Class 12 Biology – Ecosystem Notes

Class 12 Biology – Ecosystem Notes

1. Introduction to Ecosystem

  • Ecosystem: A functional unit of nature where biotic (living) and abiotic (non-living) components interact.
  • Includes:
    • Producers (autotrophs) – e.g., plants
    • Consumers (heterotrophs) – herbivores, carnivores, omnivores
    • Decomposers (saprotrophs) – bacteria, fungi
    • Abiotic components – sunlight, water, soil, temperature

2. Types of Ecosystems

  • Terrestrial ecosystems – Forest, grassland, desert, tundra
  • Aquatic ecosystems – Freshwater (pond, lake, river), Marine (ocean, estuary, coral reef)

3. Structure of Ecosystem

  • Food chains and food webs:
    • Food chain – Linear sequence of energy flow
    • Food web – Interconnected food chains
  • Trophic levels:
    1. Producers → Autotrophs (plants)
    2. Primary consumers → Herbivores
    3. Secondary consumers → Carnivores
    4. Tertiary consumers → Top carnivores
    5. Decomposers → Recycle nutrients
  • Energy flow – Unidirectional, from sun → producers → consumers → decomposers

4. Ecological Pyramids

  • Pyramid of numbers – Number of organisms at each trophic level
  • Pyramid of biomass – Biomass at each trophic level
  • Pyramid of energy – Energy content at each level; always upright

5. Biogeochemical Cycles

  • Water cycle – Evaporation, condensation, precipitation, transpiration
  • Carbon cycle – Photosynthesis, respiration, decomposition, fossil fuels
  • Nitrogen cycle – Nitrogen fixation, nitrification, denitrification, ammonification
  • Phosphorus cycle – Weathering of rocks, uptake by plants, return via decomposition

6. Ecosystem Services

  • Provisioning services – Food, water, timber, fiber
  • Regulating services – Climate regulation, flood control, water purification
  • Cultural services – Recreational, spiritual, aesthetic
  • Supporting services – Nutrient cycling, soil formation, primary production

Class 12 Biology – Ecology: Ecosystem | MCQs

Covers: ecosystem structure, food chains/webs, trophic levels, pyramids, biogeochemical cycles, and ecosystem services.

1. Basics of Ecosystem

  1. An ecosystem is defined as:
    a) Only plants in a forest
    b) Interactions between living organisms and their environment
    c) Only animals in a habitat
    d) A community of humans
    Answer: b) Interactions between living organisms and their environment
  2. Abiotic components of an ecosystem include:
    a) Plants and animals
    b) Soil, water, sunlight, and temperature
    c) Bacteria and fungi
    d) Herbivores and carnivores
    Answer: b) Soil, water, sunlight, and temperature
  3. Decomposers in an ecosystem are:
    a) Producers
    b) Consumers
    c) Bacteria and fungi
    d) Herbivores
    Answer: c) Bacteria and fungi

2. Food Chains and Food Webs

  1. Food chain shows:
    a) Energy flow between organisms in linear sequence
    b) Nutrient cycling
    c) Only predator-prey relationships
    d) Population growth
    Answer: a) Energy flow between organisms in linear sequence
  2. Which of the following is a primary consumer?
    a) Lion
    b) Deer
    c) Grass
    d) Fungi
    Answer: b) Deer
  3. Top carnivores occupy which trophic level?
    a) Primary
    b) Secondary
    c) Tertiary
    d) Producer
    Answer: c) Tertiary
  4. In a food chain, energy flow is:
    a) Bidirectional
    b) Unidirectional
    c) Random
    d) Cyclical
    Answer: b) Unidirectional

3. Ecological Pyramids

  1. Pyramid of energy is always:
    a) Inverted
    b) Upright
    c) Random
    d) Same as pyramid of numbers
    Answer: b) Upright
  2. Pyramid of numbers may be:
    a) Always upright
    b) Inverted or upright
    c) Always inverted
    d) Not related to ecosystems
    Answer: b) Inverted or upright
  3. Pyramid of biomass represents:
    a) Energy at each trophic level
    b) Nutrients recycled
    c) Mass of living material at each trophic level
    d) Population density
    Answer: c) Mass of living material at each trophic level

4. Biogeochemical Cycles

  1. Nitrogen fixation is performed by:
    a) Algae only
    b) Nitrogen-fixing bacteria
    c) Fungi only
    d) Earthworms
    Answer: b) Nitrogen-fixing bacteria
  2. Carbon in atmosphere is absorbed by plants through:
    a) Respiration
    b) Photosynthesis
    c) Decomposition
    d) Combustion
    Answer: b) Photosynthesis
  3. Phosphorus cycle does not include:
    a) Atmosphere
    b) Rocks
    c) Soil
    d) Plants
    Answer: a) Atmosphere
  4. Water cycle includes all except:
    a) Evaporation
    b) Transpiration
    c) Condensation
    d) Nitrogen fixation
    Answer: d) Nitrogen fixation

5. Ecosystem Services

  1. Supporting ecosystem services include:
    a) Nutrient cycling and soil formation
    b) Food and water supply
    c) Climate regulation
    d) Recreational activities
    Answer: a) Nutrient cycling and soil formation
  2. Cultural services of ecosystems are:
    a) Timber and fiber
    b) Climate regulation
    c) Recreation, spiritual and aesthetic benefits
    d) Decomposition
    Answer: c) Recreation, spiritual and aesthetic benefits