The ecosystem is one of the most fundamental units of ecology. It represents the interaction between living organisms (biotic components) and their physical environment (abiotic components). The study of ecosystems helps us understand how energy flows, how nutrients cycle, and how organisms depend on one another for survival. For Class 12 biology, ecosystems are an important chapter as they cover essential ecological principles like food chains, food webs, ecological pyramids, productivity, biogeochemical cycles, and ecosystem services.
In this article, we will go through every topic of the ecosystem in detail, point by point, so that you can use it as complete notes for Class 12 biology.
Complete Ecosystem Notes for Class 12 Biology | MCQs, Short & Long Questions
Definition of Ecosystem
An ecosystem can be defined as a functional unit of nature where living organisms interact with each other and with the physical environment, resulting in the exchange of energy and matter.
Example: A pond, forest, desert, grassland, and even a crop field are ecosystems.
Types of Ecosystem
Ecosystems are broadly divided into two types:
1. Natural Ecosystems
These exist naturally without human interference. They are further classified as:
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Terrestrial ecosystems: Forests, deserts, grasslands, tundra.
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Aquatic ecosystems: Freshwater (ponds, lakes, rivers) and marine ecosystems (oceans, estuaries, coral reefs).
2. Artificial Ecosystems
These are man-made ecosystems maintained by humans.
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Examples: Crop fields, aquariums, gardens.
Components of an Ecosystem
Every ecosystem has two major components:
1. Abiotic Components
These are non-living physical factors that influence organisms.
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Examples: Light, temperature, water, air, soil, nutrients.
2. Biotic Components
These are living organisms of the ecosystem and are categorized as:
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Producers (Autotrophs): Plants, algae, and photosynthetic bacteria that produce food.
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Consumers (Heterotrophs):
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Primary consumers: Herbivores (cow, deer, rabbit).
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Secondary consumers: Carnivores (fox, frog, small fish).
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Tertiary consumers: Top carnivores (lion, tiger, eagle).
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Decomposers (Saprotrophs): Bacteria and fungi that decompose organic matter into nutrients.
Functions of Ecosystem
An ecosystem performs the following key functions:
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Productivity: Conversion of solar energy into chemical energy (food).
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Decomposition: Breaking down of dead matter into nutrients.
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Energy flow: Transfer of energy from one trophic level to another.
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Nutrient cycling: Recycling of elements like carbon, nitrogen, and phosphorus.
Productivity in an Ecosystem
Productivity refers to the rate of production of biomass in an ecosystem.
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Primary Productivity:
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Gross Primary Productivity (GPP): Total energy captured by producers during photosynthesis.
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Net Primary Productivity (NPP): Energy left after respiration (GPP – Respiration = NPP).
NPP is the available energy for herbivores and consumers.
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Secondary Productivity: Biomass produced by consumers.
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Community Productivity: Rate of biomass production of an entire community.
Decomposition
Decomposition is the process of breaking down dead plants and animals into simple substances by decomposers.
Steps of Decomposition:
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Fragmentation: Breaking down of detritus (dead matter) into smaller pieces by detritivores (earthworm).
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Leaching: Nutrients in soluble form are washed into soil.
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Catabolism: Decomposers release enzymes to break down complex substances.
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Humification: Formation of humus (dark, amorphous substance rich in nutrients).
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Mineralization: Conversion of organic matter into inorganic nutrients (like nitrogen, phosphorus).
Energy Flow in Ecosystem
Energy flow in an ecosystem is unidirectional and follows the 10% law of energy transfer (proposed by Lindeman).
Food Chain
A food chain is the sequence of organisms through which energy flows.
Types:
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Grazing food chain (GFC): Starts with green plants → herbivores → carnivores.
Example: Grass → Deer → Tiger. -
Detritus food chain (DFC): Starts with dead organic matter → decomposers → detritivores.
Food Web
A food web is a network of interconnected food chains. It increases ecosystem stability because organisms have alternative food sources.
Ecological Pyramids
Ecological pyramids represent energy, biomass, or number at each trophic level.
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Pyramid of Number: Represents the number of organisms at each trophic level.
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Pyramid of Biomass: Represents total biomass.
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Upright in grassland/forest.
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Inverted in aquatic ecosystems (phytoplankton biomass is less than zooplankton).
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Pyramid of Energy: Always upright as energy decreases at higher trophic levels.
Nutrient Cycling (Biogeochemical Cycles)
Nutrients like carbon, nitrogen, phosphorus, and water move in cyclic pathways.
1. Carbon Cycle
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Source: CO₂ in atmosphere.
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Producers fix carbon by photosynthesis.
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Consumers eat plants, release CO₂ by respiration.
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Decomposers also release CO₂.
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Fossil fuels add extra carbon to the cycle.
2. Nitrogen Cycle
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Nitrogen fixation: Conversion of N₂ into ammonia (by Rhizobium, Azotobacter, lightning).
