The Ecosystem
An Introduction

Ecosystem
 A community of interdependent organisms and
the interactions with the physical environment
in which they live.
 It can also be defined as the abiotic and biotic
factors and the interactions between them.
 The interaction between organisms and the
environment is the key!

Kind of ecosystems:
[1] Natural Ecosystem: this operates by themselves
Ex: Terrestrial Ecosystem: forest, grassland, desert
Aquatic Ecosystem: Fresh water, Marine water
[2] Artificial Ecosystem: These are man-made or
man-engineered.
Ex: Croplands, rice fields, gardens, aquarium etc.

Niche vs habitat
 A habitat is a place in which an organism lives.
The habitat must provide a source of food,
water and shelter for the organism.
 Niche: The role of the organism. This is
largely to do with the trophic level of the
organism.
 For example: plants produce food for the rest
of the food chain. Tigers keep herbivore
populations under control.

Abiotic and Biotic factors
Abiotic factor: A non-living, physical factor that
may influence an organism or a system
[1] Climatic conditions:
Ex: water, soil, light, temperature, pH etc.
[2] Inorganic substances:
Ex: H2O, C, N, S, P
[3] Organic substances:
Ex: proteins, carbohydrates, lipids etc.

Biotic Factor: A living, biological factor that may
influence an organism or a system.
[1] Autotrophic component (Producers):
(i) Photoautotrophs: convert solar energy into
chemical energy. Ex: Trees, grass
(ii) Chemoautotrophs: energy generated by redox
reaction. Ex: Sulphur bacteria
[2] Heterotrophic component (consumers): they
consumes matter built by producers.

 (i) Macroconsumers/ Phagotrophs:
Herbivores, Carnivores
 (ii) Microconsumers/decomposers/saprophytes:
Decompose organic compounds of dead or
living protoplasm & release inorganic nutrients in
the environment.
Ex: micro-organisms such as bacteria, fungi etc.

Function:
1. Means how much sunlight is trapped by
plants in a year.
2. How much plant material is eaten by
herbivores.
3. How many herbivores are eaten by
carnivores.
4. Energy flow in a ecosystem is non-cyclic
(unidirectional) whereas minerals keep in
moving in a cyclic manner.

Limiting factors
 An abiotic factor can limit the population size if there
is too much or too little of it. Even if there is the
right amount of other factors
 Examples to consider:
Sunlight
Precipitation
Salinity
Nutrients in the soil

Trophic levels
 Ecosystems are often broken up and described
according to feeding relationships.
Trophic level:
 Number of links by which it is separated from
the producer, or as nth position of the organisms.
 The position of an organism in a food chain.
 The pattern of eating and being eaten forms a
linear chain called food chain.
 A group of organisms that occupy the same place
in a food chain

Trophic levels in food chains
 Primary producers (autotrophs)
 Primary consumers (herbivores)
 Secondary consumers(carnivores)
 Tertiary consumers (top carnivores)
 Decomposers
 Detrivores

Energy flow and material cycling

Food Chains

Types of food chain:
[1] Grazing Food Chain:
Autotrophic energy capture & movement of energy to
herbivores.
Solar radiation Herbivores Primary
carnivore secondary carnivore

[2] Detritus food chain:
• Organic wastes & dead matter derived from the grazing food
chain are termed as detritus.
• Energy of detritus is serve as energy for a group of organisms
(detritivores) that are separate from grazing food chain.
• Organism: Algae, Bacteria, Molluscs, Rotifer, Mites etc.
Grazing food chain
Dead & Excretory material soil organisms consumed by
carnivores in the grazing food chain
Decomposer organisms soil animals which consume
other living organisms
Organic material permanently incorporated into sediments, soil &

Significance:
1. It helps us to understand the feeding relationship and
interactions between organisms in any ecosystem.
2. Energy flow mechanisms & matter circulations.
3. Movement of toxic substances & process of
biomagnification.

Food webs
 In a given ecosystem various food chains are
linked together & intersect each other to form a
complex network called food web
 Show energy flow through an ecosystem

Food
webs

Ecological Pyramids:
It represent the trophic structure & also trophic
function of the ecosystem.
It may be of three types:
(1) Pyramid of number
(2) Pyramid of biomass
(3) Pyramid of energy

Pyramid of Number, Energy

Pyramid of biomass

Primary productivity
Primary productivity is the rate of energy capture
by producers.
= the amount of new biomass of producers, per
unit time and space

 Gross primary production (GPP)
= total amount of energy captured
 Net primary production (NPP)= GPP - respiration
 Net primary production is thus the amount of
energy stored by the producers and potentially
available to consumers and decomposers.

Secondary Productivity
 Secondary productivity is the rate of production of
new biomass by consumers, i.e., the rate at which
consumers convert organic material into new
biomass of consumers.
 Secondary production simply involves the
repackaging of energy previously captured by
producers--no additional energy is introduced into
the food chain.

