Ecosystem NEET Notes

Complete Ecosystem Notes for Class 12 Biology | NEET Revision and NCERT Solutions

Structure of Ecosystem

The interactions between the various biotic and abiotic factors of an ecosystem lead to the maintenance of the ecosystem.
Stratification is the vertical distribution of the different species occupying the different levels. For example, trees occur at a higher level then shrubs.
The various aspects taken into consideration to study the functioning of ecosystem are:
- Productivity
- Decomposition
- Energy flow
- Nutrient cycling

Productivity

A constant supply of sunlight is required for the proper functioning of any ecosystem.
The amount of biomass produced per unit area over a time period by plants during photosynthesis is defined as the primary productivity.
It is expressed as weight (g−2) or energy (Kcal m−2).
Productivity can be mainly divided into gross primary productivity (GPP) and net primary productivity (NPP). GPP is the rate of production of organic matter during photosynthesis.

NPP = GPP − Respiratory losses (R)

Secondary productivity is defined as the rate of formation of new organic matter by consumers.
Primary productivity depends upon
- type of plant species inhabiting a particular area
- photosynthetic capacity of plants
- nutrient availability

Annual net productivity for whole biosphere is about 170 b tons of organic matter.

Decomposition

It is the process of breakdown of complex organic matter into inorganic substances such as carbon dioxide, water, nutrients, etc.
Fragmentation − Breaking down of detritus (dead plant and animal remains, faecal matter) into smaller particles by detritivores (decomposers)
Leaching - Process by which these inorganic matters enter the soil
Catabolism − Process by which detritus is degraded into simpler inorganic substances by bacterial and fungal enzymes
Humification − Accumulation of humus in the soil.
Humus is resistant to microbial action and decomposes at an extremely slow rate. It acts as a reservoir of nutrients.

Mineralization − Process by which humus further degrades to release minerals into the soil
It is an oxygen consuming process and is controlled by the chemical composition of detritus and climatic conditions.

Energy Flow

Sun is the sole source of energy for all ecosystems on the earth.
Plants and other photosynthetic organisms utilize less than 50% of the solar radiation known as the photosynthetically active radiation (PAR).
In an ecosystem, plants are called producers and all animals depend upon the plants directly or indirectly for their food. Hence, they are known as consumers or heterotrophs.
The consumers can be further divided into primary consumers (herbivores), secondary consumers (primary carnivores), and tertiary consumers (secondary carnivores).
Food chain − The energy flow among the various constituent animals is known as the food chain.
Food web − The interconnection of the various food chains is called the food web.
Trophic level − Every organism occupies a specific level in their food chain known as the trophic level.
Standing crop − Each trophic level contains a certain amount of living material at a certain time known as the standing crop.
The number of trophic levels in a food chain is restricted since the energy transfer follows the 10 percent law i.e., only 10% of the energy is transferred from a lower trophic level to a higher one.

Ecological Pyramids

The energy relationship between the different trophic levels is represented by the ecological pyramids.
Their base represents the producers or the first trophic level while the apex represents the tertiary or top level consumer.
Ecological pyramids are of 3 types:
- Pyramid of number
- Pyramid of biomass
- Pyramid of energy

In most ecosystems, the three pyramids are upright except in some cases:
- The pyramid of biomass is inverted in an ocean ecosystem since a small standing crop of phytoplankton supports a large number of zooplankton.
- The pyramid of number can be inverted when, say, a large tree is eaten by small insects.
- However, the pyramid of energy is always upright.

A trophic level represents a functional level and not a single species as such. Also, a single species may become a part of more than one trophic level in the same ecosystem at the same time depending upon the role it plays in the ecosystem.
Limitations of ecological pyramids:
- The ecological pyramids do not take into account the same species belonging to more than one trophic level.
- It assumes a simple food chain that almost never exists in nature. It does not explain food webs.
- Saprophytes are not given a place in ecological pyramids even though they play a vital role in ecosystem.

