

(d) earthworm, bacteria and fungi

(e) oceans

Q.2 Which one of the following has the largest population in a food chain?
(a) Producers
(b) Primary consumers
(c) Secondary consumers
(d) Decomposers

Ans-

(d) Decomposers

Decomposers are heterotrophic organisms, which meet their energy and nutrient requirements by degrading dead organic matter. Thus, they can act on any level of the food chain.

Q.3 The second trophic level in a lake is
(a) Phytoplankton
(b) Zooplankton
(c) Benthos
(d) Fishes

Ans-

(b) Zooplankton

The first level is always plants, as they are the producers.

Q.4 Secondary producers are
(a) Herbivores
(b) Producers
(c) Carnivores
(d) None of the above

Ans-

(d) None of the above

There is no secondary producer. Plants are the only producers. They are autotrophs and synthesize food in the presence of sunlight by performing photosynthesis.

Q.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%

Ans-

(b) 50%

Sun is the main source of energy for all ecosystems on Earth. Plants along with photosynthetic and chemosynthetic bacteria (also called autotrophs) capture 2-10% of this photosynthetically active radiation and make food from simple inorganic material.

Q.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

Ans-

(a) Grazing food chain and detritus food chain

(b) Production and decomposition

(c) The upright and inverted pyramid

(d) Food chain and Food web

(e) Litter and detritus

(f) Primary and secondary productivity

Q.7 Describe the components of an ecosystem.

Ans-

An ecosystem consists of all the living populations in a given area, along with the physical environment. Thus, an ecosystem consists of both biotic or living and abiotic or nonliving components. There is a continuous interaction that happens between the two components of the ecosystem resulting in a physical structure that is unique for each type of ecosystem. The biotic and the abiotic components function as a unit with respect to the productivity, decomposition, energy flow and nutrient cycle. Let us describe the two components of the ecosystem little in detail:

1. Biotic component: The living components of the ecosystem can be categorized as either producers or consumers. Producers are autotrophic organisms with the capability of carrying on photosynthesis and making food for themselves. They indirectly make food for other populations. In terrestrial ecosystems, the producers are mainly green plants, while in an aquatic ecosystem, the varied algal species are the major producers. Consumers are heterotrophic organisms that use preformed food. There are 4 types of consumers depending upon the kind of food they consume. Herbivores feed directly on green plants and thus are termed primary consumers e.g. caterpillar feeds on leaves. Carnivores feed only on other animals and thus are secondary consumers e.g. lion feeds on other animals. Omnivores feed on both plants and animals e.g. a human being. The fourth consumers are decomposers, which feed on the remains of plants and animals following their death (detritus) e.g bacteria, fungi, earthworms and convert them into simple inorganic matter. Decomposers form the largest group of the biotic component of the ecosystem.
2. Abiotic component: The non-living parts of the ecosystem form the abiotic components. The non-living components include soil, water, light, inorganic nutrients, and weather variables. They play a major role in deciding the structure and function of the entire ecosystem.

Q.8 Define ecological pyramids and describe with examples, pyramids of number and biomass.

Ans-

Ecological pyramids are graphical representation or summarization of the trophic structure of an ecosystem. The base of the pyramid depicts the producer trophic level and is usually broad. The apex, which narrows down, is the tertiary or some higher-level consumer and the other consumer levels are depicted in between these two.

Ecological Pyramid

Q.9 What is primary productivity? Give brief description of factors that affect primary productivity.

Ans-

Primary productivity is defined as the rate at which biomass or organic matter is produced per unit area over a time period by plants during photosynthesis. It is expressed in terms of weight (g^-2 yr^-1) or energy (kcal m^-2) yr^-1. Thus, primary productivity is the rate at which solar energy is stored in organic matter. This primary productivity 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. A good amount of GPP is used by plants by themselves during the process of respiration. Net primary productivity is GPP minus respiration losses.

Factors affecting primary productivity:

1. Plant species inhabiting the area in question.
2. Environmental factors like light, temperature, soil, water, precipitation, etc.
3. Availability of nutrients and photosynthetic capacity of the plants.

Q.10 Different ecological parameters like number of organisms at each trophic level (pyramid of number), the biomass of living material (pyramid of biomass) or availability of energy (pyramid of energy) at each trophic level can be depicted by different kinds of ecological pyramids. Let us discuss pyramids of number and pyramids of biomass in detail.

Pyramid of number:

Pyramid of the number represents the number of organisms at each trophic level in a food chain. In most ecosystems, the pyramid of number is upright which means that the members at the successively higher levels are smaller in number with some exceptions where the pyramid of number is inverted. This means that producers are more in number than herbivores and herbivores are more in number than carnivores. A very simple example of a pyramid of number is a representation of the relationship between green grass/herbs/shrubs (producer) with deer (herbivore) and lion (carnivore) in a grassland ecosystem. Another example is of a pond where the lowest trophic level is represented by algae and diatoms, which are the largest in number. The second trophic level is represented by herbivorous zooplanktons which are less abundant in number. While the third and fourth trophic levels are occupied by smaller and larger fish, respectively. There is a considerable reduction in the number of individuals from the base to the top of the pyramid. Thus, as we go up the food chain, the number of organisms at each trophic level decreases in most of the ecosystem resulting in an upright pyramid of the number, if represented graphically. However, in a marine ecosystem, the number of fishes is much more than phytoplankton. Thus it is inverted. The same holds true for the parasitic food chain.

