ECOSYSTEM || Biology || Class 12 || NCERT ||

2. Ecosystem
A functional unit of nature where interactions of living organisms with
themselves and with physical environment takes place.
An ecosystem is a community of living organisms (plants, animals and
microbes) in conjunction with the nonliving components of their
environment (things like air, water and mineral soil), interacting as a
system.
The term ecosystem was coined by A.G.Tansley.
Father of ecosystem ecology – E.P. Odum
Types:
1. Artificial ecosystem: man made ecosystem. Ex: crop field,
aquarium.
2. Natural ecosystem: Terrestrial ecosystem and Aquatic ecosystem.

3. • Terrestrial ecosystem is classified into forest, grassland,
desert etc.
• Aquatic ecosystem is classified into freshwater and salt water
ecosystem.
• Freshwater ecosystem are lentic - ponds, lakes and lotic - river,
streams.
• Salt water ecosystem are wetlands, estuaries, marine

4. Stratification
Vertical distribution of different species occupying different levels in
an ecosystem is called stratification.
1. Top trees.
2. Shrubs.
3. Herbs.
4. Grass.

5. Structure of ecosystem
• The structure of ecosystem consists of biotic and abiotic components.
BIOTIC
FACTORS
PRODUCERS
CONSUMERS
DECOMPOSERS
ABIOTIC
FACTORS
TEMPERATURE
WATER
LIGHT
SOIL

6. 1. Producers-
• synthesis there own food.
• All the autotrophs of ecosystem are called Producers.
• Green plants are main Producers (Photosynthesis- main process)
• Energy Enters into Ecosystem through the producers.
• Solar energy is the ultimate source of energy
• In aquatic ecosystem Phytoplankton are major Autotrophs.
• 2 types - Photoautotroph and chemoautotroph.

7. 2. Consumers
• All the heterotrophs of ecosystem are known as consumers
• They digest their food inside the body. (First ingestion than
digestion)
• They are also known as Macro consumers/ Phagotrophs/ holozoic.
• These are of 3 types –
1. Primary consumer (herbivores) : Cow, Rabbit etc
2. Secondary consumer (primary Carnivores) : Dog, Cat, Snake etc.
3. Tertiary consumer/ Top Consumers (sec. Carnivores) : Lion, Man
etc.

8. 3. Decomposers
• Those living organism which decomposes the dead body of producers
and consumers are known as decomposers / Reducers / Transformers /
Osmotrophs
• They are also known as Micro Consumers.
• They convert complex organic material into simpler organic substance.
• Eg. Bacteria and fungi.

9. The major function of ecosystem
1. Productivity
2. Decomposition
3. Energy flow
4. Nutrient cycling

10. Productivity
• The rate of biomass production per unit area during a given period
of time is called productivity.
• It is measured in terms of weight or energy.
• It is used to compare productivity of different ecosystems.
1. Primary productivity:
• It is the amount of biomass produced per unit area in a given time
period by Plants during Photosynthesis.
• GPP-Gross Primary Productivity : It is the rate of production of
organic matter during photosynthesis.

11. GPP – R = NPP
NPP-Net Primary Productivity.
R- respiration loss.
GPP is not equal to NPP because, considerable amount of GPP is
utilized by plants in respiration.
2. Secondary productivity
It is the rate of formation of new organic matter by consumers in a
given period of time.

12. Decomposition:
It is the process of breaking down of dead organic matter into smaller
organic and inorganic molecules by Decomposers (bacteria, fungi).
DETRITUS
Raw materials of decomposition as dead plants, animals and animal
feces is called detritus.
DETRITIVORES
Animals that feed on decaying organic matter (detritus).
Examples: earthworms, termites, snails etc.

13. Steps of Decomposition:
1. Fragmentation of Detritus: It is the process of breakdown of detritus
into smaller particles. It is done by detritivores.
2. Leaching: Soluble inorganic nutrients dissolve in water and goes
down to soil, get precipitated is called leaching.
3. Catabolism: Decomposers (bacteria, fungi) release enzymes to
decompose detritus into simpler inorganic compounds.
4. Humification: Simplified detritus converted to humus.
• Humus is a Dark, Amorphous substance.
• Highly resistant to Microbial Action.
• Undergoes Decomposition very Slowly.
• Reservoir of nutrients (due to colloidal nature).
5. Mineralization: Humus is degraded to releases inorganic substances
( CO2, H2O etc) and nutrients (Ca2+, Mg2+,K + etc).

14. Factors affecting rate of Decomposition:
• Chemical composition - decomposition rate will be slow when
detritus is rich in lignin and chitin. The rate increases when detritus is
rich in nitrogen and water soluble substances like sugars.
• Climatic conditions – warm and moist environment favour
decomposition. Low temperature and anaerobiosis inhibit
decomposition.

15. Food chain
• Transfer of energy / food from the producer through a series of
organisms is known as food chain.
• Food chains represent energy flow through ecosystems.
• Different steps in a food chain are called trophic levels.
1. Grazing food chain (GFC):
• The food chain that begins with producers continued with herbivores is
called grazing food chain.
GRASS GOAT MAN

16. 2. Detritus food chain (DFC)
• The food chain that begins with dead organic matter is called detritus
food chain (DFC).
• Saprophytes: These are decomposers (fungi, bacteria) which feed on
detritus.
3. Parasitic Food chain (PFC)
DETRITUS EARTHWORM BIRD
TREE BIRD PARASITES