Ecological Succession

The composition of all ecosystems keeps on changing with change in their environment. These changes finally lead to the climax community.
Climax community − It is the community which is in equilibrium with its environment. Gradual and fairly predictable change in the species’ composition of a given area is called ecological succession.
Sere(s) − It is the sequence of communities that successively change in a given environment. The transitional communities are called seral stages or seral communities.
Succession happens in areas where no life forms ever existed as in bare rocks, cool lava, etc. (primary succession), or in areas which have lost all life forms due to destructions and floods (secondary succession).
Primary succession takes hundreds to thousands of years as developing soil on bare rocks is a slow process. Secondary succession is faster than primary succession since the nature does not have to start from scratch.
During succession, any disturbances (natural/man-made) can convert a particular seral stage to an earlier one.
Hydrarch succession − It takes place in wet areas and converts hydric conditions to mesic.
Xerarch succession − It takes place in dry areas and converts xeric conditions to mesic.
Pioneer species − These are the species that first invade a bare area. On land, these could be lichens that secrete enzymes to dissolve the rock surfaces for soil formation while in water, pioneer species could be phytoplanktons.
The ultimate result of all successions is a climax community, a mesic.

Nutrient Cycling

The amount of nutrients present in the soil at a given time is known as the standing state.
Nutrients are never lost from the ecosystem. They are only recycled from one state to another.
The movement of nutrients through the various components of the ecosystem is called nutrient cycling or biogeochemical cycles. They are of two types:
- Gaseous − Reservoir for these types of cycles exist in the atmosphere.
- Sedimentary − Reservoir for these types of cycles exist in the earth’s crust.

Carbon Cycle

About 49% of the dry weight of living organisms is made up of carbon.
The ocean reserves and fossil fuels regulate the amount of CO2 in the atmosphere.
Plants absorb CO2 from the atmosphere for photosynthesis, of which a certain amount is released back through respiratory activities.
A major amount of CO2 is contributed by the decomposers who contribute to the CO2 pool by processing dead and decaying matter.
The amount of CO2 in the atmosphere has been increased considerably by human activities such as burning of fossil fuels, deforestation.

Phosphorus Cycle

Phosphorus is an important constituent of cell membranes, nucleic acids, and cellular energy transfer systems.
Rocks contain phosphorus in the form of phosphate.
When rocks are weathered, some of the phosphate gets dissolved in the soil solution and is absorbed by plants.
The consumers get their phosphorus from the plants.
Phosphorus returns back to the soil by the action of phosphate-solubilising bacteria on dead organisms.

NCERT Solutions

An ecosystem is a functional unit of nature in which living organisms interact with one another as well as with their physical surroundings. In this chapter, we will first examine the ecosystem's structure in order to appreciate the input (productivity), energy transfer (food chain/web, nutrient cycling), and output (degradation and energy loss). We will also examine the relationships - cycles, chains, and webs - formed as a result of these energy flows within the system, as well as their interdependence. Ecosystem - Structure and Function, Productivity, Decomposition, Energy Flow, Ecological Pyramids, Ecological Succession, Nutrient Cycling, and Ecosystem Services are also covered. Ecosystems are covered in Chapter Ecosystem. Patterns and components; productivity and decomposition; energy flow; number, biomass, and energy pyramids

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.

(d) Common detritivores in our ecosystem are_________.

(e) The major reservoir of carbon on earth is_________.

Solution:

(a) Autotrophs

Plants are called as autotrophs because they fix carbon dioxide

(b) inverted

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

In aquatic ecosystems, the limiting factor for the productivity is light.

(d) earthworms

Common detritivores in our ecosystem are earthworms.

(e) Oceans

The major reservoir of carbon on earth is oceans.

2. Which one of the following has the largest population in a food chain?

(a) Producers

(b) Primary consumers

(d) Decomposers

Solution:

(d) Decomposers

Decomposers form the largest population in food chain. They include microbes such as bacteria and fungi, that derive their nutrition by disintegrating the remains of animals and dead plants.

3. The second trophic level in a lake is

(a) Phytoplankton

(b) Zooplankton

(d) Fishes

Solution:

(b) Zooplankton

Zooplankton forms the second tropic level in a lake as they are the primary consumers in food chain that is aquatic, feeding upon phytoplankton. Hence, they hold the second level.

4. Secondary producers are

(a) Herbivores

(b) Producers

(d) None of the above

Solution:

(d) None of the above

There are no secondary producers. Plants are they only producers. They are autotrophs and synthesize their own food through the process of photosynthesis.

