Ecosystem Question Answers: NCERT Class 12 Biology

(a) Plants are called as Autotrophs because they fix carbon dioxide.

(b) In an ecosystem dominated by trees, the pyramid (of numbers) is of Inverted type.

(c) In aquatic ecosystems, the limiting factor for productivity is Light.

(d) Common detritivores in our ecosystem are Earthworms.

(e) A major reservoir of carbon on Earth is Oceans.

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:

(1) 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.

(2) Leaching: It is a process where the water soluble nutrients go down into the soil layers and get locked as unavailable salts.

(3) Catabolism: It is a process in which bacteria and fungi degrade detritus through various enzymes into smaller pieces.

(4) 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.

(5) 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.

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.

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.

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.

(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.

(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.

(a) Herbivores

(b) 50%

Out of total incident solar radiation, about fifty percent of it forms photosynthetically active radiation or PAR.

 (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

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. 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. 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.

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 least.

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 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).

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.

In this question, farmer only wants to explain the productivity but all the three statements that are given in this; the meanings are very much different. So, the answer is no.

1. In the first statement, the farmer only gives the quantitative information of the wheat; not given information about area and time.
2. In this second statement, he gives information about the quantity and area but time is not expressed.
3. In the third statement, the farmer expressed all the information about the primary productivity which includes all three information like quantity, area and time as well.

Productivity refers to the rate of formation of biomass in an ecosystem and is usually expressed in the unit of mass per unit area per unit time. Primary productivity refers to the rate at which biomass is produced per unit area by the primary producers; it can be expressed as grams of carbon assimilated and dry weight of organic matter and their energy equivalents. It is a fundamental ecosystem process which shows capture, storage and transfer of energy in an ecosystem.

Ecological succession is the universal process of natural changes in the community structure on the ecological time scale. These communities are dynamic and undergoes continuous change in structure and function; important characteristic is that there structure constantly changes in response to the changing environmental conditions and these changes are directional and sequential. These changes leads to the community which is near the equilibrium with the environment is known as climax community which marks the end point of succession. The sequence of communities which replace one another level in the successional process is known as sere. The initial seral stage is Pioneer stage. More than one seral stage is evolved until the climax stage is attained. Succession slows as the community reaches the steady equilibrium stage with the environment and with the various changes in the diversity of species of organisms i.e. climax stage.

There are various attributes that have changed during late successional stage from early successional stage and these are as follows:

The Earth is a closed system with respect to matter and all matter on the Earth cycles. Nutrient cycling (or element cycling) is the movement of elements through both the biotic and abiotic components of the Earth. It is the transport and transformations of elements within and among ecosystems. It involves the incorporation of elements by living organisms and their subsequent release back into the environment through the decomposition. It binds biotic and abiotic parts of ecosystem. The movement of elements through atmosphere, hydrosphere, lithosphere and biosphere is normally termed as Biogeochemical Cycles (bio- living organisms and non- living things and geo means- rocks, air and water etc.). All the organisms require nutrients for their growth and reproduction that includes carbon, hydrogen, oxygen, nitrogen, phosphorus, sodium, Sulphur, chlorine, potassium, calcium and magnesium. The plants obtain these nutrients through the atmosphere by the gaseous cycles or from the soil through the weathering of parent rocks. The function of the reservoir is to meet with the deficit that occurs to imbalance in the rate of influx and efflux. Nutrient cycles or biogeochemical cycles are classified into two types:

1. Gaseous cycle: In this cycle, atmosphere acts as the major reservoir of the element. This cycles shows little or no permanent changes in the distribution and abundance of the element. Carbon and nitrogen are the primary representatives of the biogeochemical cycles with the prominent gaseous phase.
2. Sedimentary cycle: In this cycle, the major reservoir is the Lithosere from which the elements are released largely through the weathering of rocks. This cycle is exemplified by phosphorus, sulphur and other biologically important elements are found; these reservoirs are located in the Earth’s crust. A portion of the supply may get accumulated in the large quantities like in the deep ocean and then become inaccessible to the organisms for the continuation of the cycle.

In the comparison of gaseous cycle to the sedimentary cycle; sedimentary cycle is less perfect because the contents remains locked in the reservoir pool. (Note*: Some of the elements that are characterized by the sedimentary cycle also do have a gaseous phase like Sulphur and iodine but these phases are insignificant cause there is no gaseous large reservoir).    

