CBSE Class 12 · Biology · NCERT Biology
Ecosystem: chapter on a page
A cut of Shishya's notes for this chapter: the first paragraph of the introduction and the key ideas, formulas and facts, common mistakes and quick revision lists, word for word. Nothing is added or reworded; the worked examples are left out (they are on the chapter page).
About the notes: Written by Shishya's AI on 27 Sept 2026 from the chapter's title and class level, in Shishya's own words — not a copy or summary of the textbook. Read the official chapter for the book's own text, activities and exercises.
This note's current text has no applied fact-check on record.
Mind map
Drawn from the same notes by a fixed rule (no AI): each label is the item's own words — its bold term, its label before a colon, a short formula as written, or its first words.
The mind map as a list
- Key ideas
- An ecosystem has two components
- Producers (autotrophs) are organisms…
- Primary productivity is the rate at…
- Energy flow through an ecosystem is…
- The ten per cent law states that on…
- Decomposition is the breakdown of…
- Biogeochemical cycles (carbon cycle…
- Formulas and facts
- Net Primary Productivity (NPP)
- Ten per cent law
- Trophic levels
- Standing crop
- Detritus
- Humification
- Mineralisation
- Worked examples
- Calculating Net Primary Productivity
- Energy transfer through trophic levels
- Identifying trophic levels
- Watch out for
- Thinking energy can be recycled in…
- Confusing GPP with NPP
- Believing all ecological pyramids…
- Assuming decomposers occupy a single…
- Confusing primary succession with…
What this chapter is about
An ecosystem is a functional unit of nature where living organisms interact with each other and with their physical environment. This chapter introduces you to how ecosystems are structured and how they function through the flow of energy and cycling of nutrients. You will learn about the different components that make up an ecosystem, from producers to decomposers, and understand how energy moves through food chains and food webs.
Key ideas
- An ecosystem has two components: abiotic (non-living factors like temperature, water, soil, light) and biotic (living organisms including producers, consumers, and decomposers).
- Producers (autotrophs) are organisms like green plants and cyanobacteria that fix solar energy through photosynthesis; they form the base of every food chain.
- Primary productivity is the rate at which biomass is produced per unit area per unit time; gross primary productivity (GPP) is the total rate of photosynthesis, while net primary productivity (NPP) = GPP minus respiration.
- Energy flow through an ecosystem is unidirectional — energy enters as sunlight, passes through trophic levels, and is lost as heat at each step; it cannot be recycled.
- The ten per cent law states that on average only about ten per cent of the energy at one trophic level is available to the next trophic level; the rest is lost in respiration, undigested matter, and heat.
- Decomposition is the breakdown of dead organic matter by decomposers (bacteria and fungi) into simpler inorganic substances; this process releases nutrients back into the ecosystem.
- Biogeochemical cycles (carbon cycle, nitrogen cycle, phosphorus cycle) describe the movement of chemical elements between the biotic and abiotic components of ecosystems.
- Ecological succession is the gradual and predictable change in species composition of a community over time, leading eventually to a stable climax community.
Formulas and facts to remember
- Net Primary Productivity (NPP) = Gross Primary Productivity (GPP) − Respiration (R). NPP represents the biomass available to heterotrophs.
- Ten per cent law: Only about 10% of energy is transferred from one trophic level to the next; 90% is lost as heat or used in metabolism.
- Trophic levels: Producers → Primary consumers (herbivores) → Secondary consumers (carnivores) → Tertiary consumers (top carnivores) → Decomposers act on all levels.
- Standing crop: The mass of living material (biomass) present in an ecosystem at a given time, measured in g/m² or kg/m².
- Detritus: Dead organic matter (fallen leaves, animal remains, faeces) that serves as food for decomposers.
- Humification: Formation of humus, a dark, amorphous, colloidal organic matter resistant to microbial action, during decomposition.
- Mineralisation: Release of inorganic nutrients (like nitrates, phosphates) from humus by decomposer activity.
- Ecological pyramids: Graphical representations of trophic structure; pyramid of numbers, pyramid of biomass, and pyramid of energy. The pyramid of energy is always upright.
Common mistakes
- Thinking energy can be recycled in an ecosystem → Energy flows in one direction only and is lost as heat; only nutrients are recycled.
- Confusing GPP with NPP → GPP is total photosynthesis; NPP is what remains after plant respiration and is available to consumers.
- Believing all ecological pyramids are upright → Pyramids of numbers and biomass can be inverted in certain ecosystems (like a tree ecosystem or an ocean), but the pyramid of energy is always upright.
- Assuming decomposers occupy a single trophic level → Decomposers obtain energy from dead organisms of all trophic levels; they are not part of the linear food chain.
- Confusing primary succession with secondary succession → Primary succession occurs on bare, lifeless substrate (like bare rock); secondary succession occurs on areas where life previously existed but was disturbed (like abandoned farmland).
Quick revision
- Ecosystem = biotic community + abiotic environment functioning together.
- NPP = GPP − Respiration; this is the biomass available for consumers.
- Energy flow is unidirectional; about 90% is lost at each trophic level.
- Decomposition involves fragmentation, leaching, catabolism, humification, and mineralisation.
- Carbon cycle involves photosynthesis (uptake) and respiration/combustion (release) of CO₂.
- Succession ends in a climax community that is relatively stable.