5. What is the percentage of photosynthetically active radiation (PAR) in the incident solar radiation?

(a) 100%

(b) 50 %

(d) 2-10%

Solution:

(b) 50 %

Photosynthetically active radiation or PAR constitutes about 50% of the total incident solar radiation.

6. Distinguish between

(a) Grazing food chain and detritus food chain

(b) Production and decomposition

(d) Food chain and Food web

(e) Litter and detritus

(f) Primary and secondary productivity

Solution:

The differences are as follows:

(a) Grazing food chain and detritus food chain

Grazing food chain

Detritus food chain

Energy is derived from the sun

Energy is derived from organic matter produced in tropic levels of the grazing food chain

It typically entails a large population

It is comparatively smaller

Starts with producers at the first tropic level. The plant biomass is then consumed by herbivores which is inturn consumed by different carnivores

Starts with detritus such as dead bodies of fallen leaves and animals that are consumed by detritivores or decomposers which in turn is consumed by predators.

(b) Production and decomposition

Production

Decomposition

Rate of producing food(organic matter) by producers is known as production

Disintegration of complex organic matter from the bodies of dead animals and plants with the help of decomposers into organic raw matter namely water, carbon dioxide, other such nutrients is decomposition.

For primary production, sunlight is required

Decomposition does not require sunlight

Dependent on the photosynthetic capacity of producers

Takes places with the help of decomposers

Upright pyramid

Inverted pyramid

Pyramid of energy is always upright

Pyramid of numbers and biomass may be inverted

At the producer level of an ecosystem, this pyramid has the highest number and biomass of organisms which declines at each tropic level in a food chain

At the producer level of an ecosystem, this pyramid has the lowest number and biomass of organisms which rises at each increasing tropic level in a food chain

(d) Food chain and Food web

Food chain

Food web

Constitutes for a single linear sequence of entities

Consists of a number of interconnected food chains

Members inhabiting higher tropic levels feed only on one type of entity

Any given individual has alternate options for food sources

(e) Litter and detritus

Litter

Detritus

Comprises of all kinds of wastes above the ground level

Comprises of residues of dead animals and plants

Consists of biodegradable and non-biodegradable substances

Consists of biodegradable substances only

(f) Primary and secondary productivity

Primary productivity

Secondary productivity

It is the amount of organic matter generated by producers per unit area over a specific span of time

It is the rate of generating organic matter by consumers over a span of time

7. Describe the components of an ecosystem.

Solution:

Ecosystem is an interacting unit involving both living components and the non-living components of a region. Both of these components interact with each other functioning as a unit which is apparent in the processes of energy flow, nutrient cycling, productivity, decomposition in ecosystems such as grasslands, forest, ponds etc.

Ecosystems have two components, they are:

Abiotic components – These constitute the non-living components of an ecosystem such as temperature, light, water, wind, soil, chemical nutrients etc
Biotic components – They form the living component of an ecosystem which include biotic factors such as decomposer, consumers, and producers. Plants and some algae form the producers as they contain chlorophyll to synthesize their own food through the process of photosynthesis carried out in the presence of light, hence they are also referred to as transducers or converters. The consumers or heterotrophs are dependent on producers either directly (primary consumers) or indirectly (secondary consumers) while decomposers are constituted by the microbes in the ecosystem, such as fungi and bacteria. They form the largest population in the food chain as they derive their nutrition by disintegrating the residues of dead animals and plants.

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

Solution:

An ecological pyramid can be defined as a graphical representation of different ecological parameters, namely the number of individuals found at each tropic level, the energy quantity or the biomass found in each tropic level. These pyramids depict the producers at the base whereas the apex depicts the top level consumers found in the ecosystem.