General model of Nutrient or biogeochemical cycling:

Productivity is defined as the rate of formation of biomass in an ecosystem which is usually expressed in the unit of mass per unit area per unit time. Productivity is different from biomass as productivity is the rate at which organic matter is created by the producers whereas biomass is the amount of organic matter present at any given time which is generally expressed in wet and dry terms. In the P/R ratio P is the productivity and R is the respiration. In this the total rate of photosynthesis, including the biomass that used in respiration process during the measurement is known as Gross Primary Productivity (GPP). And, the rate of storage of biomass in the plants which exceeds the respiratory use by them is known as Net primary Productivity (NPP). This can be described as following equation:

NPP (Net primary Productivity) = GPP (Gross Primary Productivity) - R (Respiration by autotrophs)

So, according to the community energetics attribute the P/ R ratio (shows the relationship between gross production and community respiration) in the early successional species or pioneer stage or species will be greater or less than 1 and in late successional or at the climax stage, this ratio will approaches 1. 

Ecological pyramid is given by Charles Elton in 1927 it is defined as the graphical representation of the numbers or biomass and accumulated energy at each trophic level in the food chain. The relationship between numbers, biomass and energy flow in biotic community can be represented graphically by ecological pyramids. The base of the each pyramid represents the producers or first trophic level while the apex represents the next trophic levels like tertiary or high level consumers and other consumer trophic levels in between them. The shortcomings of ecological pyramids in the study of ecosystem are as follows:

1. Saprotrophs are not considered in the ecological pyramid despite of having very vital role of decomposers in the food chain.
2. More than one species can occupy multiple trophic levels but only in the case of food webs but is not applicable in the case of ecological pyramids or we can say do not consider the possibility of the existence of same species at a time at different trophic levels.
3.  Ecological pyramids do not show any concept related to variations in season and climate.

The formation of soil occurs in two stages: Weathering, Humification and Mineralization. Humification process leads to accumulation of Humus. Humus is an amorphous and a colloidal mixture of complex organic substances which is made up of humic and non- humic substances.

Humification:

• Addition of humus means the remains finally divided amorphous, incomplete decomposed black coloured organic matter to the mineral matter of the soil.
• When the living organism gets decomposed after their death in the soil, then this decomposed product is mixed with the soil. By this decomposition process, it releases various types of organic and inorganic nutrients, which then mixed into the mineral particles of the soil that converts it into the dark colour.
• And this contains various important categories: litter, Duff and leaf mould etc.
• Humus is highly resistant to microbial actions and decomposition process occurs at very slow rate.
• It is a reservoir of nutrients

Mineralization:

• The release of organically bound nutrients which changes into the inorganic form available to plants and microbes by the further degradation of humus by the microbes and then this release of inorganic nutrients occurs by this process called mineralization.
• This mineralization process occurs faster mainly in the tropical areas in comparison to temperate zone areas.

• On the basis of organic content, soil are classified into various categories like Mors, Peat, Muck, Moder soil etc.  

1. First trophic level/ Producers (Green plants): Green plants convert solar energy into the chemical energy for making the food source. The producers carry out respiration process and break down some of the organic compounds to release chemical energy which they pass onto the herbivores.
2. Second trophic level/ Primary consumers (Herbivores): Herbivores (those organisms which take organic food from the autotrophs, i.e., they are dependent on other organism for the food consumption) feeds on the green plants or producers.
3. Third trophic level/ Secondary consumers (Primary carnivores): Carnivores are the last trophic level in the food chain and are those which feed on the herbivores or the other organisms.
4. Fourth trophic level/ Tertiary consumers (Secondary carnivores): They feeds on the previous trophic level.

Decomposition is the process by which organic compounds are broken down into simple compounds which is brought about and decomposition rates are largely controlled by both biotic and abiotic agencies. Decomposition by biological agents is an oxidative process in which complex energy rich molecules are being broken down by decomposers (bacteria and fungi) eventually into water soluble inorganic nutrients. Decomposition rates are largely controlled by both biotic and abiotic factors. Biotic factors include mainly substrate quantity and quality, the type and size of microbial community; abiotic factors include such as soil temperature, moisture and pH, which determines the soil environmental conditions. If the oxygen is present, aerobic degradation occurs and if absent, there would be incomplete degradation. Decomposition is much slower where the soil contain large amount of dead organic matter and is also slower where the conditions are either too dry for the decomposers to thrive or too wet to supply them with enough oxygen.    

Self- sustainability and self- sufficiency are the overlapping states of being in which the person or the system or organization needs little or no help from the external sources or from others. It entails the self being enough to fulfill their needs and can maintain itself or we can say independent efforts.  

1. Desert Ecosystem: this biome is driest of all the biomes and covers one fifth of the Earth’s surface. Its surface receives more than twice the solar radiations, received by the humid regions and releases in twice amount of heat at night.
2. As the soil are coarse textured, shallow, rocky or gravely with good drainage and contains no subsurface water. The vegetation bears the characters of xerophytes and are modified in the specific way to conserve water. Examples of flora: succulent plants like cacti, Euphorbia found in hot and dry deserts.  

