Ecosystem | CBSE Class 12 Biology Notes

This note covers NCERT Class 12 Biology Chapter 12, Ecosystem: the structure of an ecosystem, productivity, decomposition, energy flow through food chains and trophic levels, and ecological pyramids. Nutrient cycling and ecosystem services are covered briefly at the end.

What is an ecosystem?

Definition: An ecosystem is a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment.

An ecosystem varies greatly in size, from a small pond to a large forest or a sea. Many ecologists regard the entire biosphere as a global ecosystem, a composite of all local ecosystems on Earth. Since this system is too big and complex to be studied at one time, it is divided into two basic categories.

The chapter studies an ecosystem in three parts: the input (productivity), the transfer of energy (food chain and food web, nutrient cycling) and the output (degradation and energy loss).

The NCERT summary adds that an ecosystem comprises abiotic and biotic components. The abiotic components are inorganic materials: air, water and soil. The biotic components are producers, consumers and decomposers.

What gives an ecosystem its structure?

The interaction of biotic and abiotic components results in a physical structure that is characteristic of each type of ecosystem. Two structural features are named.

The components of the ecosystem are seen to function as a unit when four aspects are considered:

1. Productivity
2. Decomposition
3. Energy flow
4. Nutrient cycling

The pond as an example

A pond is a shallow water body in which all four basic components of an ecosystem are well exhibited. It is a fairly self-sustainable unit.

The pond performs all the functions of any ecosystem, and of the biosphere as a whole:

1. Conversion of inorganic into organic material by the autotrophs, with the help of the radiant energy of the sun.
2. Consumption of the autotrophs by heterotrophs.
3. Decomposition and mineralisation of the dead matter, which releases the minerals back for reuse by the autotrophs.
4. These three events are repeated over and over again.

There is a unidirectional movement of energy towards the higher trophic levels, and its dissipation and loss as heat to the environment.

What is productivity?

A constant input of solar energy is the basic requirement for any ecosystem to function and sustain.

Definition: Primary production is the amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis. The rate of biomass production is called productivity.

A considerable amount of GPP is utilised by plants in respiration. What is left is the net primary productivity:

GPP − R = NPP

Net primary productivity is the biomass available for consumption by heterotrophs (herbivores and decomposers).

Worked example: net primary productivity

1. Suppose the gross primary productivity of a grassland is 5,000 kcal m⁻² yr⁻¹ and the plants use 2,000 kcal m⁻² yr⁻¹ in respiration. Find the net primary productivity.
2. Write the relation: NPP = GPP − R.
3. Substitute: NPP = 5,000 − 2,000.
4. NPP = 3,000 kcal m⁻² yr⁻¹. This is the amount available to the herbivores and decomposers.

What does primary productivity depend on?

• the plant species inhabiting a particular area,
• a variety of environmental factors,
• the availability of nutrients, and
• the photosynthetic capacity of plants.

Primary productivity therefore varies in different types of ecosystems.

Note: Do not mix up the two pairs. Gross and net describe whether respiration losses have been subtracted. Primary and secondary describe who makes the organic matter: plants (primary) or consumers (secondary).

How does decomposition take place?

Definition: Decomposition is the process in which decomposers break down complex organic matter into inorganic substances like carbon dioxide, water and nutrients.

Detritus is the raw material for decomposition. It consists of dead plant remains such as leaves, bark and flowers, and dead remains of animals, including fecal matter. The earthworm is called the farmer's friend because earthworms help in the breakdown of complex organic matter as well as in loosening of the soil.

The important steps in the process of decomposition are listed below. The first three operate simultaneously, so the numbers are a list order, not a time order.

1. Fragmentation: detritivores (for example the earthworm) break down detritus into smaller particles.
2. Leaching: water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts.
3. Catabolism: bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
4. Humification: a dark coloured amorphous substance called humus accumulates.
5. Mineralisation: the humus is further degraded by some microbes, and inorganic nutrients are released.

All the first three steps operate simultaneously on the detritus. Humification and mineralisation occur during decomposition in the soil.

Humus is highly resistant to microbial action and undergoes decomposition at an extremely slow rate. Being colloidal in nature, it serves as a reservoir of nutrients.