Pyramid of Biomass:

Biomass is the weight of living material at some particular time. To calculate biomass for each trophic level, first an average weight for the organisms at each level is determined, and then the number of organisms at each level is estimated. The average weight is then multiplied with the estimated number which gives the approximate biomass for each trophic level. This kind of pyramid is usually upright in most of the studied ecosystems. The pyramid of biomass in the sea is usually inverted because the biomass of fishes far exceeds that of phytoplankton.

Q.10 Define decomposition and describe the processes and products of decomposition.

Decomposition is defined as the process by which decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients. The raw material for decomposition is detritus which is nothing but the dead plant remains such as leaves, bark, flowers, dead remains of animals, faecal material, etc. This process requires oxygen and factors that affect the decomposition process are the chemical nature of detritus and climatic factors. Decomposition forms a very important step in the functioning of an ecosystem.

The various processes and products of decomposition are mentioned below:

1. Fragmentation: The process by which detritus is broken down into smaller particles by the action of detritivores (e.g. earthworm) is called fragmentation. This makes the decomposition process easier and quicker.
2. Leaching: The process by which water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts is called leaching.
3. Catabolism: The process by which bacterial and fungal enzymes degrade detritus into simpler inorganic substances is called catabolism.
4. Humification: The process by which dark coloured amorphous substance (humus) accumulates in the soil as a result of decomposition is called humification. This humus is highly resistant to the action of microbes and undergoes further decomposition at a very slow rate.
5. Mineralisation: The slow process by which humus is further degraded by microbes releasing inorganic nutrients into the soil is called mineralisation.

Q.11 Give an account of energy flow in an ecosystem.

The flow of energy in an ecosystem follows the two laws of thermodynamics. The first law says that energy cannot be created or destroyed and the second law says that when energy is transformed from one form to another, there is always some loss of energy as heat. Ecosystems need a constant input of energy from an external source. Sun is the ultimate source of energy for our planet.

Energy Flow:

Energy flows through an ecosystem from producers to consumers constituting a food chain. Producers or autotrophs capture solar energy. Of the incident solar radiations, about 50% of them are photosynthetically active radiation (PAR). Out of this, only about 2-10% of PAR is captured by photosynthetic plants and microorganisms to produce food from simple inorganic material by the process of photosynthesis. They are called producers. These producers are consumed by consumers (animals that are dependent on others for energy source) thus, passing the captured energy to the next level. The consumers that feed on the herbivores or producers are called primary consumers. Those animals that depend on the primary consumers for food are called secondary consumers and so on. This constitutes the food chain with various trophic levels which clearly indicates who eats whom in the ecosystem and how energy flows. There are various food chains in the ecosystem and each of them do not work in isolation, but are interlinked, making a food web. This unidirectional flow of energy from the sun to producers and then to consumers is what makes the ecosystem function continuously. Energy trapped into an organism does not remain in it forever. It is continuously passed to the next level of the food chain.

The energy does not remain trapped in these levels alone. Once producers and consumers die, they are acted upon by the process of decomposition by decomposers like fungi and bacteria. They break down the complex organic matter of these organisms into the simple inorganic matter and release the trapped energy in the atmosphere.

Based on the feeding relationship with other organisms and the source of nutrition, each organism occupies a particular place in the community or food chain. This place in the food chain is known as a trophic level. Producers belong to the first trophic level, herbivores (primary consumers) to the second and carnivores (secondary consumers) to the third. The amount of energy decreases as one goes up these trophic levels in the food chain.

Q.12 Write important features of a sedimentary cycle in an ecosystem.

The sedimentary cycle is the cycling of nutrients, whose reservoir is located in Earth’s crust, through various components of an ecosystem. Sulphur and phosphorus cycles are examples of sedimentary cycles. They are found in the earth crust and their release in the ecosystem is a very slow process. It is when the rocks weather, minute amounts of these minerals are dissolved in soil solutions and are absorbed by the roots of plants. Herbivores and other consumers obtain these from plants and during the process of decomposition, they are released back in the soil. Some important features of such sedimentary cycles are as follows:

• The slow movement of nutrients across various levels of the food chain.
• Involvement of very minute amounts of nutrients.
• No gaseous phase is involved in the sedimentary cycle.
• The atmospheric contribution is minimal.

Q.13 Outline salient features of carbon cycling in an ecosystem.

Carbon Cycle

The reservoir of Carbon: 71% of the total quantity of global carbon is dissolved in oceans and it is this oceanic reservoir of carbon that controls the amount of carbon dioxide in the atmosphere. At any given time, the atmosphere contains only 1% of the total global carbon. Other reservoirs of carbon are fossil fuels.

Recycling: Carbon cycling occurs through atmosphere, ocean and through the living and dead organisms. According to an estimate, 4 X 10³ kgs of carbon is fixed in the biosphere through photosynthesis annually. Carbon constitutes 49% dry weight of organisms. A good amount of carbon returns to the atmosphere as CO₂ through respiratory activities of the producers and consumers of the ecosystem. Decomposers also play an important role in the recycling of carbon through the process of decomposition. Some amount of carbon is lost to sediments and removed from circulation. Burning of wood, forest fire and combustion of organic matter, fossil fuel, volcanic activity are some more ways by which CO₂ is released in the atmosphere.

To put it more precisely, carbon recycles in the ecosystem in the following ways:

1. Carbon moves from the atmosphere to plants by the process of photosynthesis.
2. Carbon moves from plants to animals through the food chain.
3. Carbon moves from plants and animals to the ground through the process of decomposition.
4. Carbon moves back to the atmosphere from plant and animals by the process of respiration.
5. Carbon moves from fossil fuels to the atmosphere each time these fuels are burnt.
6. Carbon moves to and from water bodies.