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Nitrification: Ammonia → nitrite → nitrate (by Nitrosomonas, Nitrobacter).
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Assimilation: Plants absorb nitrate.
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Ammonification: Dead matter converted into ammonia.
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Denitrification: Nitrate → N₂ gas (by Pseudomonas).
3. Phosphorus Cycle
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Rock weathering releases phosphate.
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Plants absorb phosphate.
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Animals obtain phosphate from plants.
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Returned to soil by excretion and decomposition.
4. Water Cycle
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Evaporation, condensation, precipitation, infiltration, and transpiration.
Ecological Succession
Ecological succession is the gradual replacement of one type of community by another in an area.
Types:
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Primary succession: Occurs in lifeless areas (bare rock, sand).
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Secondary succession: Occurs in areas where life existed earlier but got disturbed (abandoned farmland, burnt forest).
Stages:
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Pioneer species → Seral communities → Climax community.
Ecosystem Services
Ecosystems provide valuable services to humans, such as:
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Provisioning services: Food, water, timber, medicine.
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Regulating services: Climate regulation, pollination, soil fertility.
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Cultural services: Aesthetic value, recreation.
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Supporting services: Nutrient cycling, soil formation.
Human Impact on Ecosystem
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Deforestation.
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Pollution (air, water, soil).
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Overexploitation of resources.
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Global warming and climate change.
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Biodiversity loss.
Multiple Choice Questions (MCQs) on Ecosystem
Q1. The functional unit of ecology is:
a) Population
b) Community
c) Ecosystem
d) Biome
Answer: Ecosystem
Q2. Which of the following is a man-made ecosystem?
a) Forest
b) Aquarium
c) Lake
d) Desert
Answer: Aquarium
Q3. The pyramid of energy is always:
a) Upright
b) Inverted
c) Straight
d) Spindle shaped
Answer: Upright
Q4. Which organism is a decomposer?
a) Cow
b) Lion
c) Bacteria
d) Deer
Answer: Bacteria
Q5. The 10% law of energy transfer was proposed by:
a) Charles Darwin
b) Lindeman
c) Odum
d) Wallace
Answer: Lindeman
Q6. The first trophic level in a food chain is occupied by:
a) Consumers
b) Producers
c) Decomposers
d) Carnivores
Answer: Producers
Q7. Net primary productivity (NPP) is equal to:
a) GPP – Respiration
b) GPP + Respiration
c) Respiration – GPP
d) Respiration only
Answer: GPP – Respiration
Q8. The process of converting nitrate into nitrogen gas is called:
a) Nitrification
b) Ammonification
c) Denitrification
d) Fixation
Answer: Denitrification
Q9. Which is a pioneer species in primary succession on rocks?
a) Grass
b) Lichens
c) Shrubs
d) Forest trees
Answer: Lichens
Q10. In an aquatic ecosystem, the pyramid of biomass is usually:
a) Upright
b) Inverted
c) Straight
d) Cylindrical
Answer: Inverted
Q11. Humus is formed during which process?
a) Photosynthesis
b) Respiration
c) Humification
d) Assimilation
Answer: Humification
Q12. Which of the following is not a greenhouse gas?
a) Carbon dioxide
b) Methane
c) Oxygen
d) Nitrous oxide
Answer: Oxygen
Q13. The main source of energy for ecosystem is:
a) Moon
b) Wind
c) Sun
d) Water
Answer: Sun
Q14. Which ecological pyramid is always upright?
a) Pyramid of biomass
b) Pyramid of number
c) Pyramid of energy
d) All of these
Answer: Pyramid of energy
Q15. The largest reservoir of carbon is:
a) Atmosphere
b) Forests
c) Oceans
d) Soil
Answer: Oceans
Q16. Mineralization is a step in:
a) Photosynthesis
b) Decomposition
c) Respiration
d) Nitrogen fixation
Answer: Decomposition
Q17. Which is an example of a detritivore?
a) Earthworm
b) Cow
c) Lion
d) Goat
Answer: Earthworm
Q18. An example of secondary succession is:
a) Bare rock colonization
b) Newly cooled lava
c) Abandoned farmland
d) Sand dunes
Answer: Abandoned farmland
Q19. The term “ecosystem” was first coined by:
a) Charles Darwin
b) Arthur Tansley
c) Raymond Lindeman
d) Eugene Odum
Answer: Arthur Tansley
Q20. In the nitrogen cycle, ammonia is converted into nitrite by:
a) Nitrosomonas
b) Nitrobacter
c) Rhizobium
d) Pseudomonas
Answer: Nitrosomonas
Short Answer Questions (Ecosystem)
Q1. Define ecosystem.
Answer: An ecosystem is a functional unit of nature where living organisms interact with each other and with the physical environment, resulting in the flow of energy and cycling of nutrients.
Q2. What is primary productivity?