They all are detritivores or saprophytes which shows detritus food chain. This food chain starts with dead organic material which is eaten by decomposers or detrivores and small carnivores that feeds on these detrivores and then they are eaten by further more organisms and food energy passes from one trophic level to the next one.

For example:

Productivity refers to the rate of formation of biomass in an ecosystem and is usually expressed in the unit of mass per unit area per unit time. Productivity is different from biomass as productivity is the rate at which organic matter is created by the producers whereas biomass is the amount of organic matter present at any given time which is generally expressed in wet and dry terms. In the P/R ratio P is the productivity and R is the respiration. In this the total rate of photosynthesis, including the biomass that used in respiration process during the measurement is known as Gross Primary Productivity (GPP). And, the rate of storage of biomass in the plants which exceeds the respiratory use by them is known as Net primary Productivity (NPP) which is the available biomass for the consumption to Heterotrophs. This can be described as following equation:

NPP (Net primary Productivity) = GPP (Gross Primary Productivity) - R (Respiration by autotrophs)

1. Nature tends to increase the gross primary productivity so that it can use much of their sunlight or solar radiations and convert it in the food.
2. Men tends to increase the net primary productivity so that enough food will be provided to the growing population of the human beings.

Productivity refers to the rate of formation of biomass in an ecosystem and is usually expressed in the unit of mass per unit area per unit time. Among all of these young forest will be more productive because Primary productivity refers to the rate at which biomass is produced per unit area by the primary producers. This grow faster than the older forest because of higher primary productivity. Other options of them contains lower number of producers, thus they shows low primary productivity.

Ecological pyramid is given by Charles Elton in 1927 it is defined as the graphical representation of the numbers or biomass and accumulated energy at each trophic level in the food chain. The relationship between numbers, biomass and energy flow in biotic community can be represented graphically by ecological pyramids. The base of the each pyramid represents the producers or first trophic level while the apex represents the next trophic levels like tertiary or high level consumers and other consumer trophic levels in between them. There are three kinds of ecological pyramid which are as follows:

1. Pyramid of numbers: Graphical representation of the total number of the individuals of different species that belongs to each trophic level in the ecosystem is known as pyramid of number. It consists of horizontal bars that shows the specific trophic levels which are arranged in the sequential manner from producer to consumers level and the length of each bar represents the total number of individuals at each trophic level. In case of grassland ecosystem or pond ecosystem, it always shows upright pyramid because the numbering of the organisms decreases at successively higher trophic level. But in the tree ecosystem it always shows inverted pyramid, in this tree is a primary producer, so it would represent the base of the pyramid and the dependent insect population will represent the second trophic level.

Diagram of Upright pyramid of numbers in grassland ecosystem:

Diagram of Inverted pyramid of numbers in tree ecosystem:

2. Pyramid of biomass: This pyramid represents the total dry weight of living beings of different species at each trophic level at the particular time. In this all kinds of organisms at a trophic level are weighted. The biomass of the producer is much greater than the herbivore’s biomass and this is greater than the carnivore’s and so on. In forest ecosystem, it is represented that the shape is upright pyramid but in some aquatic ecosystem, like large lakes and oceans this pyramid usually shows in inverted form. The reason behind the inverted pyramid is, microscopic phytoplanktonic algae are the primary producers and being single- celled they do not get accumulate much biomass; and at a given time the total weight of the standing crop of phytoplankton is less when compared to herbivores or other consumers, thus the base of the pyramid being smaller than the upper structure in the aquatic ecosystem.

Diagram of Upright pyramid of biomass in forest ecosystem:

Diagram of Inverted pyramid of biomass in pond ecosystem:

3. Pyramid of energy: It shows the rate at which energy flows from one trophic level to the next. This pyramid reflects the law of thermodynamics with the loss of energy that occurs at each transfer from one trophic level to another, hence the pyramid is always upright. In this, a given trophic level always has a smaller energy content than the trophic level which is immediately below it because some energy is lost as heat is going from one level to another. Each bar represents the amount of energy is utilized at each level in the given time annually per unit area.

Diagram of pyramid of energy:

1. Pyramid of numbers: Graphical representation of the total number of the individuals of different species that belongs to each trophic level in the ecosystem is known as pyramid of number. It consists of horizontal bars that shows the specific trophic levels which are arranged in the sequential manner from producer to consumers level and the length of each bar represents the total number of individuals at each trophic level. In case of grassland ecosystem or pond ecosystem, it always shows upright pyramid because the numbering of the organisms decreases at successively higher trophic level. But in the tree ecosystem it always shows inverted pyramid, in this tree is a primary producer, so it would represent the base of the pyramid and the dependent insect population will represent the second trophic level.