A cycle of arrows that begins and ends at a tree growing in the soil. A green leaf falls to the ground. Some leaves are eaten by insects and other animals, and their nutrients and energy enter the food web. Some nutrients leach into the soil by chemical action. Leaves are partially consumed by decomposers such as fungi and bacteria; they begin to lose form and become litter. Further decomposition by earthworms, bacteria, soil mites, fungi and so on turns the litter into organic rich soil, in which the tree grows.

See Fig. 12.1 in your NCERT textbook

What controls the rate of decomposition?

Decomposition is largely an oxygen-requiring process. Its rate is controlled by the chemical composition of the detritus and by climatic factors.

Temperature and soil moisture are the most important climatic factors, and they regulate decomposition through their effects on the activities of soil microbes.

Note: The body of the chapter lists five steps of decomposition. The NCERT summary names three processes: fragmentation of detritus, leaching and catabolism. If a question asks for the steps, give all five.

How does energy enter and flow through an ecosystem?

Except for the deep sea hydro-thermal ecosystem, the sun is the only source of energy for all ecosystems on Earth.

This small amount of energy sustains the entire living world. Plants and photosynthetic bacteria (autotrophs) fix the sun's radiant energy to make food from simple inorganic materials. All organisms depend on producers for their food, directly or indirectly. So there is a unidirectional flow of energy from the sun to producers and then to consumers.

Ecosystems are not exempt from the Second Law of thermodynamics. They need a constant supply of energy to synthesise the molecules they require, to counteract the universal tendency toward increasing disorderliness.

Producers and consumers

All animals depend on plants, directly or indirectly, for their food needs. They are hence called consumers, and also heterotrophs.

Note: Consumer and carnivore are numbered differently. A primary carnivore is a secondary consumer, and a secondary carnivore is a tertiary consumer. The primary consumer is a herbivore, not a carnivore.

What are the grazing food chain, the detritus food chain and the food web?

No energy that is trapped in an organism remains in it for ever. The energy trapped by the producer is either passed on to a consumer, or the organism dies. The death of an organism is the beginning of the detritus food chain or web.

A simple grazing food chain (GFC) given by NCERT is:

Grass (producer) → Goat (primary consumer) → Man (secondary consumer)

Decomposers meet their energy and nutrient requirements by degrading dead organic matter or detritus. They are also known as saprotrophs (sapro, to decompose). They secrete digestive enzymes that break down dead and waste materials into simple inorganic materials, which they subsequently absorb.

Definition: A food web is the natural interconnection of food chains.

The detritus food chain may be connected with the grazing food chain at some levels. Some of the organisms of the DFC are prey to the GFC animals. In a natural ecosystem, some animals, like cockroaches and crows, are omnivores.

What are trophic levels, standing crop and the 10 per cent law?

Definition: A trophic level is the specific place an organism occupies in the food chain, based on the source of its nutrition or food.

Four boxes are stacked one above the other, joined by upward arrows, with the level and examples beside each. At the bottom is the producer: first trophic level (plants); phytoplankton, grass, trees. Above it is the primary consumer: second trophic level (herbivore); zooplankton, grasshopper and cow. Next is the secondary consumer: third trophic level (carnivore); birds, fishes, wolf. At the top is the tertiary consumer: fourth trophic level (top carnivore); man, lion.

See Fig. 12.2 in your NCERT textbook

The amount of energy decreases at successive trophic levels. When any organism dies, it is converted to detritus or dead biomass, which serves as an energy source for decomposers. Organisms at each trophic level depend on those at the lower trophic level for their energy demands.

Definition: The standing crop is the mass of living material that each trophic level has at a particular time. It is measured as the mass of living organisms (biomass) or the number in a unit area.

The biomass of a species is expressed in terms of fresh or dry weight. Measurement of biomass in terms of dry weight is more accurate, because the water content of living material varies and water does not represent organic matter produced.

Definition: The 10 per cent law: only 10 per cent of the energy is transferred to each trophic level from the lower trophic level.

The number of trophic levels in the grazing food chain is restricted because the transfer of energy follows this 10 per cent law. In nature it is possible to have producer, herbivore, primary carnivore and secondary carnivore in the grazing food chain.

