An ecosystem is a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment.
- The base of each pyramid represents the producers (first trophic level) while the apex represents tertiary or top level consumer.
Ecological pyramids are 3 types:
(a) Pyramid of number
(b) Pyramid of biomass
(c) Pyramid of energy
Types of ecosystems
· Terrestrial ecosystem: Forest, grassland, desert etc
· Aquatic ecosystem: Pond, lake, wetland, river & estuary
·
Man-made ecosystem:
Crop fields and aquarium
The entire biosphere can be regarded as a
global ecosystem.
ECOSYSTEM
– STRUCTURE & FUNCTION
- In an ecosystem, interaction of biotic and
abiotic components occurs. These components function as a unit.
- Vertical distribution of different species
occupying different levels is called stratification. E.g. trees occupy top vertical strata (layer) of a forest,
shrubs the second and herbs and grasses occupy the bottom layers.
4 basic components of functioning of an ecosystem:
i. Productivity ii. Decomposition
iii.
Energy flow iv. Nutrient cycling
Pond (Aquatic ecosystem)
- A pond is a shallow, simple,
self-sustainable water body that exhibits all basic components of an ecosystem.
- Abiotic components in pond: water and the rich soil deposit at the bottom.
Climatic conditions: The solar input, the cycle of temperature, day-length etc.
- Autotrophic components: phytoplankton, some algae and the floating, submerged and
marginal plants.
- Consumers (heterotrophs): zooplankton, free swimming and bottom dwelling forms.
- Decomposers: fungi, bacteria and flagellates.
- Pond performs all the functions of an
ecosystem such as
o Conversion of inorganic into organic
material with the help of the radiant energy of the sun by the autotrophs.
o Consumption of the autotrophs by
heterotrophs.
o Decomposition and mineralization of the
dead matter to release them back for reuse by the autotrophs.
PRODUCTIVITY
- A constant input of solar energy is the
basic requirement for any ecosystem to function and sustain.
- The amount of biomass or organic matter
produced per unit area over a time period by plants during photosynthesis is called primary
production. It is expressed in terms of weight (g–2)
or energy (kcal m–2).
- The rate of biomass production is called productivity. It is expressed in terms of g–2 yr–1 or
(kcal m–2) yr–1.
- It can be divided into gross primary
productivity (GPP) and net primary productivity (NPP).
- Gross
primary productivity: It is the rate of
production of organic matter during photosynthesis. A considerable amount of
GPP is utilized by plants in respiration.
- Gross primary productivity minus
respiration losses (R) is the net primary productivity (NPP), i.e. NPP is the available biomass for the consumption to
heterotrophs (herbivores and decomposers).
NPP = GPP
– R
- Secondary
productivity: It
is the rate of formation of new organic
matter by consumers.
- Primary productivity depends on
o The plant species inhabiting a particular area
o Environmental factors
o Availability of nutrients
o Photosynthetic capacity of plants
Therefore, it varies in different types of
ecosystems.
-
The
annual net primary productivity of the whole biosphere is approximately 170
billion tons (dry weight) of organic matter. Of this, despite occupying
about 70 % of the surface, the productivity of the oceans is only 55 billion
tons. Rest of course, is on land.
DECOMPOSITION
- It is the breakdown of complex organic
matter by decomposers into inorganic substances like carbon dioxide, water and
nutrients.
- It is largely an oxygen-requiring process.
- Detritus (dead plant remains such as leaves, bark, flowers and dead
remains of animals, including fecal matter) is the raw material for
decomposition.
Steps
of decomposition
a. Fragmentation: It is the breakdown of detritus into smaller particles by detritivores
(e.g. earthworm).
b. Leaching: By this process, water soluble inorganic nutrients go down into
the soil horizon and get precipitated as unavailable salts.
c. Catabolism: Degradation of detritus into simpler inorganic substances by bacterial
and fungal enzymes.
Fragmentation, leaching and catabolism operate
simultaneously on the detritus.
d. Humification: Accumulation of humus (dark amorphous
substance) in soil. Humus is
resistant to microbial action and so decomposes very slowly. Being colloidal in
nature it serves as a reservoir of nutrients.
e. Mineralization: It is the release of inorganic nutrients due to the degradation
of humus some microbes.