Answer: Primary productivity is the rate at which energy is captured by producers through photosynthesis in a given area and time.
Q3. Name the steps of decomposition.
Answer: The main steps of decomposition are fragmentation, leaching, catabolism, humification, and mineralization.
Q4. What is a food chain?
Answer: A food chain is a linear sequence of organisms where energy and nutrients are transferred from one trophic level to another.
Q5. Give one example of an inverted biomass pyramid.
Answer: An aquatic ecosystem like a pond or ocean shows an inverted biomass pyramid because phytoplankton biomass is less than zooplankton biomass.
Q6. What are pioneer species?
Answer: Pioneer species are the first organisms to colonize barren or lifeless areas during primary succession, such as lichens and mosses.
Q7. What is humus?
Answer: Humus is a dark, amorphous, nutrient-rich organic matter formed during decomposition and plays a vital role in soil fertility.
Q8. Define ecological succession.
Answer: Ecological succession is the natural, gradual, and predictable change in the species composition of a given area, leading to the formation of a stable climax community.
Q9. Name two gaseous biogeochemical cycles.
Answer: The carbon cycle and nitrogen cycle are gaseous biogeochemical cycles.
Q10. Who coined the term “ecosystem”?
Answer: The term “ecosystem” was coined by Arthur Tansley in 1935.
Long Answer Questions (Ecosystem)
Q1. Explain energy flow in an ecosystem.
Answer: Energy flow in an ecosystem is unidirectional. Solar energy is captured by producers (plants) through photosynthesis and converted into chemical energy. This energy passes to herbivores (primary consumers) and then to carnivores (secondary and tertiary consumers). At each trophic level, about 90% of energy is lost as heat, and only 10% is transferred to the next level, known as Lindeman’s 10% law. The flow of energy can be represented through food chains, food webs, and ecological pyramids.
Q2. Describe the steps of decomposition.
Answer: Decomposition begins with fragmentation, where detritivores like earthworms break dead matter into smaller pieces. Next, leaching occurs where water-soluble nutrients are washed away into the soil. Then, decomposers perform catabolism, breaking complex organic matter into simple compounds. This is followed by humification, forming humus, which is rich in nutrients. Finally, mineralization occurs, releasing inorganic nutrients such as nitrogen and phosphorus into the soil.
Q3. Write a note on ecological pyramids.
Answer: Ecological pyramids are graphical representations showing the relationship between producers and consumers in terms of number, biomass, or energy.
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Pyramid of number: Shows the number of individuals at each trophic level; usually upright but can be inverted.
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Pyramid of biomass: Represents total biomass; upright in terrestrial ecosystems but inverted in aquatic systems.
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Pyramid of energy: Always upright, as energy decreases progressively at each trophic level.
Q4. Explain the nitrogen cycle in detail.
Answer: The nitrogen cycle involves the following steps:
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Nitrogen fixation: Conversion of atmospheric nitrogen into ammonia by nitrogen-fixing bacteria (Rhizobium) or lightning.
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Nitrification: Ammonia is converted to nitrite by Nitrosomonas and then to nitrate by Nitrobacter.
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Assimilation: Plants absorb nitrate, and it enters food chains.
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Ammonification: Decomposers convert dead matter into ammonia.
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Denitrification: Pseudomonas bacteria convert nitrate back into nitrogen gas, releasing it into the atmosphere.
Q5. Discuss ecological succession with an example.
Answer: Ecological succession is the natural process of gradual replacement of one community by another until a stable climax community is formed. For example, in primary succession on bare rock, lichens act as pioneer species, breaking rock into soil. Mosses and small herbs then colonize, followed by grasses, shrubs, and trees, leading to a forest climax community. Secondary succession, on the other hand, occurs in areas where a community existed earlier but was disturbed, such as abandoned farmland.
Frequently Asked Questions (FAQs)
Q1. What is the difference between primary and secondary productivity?
Answer: Primary productivity is the rate of production of organic matter by producers, whereas secondary productivity is the rate of biomass production by consumers.
Q2. Why is the pyramid of energy always upright?
Answer: The pyramid of energy is always upright because energy decreases at each trophic level due to loss as heat, so less energy is available for higher levels.
Q3. Which cycle does not have a gaseous phase?
Answer: The phosphorus cycle does not have a gaseous phase.
Q4. How is the ecosystem self-sustaining?
Answer: An ecosystem is self-sustaining because producers capture energy, consumers utilize it, decomposers recycle nutrients, and energy flow along with nutrient cycling maintains balance.
Q5. What is ecosystem services?
Answer: Ecosystem services are the benefits humans derive from ecosystems, such as food, clean air, climate regulation, pollination, nutrient cycling, and cultural values.
Conclusion
The ecosystem is a self-sustaining unit that maintains balance in nature by energy flow, nutrient cycling, and ecological interactions. Understanding the structure, function, and importance of ecosystems is crucial for sustainable development and conservation of biodiversity.
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