Ecosystem Class 12th Biology CBSE Solution

Class 12th Biology CBSE Solution

Exercises
Question 1.

Fill in the blanks.

A. Plants are called as ____________ because they fix carbon dioxide.


Answer:

Autotrophs

Auto means self and trope means nourishing. So, since plants can synthesise their own food through the process of photosynthesis they are termed as autotrophs.



Question 2.

In an ecosystem dominated by trees, the pyramid (of numbers)

Is _____ type.


Answer:

Inverted

A pyramid of number represents the total no. of organisms at each tropic level. In this pyramid, the number of individuals is increased from lower level to higher trophic level and the whole ecosystem is dependent on trees.




Question 3.

In aquatic ecosystems, the limiting factor for the productivity is_______.


Answer:

Sunlight

Limiting factors are environmental influences that constrain the productivity of organisms, populations, or communities and thereby prevent them from achieving their full biological potential which could be realized under optimal conditions. So if the amount of light entering in the pond decreases it will affect the productivity of the ponds as the phytoplankton and other aquatic plants would not be able to perform the photosynthesis and ultimately the productivity of pond will decrease.



Question 4.

Common detritivores in our ecosystem are_________.


Answer:

Earthworms, bacteria and fungi

The organisms that feed on dead and decaying matter are called detritivores. They help in breaking down of organic matter.



Question 5.

The major reservoir of carbon on earth is_________.


Answer:

Oceans

About 71% of carbo is present on oceans. The oceanic reservoir regulates the cycling of carbon in the atmosphere. Carbon is present in oceans in dissolved forms, in organic forms (present in living organisms), as calcareous sediments, fossils.



Question 6.

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: Decomposer

Decomposers are the organisms which obtain nutrients by breaking down the organic matter. It includes micro-organisms such as bacteria, fungi.



Question 7.

The second trophic level in a lake is

A. Phytoplankton

B. Zooplankton

C. Benthos

D. Fishes


Answer:

B: zooplanktons

Zooplanktons are the part of aquatic food chain and consumes phytoplankton. As they are present at the second level in a food chain they form the second trophic level.



Question 8.

Secondary producers are

A. Herbivores

B. Producers

C. Carnivores

D. None of the above


Answer:

D: None of the above

Plants are considered as the primary producers as they entrap the solar energy through the process of photosynthesis and convert inorganic carbon di-oxide into organic compounds. This energy is then transferred to the second tropic level i.e. primary consumer. So there are no secondary producers in the food chain.



Question 9.

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%

50% of the total incident solar radiation is PARs but only 2-10% of PARs is captured by plants. So 50% of the incident rays are PARs in total, however only 2-10% is CAPTURED by them.


Question 10.

Distinguish between

Grazing food chain and detritus food chain


Answer:



Question 11.

Distinguish between

Production and decomposition


Answer:



Question 12.

Distinguish between

Upright and inverted pyramid


Answer:



Question 13.

Distinguish between

Food chain and Food web


Answer:



Question 14.

Distinguish between

Litter and detritus


Answer:



Question 15.

Distinguish between

Primary and secondary productivity


Answer:



Question 16.

Describe the components of an ecosystem.


Answer:

An ecosystem can be visualised as a functional unit of nature, where living organisms i.e. producers, consumers and decomposers interact among themselves and also with the surrounding physical environment.

The biotic and abiotic 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 which are examples of natural ecosystem. Apart from this artificial ecosystem includes cultivated land, aquarium.


The two components of an ecosystem are:


1. Biotic Components: It is the living component of an ecosystem that includes biotic factors such as producers, consumers, decomposers, etc.


a. Primary producers: Primary producers are basically green plants, certain bacteria and algae that carry out photosynthesis. They 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.


b. Consumers: Consumers are incapable of producing their own food. They depend on organic food derived from plants, animals or both.


Consumers can be divided into two groups namely:


I. Primary Consumers: Herbivores are primary consumers which feed mainly on plants.


II. Secondary consumers: They feed on primary consumers it includes carnivores and omnivores. Carnivores which feed on both primary and secondary consumers are called tertiary consumers e.g. lion which can eat wolves, snakes etc. Omnivores are organisms which consume both plants and animals e.g. man, bear, etc.


c. decomposers: They are bacteria and fungi which obtain energy and nutrients from dead organic substances (detritus) of plant and animals. The products of decomposition such as inorganic nutrients which are released in the ecosystem are reused by producers and thus recycled. Earthworm and certain soil organisms (such as nematodes, and arthropods) are detritus feeders and help in the decomposition of organic matter and are called derivers.




Question 17.

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


Answer:

An ecological pyramid is defined as a graphical representation of various ecological parameters present at each trophic level. Ecological pyramids represent producers which are placed at the base, while the apex represents the consumers. There are three types of pyramids:

1. pyramid of number


2. pyramid of biomass


3. pyramid of energy


PYRAMID OF NUMBER: The pyramid of numbers deals with the relationship between the numbers of primary producers and consumers of different orders. In all cases, the base of such a pyramid always represents the numbers of primary producers and the subsequent structures on this base are represented by the number of consumers of successive levels, the top representing the number of top carnivores in that ecosystem. In the ecological pyramid so formed, the higher the step in the pyramid, the lower the number of individuals and the larger their size.


