🌸Chapter 12
Ecosystem
(Biology Notes)
1. Introduction to Ecosystem
Definition:
- An ecosystem is a functional
unit of nature in which living organisms (biotic components) interact
with each other and with the physical environment (abiotic components).
- It is the basic functional
unit of ecology, capable of self-sustenance.
Key Points:
- First coined by A.G. Tansley
(1935).
- Ecosystems can be natural
(forests, ponds, oceans) or artificial/man-made (crop fields,
gardens, aquariums).
- Components: Biotic + Abiotic.
Keywords: Functional unit, biotic, abiotic, self-sustaining.
2. Components of an Ecosystem
A. Biotic Components (Living)
1.
Producers
(Autotrophs)
o Organisms that produce their own
food via photosynthesis.
o Examples: Green plants,
phytoplankton, algae.
o Role: Form the basis of energy
in an ecosystem.
2.
Consumers
(Heterotrophs)
o Organisms that depend on
producers or other consumers for food.
o Types:
§ Primary consumers: Herbivores (e.g., deer, cow)
§ Secondary consumers: Carnivores (e.g., snake, fox)
§ Tertiary consumers: Top carnivores (e.g., tiger, eagle)
§ Omnivores: Eat both plants and animals (e.g.,
bear, humans)
3.
Decomposers
(Saprotrophs)
o Break down dead organisms and
recycle nutrients back to soil.
o Examples: Bacteria, fungi, some
protozoa.
B. Abiotic Components (Non-living)
- Physical factors: Temperature, light, wind,
rainfall.
- Chemical factors: Water, minerals, soil, pH,
salinity.
- Importance: Determine the structure,
function, and productivity of the ecosystem.
Keywords: Autotrophs, heterotrophs, saprotrophs, abiotic factors.
3. Structure of an Ecosystem
A. Physical Structure
1.
Abiotic
structure: Soil,
water, climate
2.
Biotic
structure:
Producers, consumers, decomposers
B. Functional Structure
1.
Food
chain and Food web
o Food chain: Linear flow of energy (producer →
herbivore → carnivore → decomposer)
o Example: Grass → Grasshopper → Frog → Snake
→ Hawk
o Food web: Interconnected food chains
2.
Trophic
Levels:
o Level of nourishment in a food
chain:
§ 1st trophic level: Producers
§ 2nd trophic level: Primary consumers
§ 3rd trophic level: Secondary consumers
§ 4th trophic level: Tertiary consumers
3.
Ecological
Pyramid:
o Graphical representation of number,
biomass, or energy at each trophic level.
o Types:
§ Pyramid of Number
§ Pyramid of Biomass
§ Pyramid of Energy (always upright)
4. Energy Flow in Ecosystem
- Energy flows in one
direction: Sun → Producer → Consumer → Decomposer
- Laws of energy flow:
- Liebig’s Law of Minimum: Productivity depends on the
scarcest nutrient.
- Energy transfer efficiency: ~10% from one trophic level
to the next (10% law of Lindeman).
Keywords: One-way flow, 10% law, autotroph, heterotroph.
5. Ecological Succession
Definition:
- Gradual, predictable change
in species composition of a community over time.
Types:
1.
Primary
Succession – Occurs
on bare, lifeless areas (e.g., bare rock → lichen → moss → grass → shrubs →
trees).
2.
Secondary
Succession – Occurs
in disturbed habitats (e.g., after fire or flood) where soil already
exists.
Climax Community:
- Stable, mature, self-sustaining
ecosystem (e.g., tropical rain forest, desert).
Keywords: Succession, climax community, pioneer species.
6. Ecosystem Productivity
1.
Primary
Productivity:
Rate of conversion of solar energy to chemical energy by producers.
o Gross Primary Productivity (GPP): Total energy produced
o Net Primary Productivity (NPP): Energy available to consumers (NPP
= GPP – Respiration)
2.
Secondary
Productivity:
Energy stored in herbivores or carnivores.
3.
Factors
affecting productivity:
Light, temperature, water, nutrients.
Keywords: NPP, GPP, solar energy, photosynthesis.
7. Nutrient Cycling (Biogeochemical
Cycles)
Definition: Recycling of nutrients between biotic and abiotic
components.
Major Cycles
1.
Carbon
Cycle:
Photosynthesis → Respiration → Decomposition → Fossil fuels → Combustion
2.
Nitrogen
Cycle:
N-fixation → Nitrification → Assimilation → Ammonification → Denitrification
3.
Phosphorus
Cycle: Rock →
Soil → Plants → Animals → Soil
4.
Water
Cycle (Hydrological):
Evaporation → Condensation → Precipitation → Runoff
Keywords: Biogeochemical cycles, decomposition, fixation,
mineralization.
8. Ecosystem Types
A. Aquatic Ecosystems
1.
Freshwater:
Pond, lake, river
2.
Marine:
Ocean, sea, estuary
B. Terrestrial Ecosystems
1.
Forests:
Tropical, temperate, boreal
2.
Grasslands
3.
Deserts
4.
Tundra
Keywords: Terrestrial, aquatic, biome.
9. Importance of Ecosystem
1.
Ecological
services: Oxygen,
climate regulation, water purification
2.
Economic
services: Timber,
fish, medicinal plants, fodder
3.
Biodiversity
conservation:
Maintains genetic and species diversity
10. Human Impact on Ecosystems
- Pollution: Air, water, soil
- Deforestation: Habitat loss, climate change
- Overexploitation: Wildlife hunting, overfishing
- Global warming & climate
change:
Rise in CO₂, melting glaciers
- Conservation measures: Afforestation, wildlife
sanctuaries, national parks, sustainable use of resources
Keywords: Anthropogenic, biodiversity loss, conservation.
Important Diagrams for Exams
1.
Structure
of Ecosystem:
Biotic + Abiotic components
2.
Food
Chain & Food Web
3.
Pyramids: Number, Biomass, Energy
4.
Carbon,
Nitrogen, and Water Cycles
5.
Succession
Flow Chart (Pioneer
→ Intermediate → Climax)
Quick Tips for Board & NEET
- Learn definitions and keywords
thoroughly.
- Focus on diagrams: 1–2
marks often asked.
- Memorize 10% energy transfer
and trophic levels.
- Be able to distinguish primary
vs secondary succession, and pyramids types.
- Revise nutrient cycles
carefully; diagram-based questions are common.
- Understand examples of
ecosystems (forest, pond, ocean, grassland).
NCERT Keywords to Remember:
Ecosystem, biotic, abiotic, autotroph, heterotroph, saprotroph, trophic levels,
food chain, food web, energy flow, primary productivity, secondary
productivity, succession, climax community, biogeochemical cycles,
anthropogenic impact, biodiversity.
Sample Diagrams for Quick Revision
1.
Food
Chain Example:
Grass → Grasshopper → Frog → Snake →
Hawk
2.
Energy
Pyramid (upright):
Hawk
Snake
Frog
Grasshopper
Grass
3.
Succession
Flow Chart (Primary):
Bare Rock → Lichens → Moss → Grass →
Shrubs → Trees → Climax Forest
