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Ecosystem – Class 12 Biology notes, important questions and MCQs

NCERT Class 12 Biology, chapter 12. Notes, key terms, important questions with answers and MCQs.

Chapter overview

An ecosystem is a functional unit of living and non-living components. Energy flows one way through food chains, nutrients cycle, and ecological pyramids show structure.

Structure and productivity

Producers (autotrophs), consumers (herbivores, carnivores) and decomposers.
Gross primary productivity (GPP) − respiration = net primary productivity (NPP).
Decomposition: fragmentation, leaching, catabolism, humification, mineralisation.

Energy and pyramids

10% law (Lindeman): only about 10% of energy passes to the next trophic level.
Pyramid of energy is always upright; pyramid of biomass is inverted in the sea; pyramid of numbers is inverted for a tree ecosystem.
Succession: pioneer species (lichens on rocks) → climax community; carbon and phosphorus cycles.

Key terms

Trophic level: position of an organism in a food chain
Ecological succession: gradual, predictable change in community composition
Humus: dark, amorphous organic matter resistant to decay

Important questions with answers

Why is the pyramid of energy always upright?2 marks

Energy decreases at each trophic level because only about 10% passes on and the rest is lost as heat, so lower levels always have more energy than higher levels.

Differentiate between primary and secondary succession with examples.3 marks

Primary succession starts on bare areas where no soil exists (bare rock, newly cooled lava) and takes a very long time, beginning with lichens. Secondary succession starts where a community was destroyed but soil remains (abandoned farmland, burnt forest) and is faster, beginning with grasses or herbs.

Describe the process of decomposition and the factors that affect its rate.5 marks

Decomposition breaks complex organic matter into inorganic substances like CO₂, water and nutrients. Steps: fragmentation (detritivores break detritus into small pieces), leaching (water-soluble nutrients move into the soil), catabolism (bacterial and fungal enzymes break detritus into simpler substances), humification (formation of dark, resistant humus) and mineralisation (humus is slowly broken down to release inorganic nutrients). Rate depends on the chemical nature of detritus (fast if rich in nitrogen and sugars, slow if rich in lignin and chitin) and on climate: warm and moist conditions speed it up; low temperature and lack of oxygen slow it down.

Quick revision

Energy flow is unidirectional.
Pyramid of energy is never inverted.
Lichens are pioneers on bare rock.

Exam tips

If producers have 10,000 J, the tertiary consumer gets about 10 J.
Standing crop is the mass of living material at a time; productivity is a rate (per unit area per unit time) — do not mix them up.

Practice MCQs

Organisms that make their own food are
1. consumers
2. parasites
3. decomposers
4. producers
Show answer
Answer: (d) producers. Autotrophs.
Energy flow in an ecosystem is
1. cyclic
2. random
3. bidirectional
4. unidirectional
Show answer
Answer: (d) unidirectional. Lost as heat.
Pyramid of biomass in the sea is usually
1. upright
2. spindle
3. inverted
4. absent
Show answer
Answer: (c) inverted. Phytoplankton small biomass.
Pioneer species on a bare rock are usually
1. lichens
2. mosses
3. shrubs
4. trees
Show answer
Answer: (a) lichens. Lichens.
Detritivores like earthworms break detritus into small pieces. This step is called
1. leaching
2. mineralisation
3. humification
4. fragmentation
Show answer
Answer: (d) fragmentation. First step of decomposition.
If producers fix 20,000 J, energy at the secondary consumer is about
1. 2000 J
2. 2 J
3. 20 J
4. 200 J
Show answer
Answer: (d) 200 J. 10% twice.
NPP equals
1. GPP + respiration
2. GPP − respiration
3. respiration − GPP
4. GPP × 10%
Show answer
Answer: (b) GPP − respiration. Net after respiration.
The pyramid of numbers in a single tree ecosystem (tree → insects → birds) is
1. upright
2. absent
3. always rectangular
4. inverted (spindle-shaped)
Show answer
Answer: (d) inverted (spindle-shaped). One tree supports many insects.

All chapters of Class 12 Biology

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.