The shape of the pyramid of numbers may be upright or inverted.


EXAMPLE 1: UPRIGHT: 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 least.



EXAMPLE 2: INVERTED: 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 BIOMASS: A pyramid of biomass takes into account, for a given unit area, the biomass of the producers, the biomass of the herbivores, the biomass of the first-level carnivores, and so on. SO, 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.


EXAMPLE 1: UPRIGHT: 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.



EXAMPLE 2: INVERTED: 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).



Question 18.

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


Answer:

Primary productivity can be defined as the amount of organic matter or biomass which is produced by producers per unit area over a period of time. Various actors affect the Primary productivity of an ecosystem. It includes environmental factors such as light, temperature, water, pH of soil etc. It also depends on the availability of nutrients and photosynthetic capacity of plant.



Question 19.

Define decomposition and describe the processes and products of decomposition.


Answer:

Decomposition is the process of breaking down of complex organic matter with the help of decomposers into smaller inorganic raw material such as CO2, H2O, and other nutrients. It constitutes of various processes:

1. Fragmentation: The breakdown of detritus (Organic matter) into smaller pieces by the action of detritivores (earthworms).


2. Leaching: The water-soluble nutrients move down into the layers of soil and get locked as unavailable salts.


3. Catabolism: Further the detritus is degraded into smaller pieces by bacteria and fungi through various enzymes.


4. Humification: Humification leads to the formation of a dark coloured colloidal substance called humus, which acts as reservoir of nutrients for plants.


5. Mineralization: The process of releasing inorganic nutrients from the humus in the soil is done by microbes and this process is known as mineralization


Decomposition thus leads to the production of a dark coloured, nutrient-rich substance called humus. Degradation of humus releases inorganic raw materials such as CO2,water, and other nutrient in the soil.




Question 20.

Give an account of energy flow in an ecosystem.


Answer:

Sun is considered as the ultimate source of energy. Solar radiations from sun passes through the atmosphere and are absorbed by the Earth’s surface. These radiations help to maintain the ecosystem as they:

1. Carry out the process of photosynthesis.


2. Maintain the Earth’s temperature for the survival of living organisms.


Few solar radiations are reflected by the Earth’s surface. So, only 2-10% of solar energy is captured by producers. This energy is used by the plant for the production of the biomass. This is called primary production.


Unidirectional flow of energy takes place from the sun to producers and then to consumers. When the plants are consumed by the herbivores, only 10% of the stored energy is transferred to herbivores. The remaining 90 % of this energy is utilized by plants for its own processes which includes respiration, growth, and reproduction.


Similarly, to the next tropic level i.e. carnivores, only 10% of the energy is transferred. This law is termed as ten percent law of energy flow.




Question 21.

Write important features of a sedimentary cycle in an ecosystem.


Answer:

The important features of a sedimentary cycle in an ecosystem are following:

1. Elements such as sulphur, phosphorus, potassium, and calcium have sedimentary cycles and their reservoirs is in the Earth’s crust or rocks.


2. Sedimentary cycles are very slow as they take a long time to complete their circulation. During recycling the nutrient elements get locked in the reservoir pool, thereby taking a long time to come out and continue circulation.


3. Rock minerals once released into the soil solution do not return to the rock itself.



Question 22.

Outline salient features of carbon cycling in an ecosystem.


Answer: 


Carbon is the most important element in the biological system and constitutes about 50% of all living organisms. It occurs through atmosphere, oceans and through living and dead organisms.


1. from the atmosphere to plants: plants take in carbon dioxide from the atmosphere and release oxygen during the process of photosynthesis. The reaction is:
6 CO2 + 6 H2O + solar energy → C6H12O6 + 6 O2


2. From plants to animals: Consumption of plants by animals directly or indirectly transfers the carbon molecules as glucose through the food chains.


3. From plants to animals: From living things to the environment: When plants and animals die, they decompose into the soil. During this decomposition, the carbon compounds in their bodies re-join the soil.


4. From living things into the atmosphere: When we exhale, we give out carbon dioxide into the atmosphere. Through the process of respiration, carbon is cycled through the atmosphere from living things.


5. From fossil fuels into the atmosphere when fuels are burned: Our species relies on fossil fuels for a large portion f our energy needs even though the trend is gradually changing. When we burn fossil fuels, we are releasing carbon compounds like carbon dioxide and carbon monoxide into the atmosphere. Every year, around five and a half billion tons of carbon is released by the burning of fossil fuels. Out of the fossil fuels burned, around three billion tons of it goes into the atmosphere whereas most of the rest get mixed with the oceans or settle down on the ground.


6. Carbon moves from the atmosphere to the oceans: The oceans soak up carbon through the surface and deposits from rivers and estuaries.


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