Diagram of Upright pyramid of numbers in grassland ecosystem:

Diagram of Inverted pyramid of numbers in tree ecosystem:

2. Pyramid of biomass: This pyramid represents the total dry weight of living beings of different species at each trophic level at the particular time. In this all kinds of organisms at a trophic level are weighted. The biomass of the producer is much greater than the herbivore’s biomass and this is greater than the carnivore’s and so on. In forest ecosystem, it shows the shape of upright pyramid but in some aquatic ecosystem, like large lakes and oceans this pyramid usually shows in inverted form. The reason behind the inverted pyramid is, microscopic phytoplanktonic algae are the primary producers and being single- celled they do not get accumulate much biomass; and at a given time the total weight of the standing crop of phytoplankton is less when compared to herbivores or other consumers, thus the base of the pyramid being smaller than the upper structure in the aquatic ecosystem.

Diagram of Upright pyramid of biomass in forest ecosystem:

Diagram of Inverted pyramid of biomass in pond ecosystem:

The complicated network of the food chains which are interconnected at the different trophic levels to form a number of feeding connections is known as food web. Food webs provide the pattern of basic ecological interactions among the species and the trophic levels. They consume producer, primary consumer/ carnivores and so on levels which are interconnected with each other. Some names are given in question in which some are producers, herbivores and others are carnivores and top carnivores.

Producers (first trophic level): Algae, Hydrilla, Maize Plant, Phytoplanktons, Nymphaea, Spirogyra.

Primary carnivores/ primary consumers (second trophic level): Grasshopper, Crustaceans, deer, rat, squirrel, rabbit, elephant, Goat, ducks.

Secondary carnivores/ secondary consumers (third trophic level): Spider, cockroach, lizard, wolf, snake, toad, fish, crane, predatory birds like crow or sparrow, whale, peacock. 

Tertiary carnivores/ tertiary consumers (fourth trophic level): Lion, tiger.

Diagram of food web:

All the organisms require matter for their energy for the activities. Organisms in the ecosystem capture solar radiations, transforms, transfers and stores energy. Energy flow is the important function in the ecosystem. The ecosystem functions is consistent with the laws of thermodynamics which deals with the relationship between energy and matter. There are two basic laws of thermodynamics on which the behaviour of energy in the ecosystem is based. The First Law of Thermodynamics is also called law of conservation of energy which states that energy cannot be created or nor be destroyed but can only transformed. Energy in the form of heat that does not do any work goes to increase the randomness or disorder of the universe. The degree of randomness or disorder in a system is known as entropy. The Second Law of Thermodynamics states that every energy transformation cannot be 100% efficient; some energy is always lost as heat; or the energy accumulated by the producers pass on to the consumers in the form of food chain with the gradual loss of energy. Furthermore, this energy is used for performing the metabolic activities for the formation of food and is stored as biomass. This energy is then transferred to next trophic level as we have discussed in Lindeman’s law of 10 %. Ecosystem needs constant supply of energy for the synthesis of molecules, to counteract the tendency towards the increasing randomness. Thus, ecosystem is not exactly exempt from the second law of thermodynamics.

1. As all the green plants are autotrophs they play important role as the source of energy for all the heterotrophs because heterotrophs are dependent on the autotrophs. Green plants convert solar energy into the chemical energy for making the food source. If the plants get disappear from the earth it will lead to imbalance in the atmosphere. Primary productivity will be reduced and the producers that carry out respiration process which they carry out and break down some of the organic compounds to release chemical energy which they pass onto the herbivores. So if the producers are removed, there will be no energy available for the higher trophic level because the herbivores are dependent on the producer level and so on.
2. Herbivores (those organisms which take organic food from the autotrophs, i.e., they are dependent on other organism for the food consumption) feeds on the green plants or producers. So if the herbivore level are eliminated from the food chain, primary productivity will increase.
3. Carnivores are those which feed on the herbivores or the other organisms. Carnivores are the last trophic level in the food chain so, if the carnivore population is removed; the herbivores level will increased, overgrazing will occurs and there will be shortage of primary productivity. And, by the overgrazing of animals will leads to desertification.

An ecosystem is a functional unit consists of all the organisms in the particular place that interacts with one another or with their physical environment that are interconnected by the flow of energy and a cycling of materials. It consists of various biotic and abiotic components. There are two different types of ecosystem: Natural ecosystem and Artificial or domesticated ecosystem.

 Artificial or domesticated ecosystem is maintained artificially by the man by the addition of energy. For example: crop lands or crop fields like maize, wheat, rice field etc. where the man tries to control the biotic community as well as the physio- chemical environment.

Difference between natural and man- made or artificial ecosystem are as follows:

Natural ecosystem occurs naturally and includes two types of ecosystem: Terrestrial and Aquatic ecosystem whereas artificial or domesticated ecosystem is maintained artificially by the man by the addition of energy. In the Natural ecosystem, biotic and abiotic components are self- sustained; on the other hand, in artificial ecosystem the components are sustained artificially by regular feeding and cleaning of aquarium etc.