Worked example: the 10 per cent law

1. Suppose the producers of a food chain hold 10,000 J of energy. Find the energy available to the tertiary consumer.
2. Primary consumers receive 10 per cent of 10,000 J = 1,000 J.
3. Secondary consumers receive 10 per cent of 1,000 J = 100 J.
4. Tertiary consumers receive 10 per cent of 100 J = 10 J.

These are the figures in NCERT's ideal pyramid of energy (Figure 12.4 d).

Energy from the sun passes to the first trophic level, the producers (plants). From there arrows pass to the second trophic level, the primary consumers (herbivores), then to the third trophic level, the secondary consumers (carnivores), and then to the fourth trophic level, the tertiary consumers (top carnivores). At every level an arrow marked heat leaves the chain, and from every level another arrow leads down to the decomposers at the bottom; each of these arrows to the decomposers also gives off heat.

See Fig. 12.3 in your NCERT textbook

What are ecological pyramids?

The base of a pyramid is broad and it narrows towards the apex. One gets a similar shape when the food or energy relationship between organisms at different trophic levels is expressed. The base of each pyramid represents the producers, or the first trophic level, and the apex represents the tertiary or top level consumer. Three types of ecological pyramids are usually studied.

Each pyramid is drawn as a stack of bars, with the producers (P) at the base, then the primary consumers (PC), the secondary consumers (SC) and the tertiary consumers (TC) at the top. In (a), the pyramid of numbers in a grassland, the bars narrow from 5,842,000 producers to 3 top carnivores. In (b), the pyramid of biomass, the bars narrow sharply from 809 to 1.5 kg m⁻². In (c) there are only two bars and the pyramid is inverted: the producer bar (4) is narrower than the primary consumer bar (21) above it. In (d), the pyramid of energy, 1,000,000 J of sunlight is written below the base, and the bars fall from 10,000 J to 10 J.

See Fig. 12.4 a, b, c and d in your NCERT textbook

NCERT draws three conclusions from these figures.

• Only three top carnivores are supported in an ecosystem based on the production of nearly 6 million plants.
• The pyramid of biomass shows a sharp decrease in biomass at higher trophic levels.
• Primary producers convert only 1 per cent of the energy in the sunlight available to them into NPP: 10,000 J out of 1,000,000 J.

Two rules apply to every pyramid. Any calculation of energy content, biomass or numbers has to include all the organisms at that trophic level. And a trophic level represents a functional level, not a species as such. A given species may occupy more than one trophic level in the same ecosystem at the same time. For example, a sparrow is a primary consumer when it eats seeds, fruits and peas, and a secondary consumer when it eats insects and worms.

When is a pyramid inverted, and what are the limitations of pyramids?

In most ecosystems all the pyramids, of number, of energy and of biomass, are upright. Producers are more in number and biomass than the herbivores, and herbivores are more in number and biomass than the carnivores. Energy at a lower trophic level is always more than at a higher level. There are exceptions.

The pyramid of energy is always upright because, when energy flows from a particular trophic level to the next, some energy is always lost as heat at each step. Each bar in the energy pyramid indicates the amount of energy present at each trophic level in a given time, or annually per unit area.

Note: The inverted biomass pyramid of the sea can look like a paradox. The standing crop is the biomass present at one moment. Phytoplankton are small and short-lived but reproduce very fast, so a small standing crop can keep feeding a larger standing crop of consumers.

Limitations of ecological pyramids

• A pyramid does not take into account the same species belonging to two or more trophic levels.
• It assumes a simple food chain, something that almost never exists in nature. It does not accommodate a food web.
• Saprophytes are not given any place in ecological pyramids, even though they play a vital role in the ecosystem.

What does the NCERT summary add on nutrient cycling and ecosystem services?

Nutrient cycling is one of the four functional aspects of an ecosystem.

Definition: Nutrient cycling is the storage and movement of nutrient elements through the various components of the ecosystem. Nutrients are repeatedly used through this process.

The products of ecosystem processes are named ecosystem services, for example the purification of air and water by forests.

The summary also restates two points in words worth knowing. Energy flow is unidirectional: first, plants capture solar energy, and then food is transferred from the producers to the decomposers. And secondary productivity is described there as the rate of assimilation of food energy by the consumers.