17. Trophic level
• Each Step of the food chain is called trophic level.
• 4 trophic level are present in ecosystem because level of energy
decreases during the flow of energy from one trophic level to
another trophic level.
Standing crop:
• Each trophic level has a certain mass of living material at a
particular time is called standing crop.
• The standing crop is measured as the mass of living organisms
(biomass) or the number in a unit area.
First trophic level T1 Producers 100%
Second trophic
level
T2 Primary
Consumers
10%
Third trophic level T3 Secondary
consumers
1%
Fourth trophic level T4 Top consumers 0.1%

18. Food web:
The natural interconnection of food chains forms a web called food web.

19. Energy flow
The storage, expenditure, transformation of energy is based on 2 basic
laws of thermodynamics.

20. Ecological pyramid
Graphical representation of ecological parameters at different trophic level
in ecosystem is called Pyramids.
These are energy, biomass or number of different organisms.
Characteristic features of ecological pyramids.
• The base of the pyramid is broad and it is narrow at the apex.
• The relationship can be expressed in terms of number, energy or
biomass.
• The base of the pyramid represented by producer and apex is the top
consumer, other trophic levels are in between.
• In most ecosystems, all the pyramids of number, energy and biomass
are upright.

24. Inverted pyramid:
• The pyramid of number in a tree ecosystem is inverted.
• The pyramid of biomass in sea also inverted because the biomass of
fishes is more than phytoplankton.
• Pyramid of energy is always upright, can never be inverted.
Because when energy flows from a particular trophic level to the
next, some energy is always lost as heat at each step.

26. Limitations of Ecological Pyramids
• It does not take into account the same species belonging to two or
more trophic levels.
• It does not accommodate a food web.
• Saprophytes do not have any place.

27. Ecological succession
• Development of plant community on a barren area is called Ecological
succession or Biotic succession.
• There is a gradual and predictable change in the species composition of
a given area.
• Biotic communities are never stable.
Pioneer species - The first species that invade a bare area.
Climax species – The last and stable community in an area.
Seral stages – In succession stages which comes between pioneer
community and climax community is called transitional or seral
community.

28. Characteristics –
• Gradual replacement
• Continuous change towards stability
• Increase in species diversity
• Decrease in net productivity
• Parallel evolution
• Change in number and type of animals and decomposers.

29. Cause of succession
1. Biotic factors – favorable for next community
2. Physiological factors – unfavorable conditions
Types of Succession
1. Primary Succession - A Succession that starts where no living
organisms are there.
E.g. Volcanic lava, land slides etc.
2. Secondary succession - A Succession where vegetation was present
previously but destroyed due to natural or artificial causes.
E.g. Fire, Flood etc.

32. Hydrarch Succession
Stages of hydrosere or hydrarch succession in the newly formed lake or
pond.
1. Phytoplankton stages – pioneer community, first coming minute
autotrophic organism, These produce organic matter.
E.g. Cyanobacteria
2. Rooted submerged stages – E.g. Vallisneria
3. Free floating stages – E.g. Nymphea
4. Reed swamp stages (Amphibious stages) – E.g. Azolla
5. Sedge meadow stage (marsh meadow stage) – E.g. muddy plant
6. Scrub stage – E.g. Woody shrubs
7. Forest stage – E.g. Tree

34. Xerarch Succession
Stages of Lithoosere or Xerarch succession on the rocks.
1. Crustose lichen stages – pioneer community, Produce organic acid
which cause weathering of rocks.
2. Foliose lichen stages – large lichen with leafy thalli
3. Moss stages
4. Herb stages
5. Shrub stage
6. Forest stage

36. Nutrient cycling/ biogeochemical cycling
A nutrient cycle (or ecological recycling) is the movement and exchange
of organic and inorganic matter back into the product ion of living
matter.
The process is regulated by food web pathways that decompose matter
into mineral nutrients.
Nutrient cycles occur within ecosystems. Ecosystems are inter connected
systems where matter and energy flows and is exchanged as organisms
feed, digest , and migrate.
Minerals and nutrients accumulate in varied densities and uneven
configurations.

37. There are two types of cycles –
1. Gaseous cycle –
Cycle which have its reservoir of the nutrient in atmosphere.
Ex: Carbon cycle, Nitrogen cycle.
2. Sedimentary cycle –
Cycle which have its reservoir of the nutrient is earth crust.
Ex:, Sulphur cycle

38. 1. Carbon constitutes 49 % of dry weight of organism.
2. 0.003% of CO2 is present in atmosphere.
3. Carbon is fixed in the biosphere through photosynthesis.
4. Carbon returns to the atmosphere as CO2 through respiratory
activities of the producers and consumers.
5. Decomposers also contribute Some amount of CO2 from dead
organic matter
6. Burning of wood, forest fire , combustion of organic matter, fossil
fuel, and volcanic activity are additional sources for releasing CO2
in the atmosphere.
Carbon cycle

40. Phosphorus cycle
1. Phosphorus is a major constituent of organism.
2. It found in nucleic acids and cellular energy transfer systems (ATP).
3. Many animals also need large quantities of this element to make
shells, bones and teeth.
4. The natural reservoir of phosphorus is rock. In which it contains in
the form of phosphates.
5. When rocks are weathered, minute amounts of phosphates dissolve in
soil solution and are absorbed by the roots of the plants.
6. Herbivores and other animals obtain this element from plants.
7. The waste products and the dead organisms are decomposed by
phosphate-solubilising bacteria releasing phosphorus.

42. Ecosystem services
• The products of ecosystem processes are called as ecosystem services.
Ex: Forest ecosystems purify air and water,
• It avoids droughts and flood.
• It cycle nutrients and generate fertile soils.
• It provide wildlife habitat, maintain biodiversity.
• It provide storage site for carbon.
• It also provide aesthetic, cultural and spiritual values.

43. • THANK YOU •