Energy flow in ecosystems:
• Energy flow is the movement of energy through
an ecosystem : from the external environment
through organisms and back to the external
environment.
• Energy flow can be explained by various energy
flow models.

[1] Single Channel Energy Flow
Model:
• Unidirectional flow of energy.
• In each trophic level, there occurs progressive
decrease in energy

[2] Y-Shaped or double channel energy flow model:
• Given by H.T. Odum in 1956.
• Shows a common boundary, light and heat flows as well as the
import, export & storage of organic matter.
• Decomposers are placed in separate box.
• Explain stratified structure of ecosystems.
• Separates the food chain.

[3] Universal Energy Flow model:
• Given by E.P. Odum.
• Energy was lose at each energy level, thereby resulting in less
energy available at next trophic level as indicated by narrow
pipes (energy flow) & smaller boxes (stores energy in
biomass).

Energy transfer in a food chain:
First law of thermodynamics

Ecological Succession
• The development of the community by the action of
vegetation on the environment leading to the establishment of
new species is termed succession.
• Final & stable community is termed as climax community.
• Traditional community is called sere or seral stage. Its
directional change in vegetation.

Causes
1. Initiating Causes: climatic (erosion, deposits, wind,
wire etc.), Biotic
2. Ecesis or continuing causes: migration, competition,
reaction etc.
3. Stabilizing causes: climate of area which stabilize the
community.

Changes during succession
1. Continuous change occurs in the kinds of plants & animals.
2. Increase in diversity of species takes place.
3. Progressive increase in the amount of living biomass & dead.
4. Green pigment go on increasing during primary succession.
5. Food chain become more complex.
6. Role of detritus becomes progressively more & more important.
7. Quality of the habitat gets modified.
8. Life cycle of mature community species are longer & complex.

Types of succession
1. Primary: its an area in any of the basic environments is
colonized by organisms for the first time.
2. Secondary: if the area under colonization has been cleared
by an agency as burning, grazing, clearing, sudden change in
climatic factor etc. of the previous plants
3. Autogenic: community itself modifies its own environment
and, thus, causing its own replacement by new community.

4. Allogenic: replacement of one community by another is largely
due to force other than the effects of communities on the
environment.
5. Autotrophic: its characterized by early & continued dominance
of autotrophic organisms such as green plants.
6. Heterotrophic: it is characterized by early dominance of
heterotrophic organisms as bacteria, fungi & animals.

7. Induced: activities such as overgrazing, frequent scrapping,
shifting cultivation or industrial pollution may cause
deterioration of an ecosystem.
8. Retrogressive: return to simpler & less dense or even
improvised form of community from an advanced or climax
community causes are allogenic.
9. Cyclic: repeated occurrence of certain stages of succession
whenever there is an open condition created within a large
community.

Process of succession
1. Nudation: development of a bare area without any form of
life.
It may occur due to landslides, erosion, deposition.
Three types are there:
(a) Topographic: soil erosion by gravity, water or wind causes
disappearance of existing community.
(b) Climatic: Glaciers, dry period etc. may destroy community.
(c) Biotic: man-made destruction of forest, grasslands for
housing, industry, agriculture etc. disease epidemics caused
by fungi, bacteria, virus etc.

2. Invasion: Successful establishment of a species in a
bare area.
(a) Migration (Disposal): seeds, spores etc. may reach to
bare area by air, water etc.
(b) Ecesis (Establishment): After reaching new areas, the
process of successful establishment of the species
means adjustment with condition there. For ex: seeds-
---------germinate---------grow---------adult start to
reproduce
(c) Aggregation: colonization by successive offsprings &
new migrants help increase the population. Number
of individuals of the species increases in number.

3. Competition and coaction:
Due to aggregation of a large number of individuals of the
species at the limited place, there develops competition (i.e.
interspecific & intraspecific) for space & nutrition.
Individuals of a species affect each other’s life in various ways
this is called coaction.
The species which fail to compete with other species are
discarded.

4. Reaction:
 It includes mechanisms of the modification of the
environment through the influence of living organisms on
it.
 As a result of reaction, the environment is modified &
become unusable for existing community which sooner or
later replaced by another community.
 Whole sequence of communities that replaces one another
in the given area called sere and different communities
constituting the sere are called seral stages or development
stages.

5. Stabilization (Climax):
 Finally, there occur a stage in the process, when the final
community become more & less established for a longer period
of time & it can maintain itself with climate.
 The final community is not replace & known as climax
community & stage is climax stage.

Primary succession-

Secondary succession-
 Ex:
 A fire levels
portions of a
forest

Secondary succession-
 Ex:
 A farmer
plows his
field

Secondary succession-