Short answers to the NCERT exercise

Glossary

• Ecosystem — A functional unit of nature where living organisms interact among themselves and with the surrounding physical environment.
• Stratification — The vertical distribution of different species occupying different levels in an ecosystem, such as trees, shrubs and herbs in a forest.
• Primary production — The amount of biomass or organic matter produced per unit area over a time period by plants during photosynthesis.
• Gross primary productivity — The rate of production of organic matter during photosynthesis in an ecosystem.
• Net primary productivity — Gross primary productivity minus respiration losses; the biomass available to heterotrophs.
• Secondary productivity — The rate of formation of new organic matter by consumers.
• Detritus — Dead plant remains and dead remains of animals, including fecal matter; the raw material for decomposition.
• Humus — A dark coloured amorphous substance, highly resistant to microbial action, that serves as a reservoir of nutrients.
• Mineralisation — The release of inorganic nutrients when humus is further degraded by some microbes.
• Saprotrophs — Decomposers, mainly fungi and bacteria, which obtain energy and nutrients by degrading dead organic matter.
• Trophic level — The specific place an organism occupies in the food chain, based on the source of its nutrition.
• Standing crop — The mass of living material present at each trophic level at a particular time.
• Food web — The natural interconnection of food chains in an ecosystem.
• Photosynthetically active radiation — The part of incident solar radiation, less than 50 per cent, that can be used in photosynthesis.

Common errors and misconceptions

• Misconception: Plants capture 50 per cent of sunlight. Correct: Less than 50 per cent of incident solar radiation is PAR, and plants capture only 2 to 10 per cent of the PAR.
• Misconception: NPP is the total organic matter made in photosynthesis. Correct: That is GPP. NPP is what remains after respiration losses: GPP − R = NPP.
• Misconception: Secondary productivity is the productivity of secondary consumers only. Correct: It is the rate of formation of new organic matter by consumers.
• Misconception: The grazing food chain carries most of the energy in every ecosystem. Correct: The GFC is the major conduit in an aquatic ecosystem. In a terrestrial ecosystem a much larger fraction flows through the detritus food chain.
• Misconception: A primary carnivore is a primary consumer. Correct: The primary consumer is a herbivore. A primary carnivore is a secondary consumer.
• Misconception: The pyramid of energy can be inverted in the sea. Correct: It is the pyramid of biomass that is generally inverted in the sea. The pyramid of energy is always upright.
• Misconception: Decomposition is fastest in cold, waterlogged soil. Correct: A warm and moist environment favours decomposition. Low temperature and anaerobiosis inhibit it.
• Misconception: A species always belongs to one trophic level. Correct: A trophic level is a functional level. A sparrow is a primary consumer when it eats seeds and a secondary consumer when it eats insects.

Exam-style questions with model answers

Q1. Define stratification with an example. [1 mark]

1. Stratification is the vertical distribution of different species occupying different levels. In a forest, trees occupy the top layer, shrubs the second, and herbs and grasses the bottom layers.

Q2. Distinguish between gross primary productivity and net primary productivity. [2 marks]

1. Gross primary productivity (GPP) is the rate of production of organic matter during photosynthesis.
2. Net primary productivity (NPP) is GPP minus the respiration losses of the plants (GPP − R = NPP). It is the biomass available for consumption by heterotrophs.

Q3. Why is the pyramid of energy always upright? [2 marks]

1. When energy flows from a particular trophic level to the next trophic level, some energy is always lost as heat at each step.
2. So the energy at a lower trophic level is always more than at a higher level, and the pyramid can never be inverted.

Q4. Distinguish between the grazing food chain and the detritus food chain. [3 marks]

1. The grazing food chain begins with the producers, for example grass, goat, man. The detritus food chain begins with dead organic matter.
2. The detritus food chain is made up of decomposers, which are heterotrophic organisms, mainly fungi and bacteria, also called saprotrophs.
3. In an aquatic ecosystem the grazing food chain is the major conduit for energy flow. In a terrestrial ecosystem a much larger fraction of energy flows through the detritus food chain.

Q5. State the factors that control the rate of decomposition. [3 marks]

1. Chemical composition of detritus: decomposition is slower if the detritus is rich in lignin and chitin, and quicker if it is rich in nitrogen and water-soluble substances like sugars.
2. Climatic factors: temperature and soil moisture are the most important, and act through their effects on soil microbes. A warm and moist environment favours decomposition.
3. Low temperature and anaerobiosis inhibit decomposition, resulting in a build up of organic materials. Decomposition is largely an oxygen-requiring process.