Pyramids are of three types:

Pyramid of numbers – it gives the graphical representation of the number of individuals found at each tropic level in a food chain of an ecosystem. This pyramid can be inverted or upright depending on the crowd of producers. Example – In a Greenland ecosystem, this pyramid is upright where in the food chain, the number of producers is followed by the number of herbivores, which in turn is followed by number of secondary and tertiary consumers. Therefore, the number of individuals at the level of producers will be maximum, whereas the number of individuals at the top are carnivores will be the least. The pyramid of numbers in a parasitic food chain is inverted, where in the food chain, producers provide food to fruit eating birds which in response support few species of insects

Pyramid of biomass – it is a graphical representation of the total quantity of living matter found at each tropic level of an ecosystem and can either be inverted or upright. In grasslands and forest ecosystems it is upright as the quantity of biomass at the producer level is higher than at the carnivore level, at the top. This pyramid is inverted for a pond ecosystem as the biomass of fishes far surpass the biomass of zooplankton on which they feed.

Pyramid of energy

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

Solution:

Primary productivity can be defined as the amount of organic matter or biomass that is generated by producers per unit area over a span of time.

It can be affected by factors such as rain, temperature, water, light etc. Primary productivity also depends on the availability of nutrients and that of plants to carry out the process of photosynthesis.

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

Solution:

Decomposition can be defined as a process that involves the disintegration of complex organic matter from the body of dead animals and plants with the action of decomposers such as microbes into inorganic raw materials such as water, carbon dioxide and other such nutrients. The different processes in decomposition are as follows:

Fragmentation – the first phase in the decomposition process, it includes the disintegration of detritus into fine particles through the action of detritivores such as earthworms.
Leaching – here, the water-soluble nutrients penetrate into the layers of soil and get sealed as unavailable salts
Catabolism – Fungi and bacteria degrade detritus into finer particles through different enzymes
Humification – this phase causes the formation of humus – a dark colored colloidal substance that serves as a reservoir of nutrients for plants
Mineralization – further degradation of humus through the activity of microbes takes place in this stage wherein inorganic nutrients are finally released in to the soil. This phenomena of releasing inorganic nutrients from the humus is known as mineralization. Humus degrades to release inorganic raw materials such as water, carbon dioxide, and other nutrients in the soil.

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

Solution:

In an ecosystem, energy enters from the ultimate source of energy – the Sun. These solar rays pass through the atmosphere to be absorbed by the surface of the earth, which help plants in performing photosynthesis. Additionally, they also help in maintaining the temperature of the earth so that living entities survive. Some of these incident rays are reflected by the surface of the earth and only close to 2-10% of the solar energy is captured by the producers (green plants) during the photosynthesis process in order to convert it into food.

Gross primary productivity is the rate at which the biomass is generated by plants during the process of photosynthesis. Just 10% of the stored energy is transferred to herbivores from the producers when plants are consumed by herbivores. The rest of 90% of the energy is used for different processes by plants such as growth, respiration and reproduction. Likewise, a mere 10% is conveyed to carnivores. This is referred to as the ten percent law of energy flow.

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

Solution:

Sedimentary cycles have reservoirs in the crust of earth or rocks. Nutrient elements such as phosphorus, sulphur, potassium and calcium are present in the sediments of the earth. These sedimentary cycles are unhurried, and slow. It takes a long time to complete their circulation and are deemed to be less perfect cycles. It is because when recycling, the nutrient elements may get sealed in the reservoir pool taking a long time to come out and continue circulation. Hence, they typically go out of circulation for a long period of time.

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

Solution:

The carbon cycle is an essential gaseous cycle which has its reservoir pool in the atmosphere. All of the living entities consist of carbon as a major constituent of the body. This carbon is a basic element present in all the living forms. Biomolecules such as lipids, carbohydrates, proteins etc that are crucial for life processes are made of carbon. Living forms are incorporated with carbon through the basic process of photosynthesis that is carried out by plants, the primary producers. The process of photosynthesis uses up atmospheric carbon dioxide and sunlight to produce a carbon compound known as ‘glucose’, this in turn is used by other living entities.

Hence, atmospheric carbon gets incorporated into life forms. It now becomes necessary to recycle this absorbed carbon dioxide back to the atmosphere completing the cycle. For this recycling of carbon back to the atmosphere in the form of carbon dioxide gas, various processes can be carried out. The respiration process disintegrates glucose molecules to produce carbon dioxide gas. The decomposition process gives out carbon dioxide from dead bodies of animals and plants into the atmosphere. Some other sources of carbon dioxide are industrialization, combustion of fuels, deforestation, forest fires, volcanic eruptions and so on.