Download Lecture 4.

Dr. Syed Zulifiqar Andrabi
Ecosystem
The existence of living organisms depends on a system of complex interactions within
and in between the components of the environment the lithosphere hydrosphere
atmosphere and the biosphere itself. It is these interactions which satisfy the needs of all
living organisms such as food, shelter, water, and oxygen to respire, mates to reproduce
etc, which are essential for sustained life on this planet. The complex system in which
interactions between the different components of the environmental occur is referred to as
an ecosystem.
A biotic community cannot live in isolation. It lives and flourishes in an environment
which supplies and fulfills its material and energy requirements and provides other living
conditions. The biotic community and its physical environment in which matter (chemical
elements) cycle and energy flows is called the ‘ecosystem’.
Ecosystem is a self sustaining structural and functional unit of biosphere which includes
the living organisms and their non- living environment that interact by means of food
chains resulting in the flow of energy and material cycling to form a self supporting
system. Ecosystem is a combination of two words “Eco” a Greek word means
environment and system means collection of parts and each part having its own function
to perform.
The term ‘Ecosystem’ was first proposed by A.G. Tansley in 1935. He defined it as
“the system resulting from the integration of all the living and non-living factors of the
environment”. He, thus, regarded the ecosystem as including not only organism- complex
but also the whole complex of physical factors forming the environment. Keeping this in
view, we can think of the earth as a giant ecosystem where biotic (living) and abiotic
(non-living) components are constantly acting and reacting up on each other bringing
forth structural and functional changes in it. This vast ecosystem the biosphere is however
difficult to handle. Thus for convenience the study of nature is carried out by making its
artificial sub-divisions into smaller units i.e. ecosystem. An ecosystem thus is the smallest
unit of biosphere that has all the characteristics to sustain life.
According to Odum and Clarke 1962 an ecosystem may be defined as “A dynamic
system which includes both biotic and abiotic components influencing the properties of
each other they both are necessary for the maintenance of life”. As there is transfer of
energy as well as materials in an ecosystem thus it is an open system.
Structure of Ecosystem
The two main aspects of an ecosystem are structure and function. Together they illustrate the
organization of an ecosystem. The structure of an ecosystem means the composition of
biological community including species population (numbers), biomass, life history,
distribution in space etc. the quantity and distribution of abiotic (non-living) materials such as
water, soil, nutrients, etc and the sense of conditions of existence such as temperature light,
humidity, wind, wave action etc.
Every ecosystem has two major components which are abiotic and biotic.
(i)
Abiotic components: The physical and chemical components of an ecosystem
constitute its abiotic structure.
(a) Physical factors: These factors include soils, temperature, wind, sunlight, water,
currents, humidity etc. these are very important and have strong influence on the
ecosystem.
(b) Chemical factors: These constitute inorganic and organic substances. The inorganic
substances include Carbon, Hydrogen, Nitrogen, Potassium Phosphorus, and Sulphur
etc these are involved in nutrient cycling and are present in an ecosystem at any given
time. The organic substances include carbohydrates, lipids, and proteins and are
present in the biomass or in the environment.
(ii)
Biotic component: The living organisms present in an ecosystem all plants
animals and microorganisms constitute the biotic component of an ecosystem.
These organisms have different nutritional behaviour and status in the ecosystem
and are accordingly known as producers, consumers and decomposers based on
how do they get their food.
(a) Producers/ Autotrophs: These organisms manufacture their own food by
using sunlight, simple inorganic substances and convert them into complex
organic substances. These are self nourishing organisms such as algae green
plants and photosynthetic bacteria.
Autotrophs are of two types-organisms which utilize sunlight and manufacture
food are known as photo autotrophs (Photo= light, auto= self, troph= food) such
as photosynthetic bacteria, green plants, algae.
Some autotrophs produce organic matter by the oxidation of certain chemicals in
absence of sunlight. They are known as chemoautotroph’s for instance sulphur
bacteria living in deep oceans use heat generated by the decay of radioactive
elements and manufacture food for themselves.
(b) Consumers/ Heterotrophs: All organisms which get their organic food by
feeding upon other organisms are called consumers which are of following
types:
(i)
Herbivores (plant eaters): Feeding directly on producers and thus known
as primary consumers e.g. rabbit insect etc.
(ii)
Carnivores (meat eaters): They feed on other consumers. If they feed on
herbivores they are called as secondary consumers and if they feed on
other carnivores (snake, big fish etc) they are known as tertiary consumers.
(iii) Omnivores: They feed on both plants and animals e.g. rat, fox humans
etc.
(iv)
Detritivores: They feed on the parts of dead organisms, wastes of living
organisms and partially decomposed matter. They are also known as
detritus feeders e.g. beetles termites, ants, earthworms etc.
(c) Saprotrophs: These are micro consumers popularly known as decomposers
such as bacteria, fungi etc. they derive their energy nutrition by breaking down
the complex organic molecules to simpler organic compounds and ultimately
into inorganic nutrients making them available again to producers thus they
act as a bridge between biotic and abiotic components of Ecosystem.