Q6. What are the limitations of ecological pyramids? [3 marks]

1. A pyramid does not take into account the same species belonging to two or more trophic levels.
2. It assumes a simple food chain, which almost never exists in nature, and does not accommodate a food web.
3. Saprophytes are not given any place in ecological pyramids, even though they play a vital role in the ecosystem.

Q7. Define decomposition and describe the steps in the process. [5 marks]

1. Decomposition is the breakdown of complex organic matter by decomposers into inorganic substances like carbon dioxide, water and nutrients. Its raw material is detritus: dead plant and animal remains, including fecal matter.
2. Fragmentation: detritivores such as the earthworm break down detritus into smaller particles.
3. Leaching: water-soluble inorganic nutrients go down into the soil horizon and get precipitated as unavailable salts. Catabolism: bacterial and fungal enzymes degrade detritus into simpler inorganic substances.
4. Humification: a dark coloured amorphous substance, humus, accumulates. It is highly resistant to microbial action, decomposes extremely slowly and, being colloidal, serves as a reservoir of nutrients.
5. Mineralisation: the humus is further degraded by some microbes and inorganic nutrients are released.

Q8. Give an account of energy flow in an ecosystem. [5 marks]

1. The sun is the only source of energy for all ecosystems, except the deep sea hydro-thermal ecosystem. Less than 50 per cent of incident solar radiation is PAR, and plants capture only 2 to 10 per cent of it.
2. Energy flows in one direction, from the sun to the producers and then to the consumers: primary consumers (herbivores), secondary consumers (primary carnivores) and tertiary consumers (secondary carnivores).
3. Each of these occupies a trophic level: producers the first, herbivores the second, carnivores the third and top carnivores the fourth. The amount of energy decreases at successive trophic levels.
4. The transfer follows the 10 per cent law: only 10 per cent of the energy is transferred to each trophic level from the lower one, which restricts the number of trophic levels. The rest is lost, mainly as heat.
5. When an organism dies it becomes detritus, which is the energy source for the decomposers of the detritus food chain.

Key takeaways

• An ecosystem is a functional unit of nature; its four functional aspects are productivity, decomposition, energy flow and nutrient cycling.
• Species composition and stratification are the two main structural features of an ecosystem.
• GPP minus respiration losses is NPP; the biosphere's annual NPP is about 170 billion tons, of which the oceans give only 55 billion tons.
• Decomposition has five steps: fragmentation, leaching, catabolism, humification and mineralisation; it is favoured by warm, moist, oxygen-rich conditions.
• Less than 50 per cent of incident solar radiation is PAR, and plants capture only 2 to 10 per cent of the PAR.
• The grazing food chain starts from producers and the detritus food chain from dead organic matter; their interconnection makes a food web.
• Only 10 per cent of the energy passes from one trophic level to the next, which restricts the number of trophic levels.
• The pyramid of energy is always upright; the pyramid of biomass is generally inverted in the sea, and the pyramid of numbers is inverted for a big tree.

Test yourself

What are the two basic categories of ecosystems?

Ecosystems are divided into two basic categories, terrestrial and aquatic; crop fields and an aquarium are man-made ecosystems.

What is the annual net primary productivity of the whole biosphere?

It is approximately 170 billion tons (dry weight) of organic matter, of which the oceans contribute only 55 billion tons.

What is detritus?

Detritus is dead plant remains such as leaves, bark and flowers, and dead remains of animals including fecal matter; it is the raw material for decomposition.

Which kinds of detritus decompose slowly?

Detritus rich in lignin and chitin decomposes slowly, while detritus rich in nitrogen and sugars decomposes quickly.

What share of PAR do plants capture?

Plants capture only 2 to 10 per cent of the photosynthetically active radiation.

Which food chain is the major conduit for energy flow in an aquatic ecosystem?

In an aquatic ecosystem the grazing food chain is the major conduit for energy flow.

To which trophic level do herbivores belong?

Herbivores are primary consumers and belong to the second trophic level of the food chain.

How many top carnivores does NCERT's grassland pyramid of numbers show?

It shows only three top carnivores, supported by the production of nearly 6 million plants.