Factors
influencing decomposition
· Chemical composition of detritus: Decomposition rate is slower if detritus is rich in lignin &
chitin, and quicker, if detritus is rich in nitrogen and water-soluble
substances like sugars.
· Climatic factors like temperature and soil
moisture: Warm and moist environment favour
decomposition whereas low temperature and anaerobiosis inhibit decomposition
resulting in buildup of organic materials.
ENERGY
FLOW
- Sun is the only source of energy for all
ecosystems (except deep sea hydro-thermal ecosystem).
- Of the incident solar radiation less than
50% of it is photosynthetically
active radiation (PAR).
- Plants and photosynthetic &
chemosynthetic bacteria (autotrophs), fix solar radiant energy to make food.
-
Plants capture only 2-10% of the PAR and this small
amount of energy sustains the entire living world. So, it is very important to
know how the solar energy captured by plants flows through different organisms
of an ecosystem.
- Ecosystems obey 2nd Law of thermodynamics.
They need a constant supply of energy to synthesise the molecules they require,
to counteract the universal tendency toward increasing disorderliness.
-
Producers: All organisms are dependent for their food on producers
(green plants), either directly or indirectly. In a terrestrial ecosystem,
major producers are herbaceous and woody plants. Primary producers in an
aquatic ecosystem are phytoplankton, algae and higher plants.
- The energy trapped by the producer is
either passed on to a consumer or the organism dies. Death of organism is the
beginning of the detritus food chain/web.
- Consumers (heterotrophs): These are all animals that depend on plants (directly or indirectly)
for their food. They include:
o Primary consumers (herbivores- feed on plants). E.g. insects, birds and mammals in terrestrial ecosystem and
molluscs in aquatic ecosystem.
o Secondary consumers (primary
carnivores-
feed on herbivores). E.g. frog, fox, man etc.
o Tertiary consumers (secondary carnivores- feed on primary carnivores).
- A simple grazing food chain (GFC) is
depicted below:
- Detritus
food chain (DFC)
begins with dead organic matter. It is made up of decomposers
(saprotrophs) which are heterotrophic organisms. E.g. fungi
& bacteria. They meet their energy and nutrient requirements by degrading dead
organic matter or detritus.
- Decomposers secrete digestive enzymes that
breakdown dead and waste materials into simple, inorganic materials, which are
subsequently absorbed by them.
- In an aquatic ecosystem, GFC is the major
conduit for energy flow.
- In a terrestrial ecosystem, a much larger
fraction of energy flows through the DFC than through the GFC.
- DFC may be connected with GFC at some
levels: some of the organisms of DFC are prey to the GFC animals. Some animals (cockroaches,
crows etc.) are omnivores. These interconnections of food chains make a food web.
- Organisms occupy a place in the natural surroundings or in a community according to their feeding relationship. A specific place of organisms in the food chain is known as their trophic level. Producers belong to the first trophic level, herbivores to the second and carnivores to the third.
- The number of trophic levels in the grazing food chain is restricted as the transfer of energy follows 10 % law – only 10% of the energy is transferred to each trophic level from the lower trophic level. In nature, it is possible to have so many levels – producer, herbivore, primary carnivore, secondary carnivore in the grazing food chain.
- The number of trophic levels in the grazing food chain is restricted as the transfer of energy follows 10 % law – only 10% of the energy is transferred to each trophic level from the lower trophic level. In nature, it is possible to have so many levels – producer, herbivore, primary carnivore, secondary carnivore in the grazing food chain.
ECOLOGICAL PYRAMIDS
- The representation of a food chain in the form of a pyramid is called ecological pyramid. - The base of each pyramid represents the producers (first trophic level) while the apex represents tertiary or top level consumer.
Ecological pyramids are 3 types:
(a) Pyramid of number
(b) Pyramid of biomass
(c) Pyramid of energy
Carbon
cycle
|
Phosphorous cycle
|
Atmospheric input is higher
|
Much smaller
|
There is gaseous exchange b/w organism
& environment
|
Gaseous exchange is negligible
|
Differences
between carbon and phosphorous cycles
ECOSYSTEM
SERVICES
- The products of ecosystem processes are called
ecosystem
services.
- E.g. healthy forest ecosystems purify air
and water, mitigate droughts and floods, cycle nutrients, generate fertile
soils, provide wildlife habitat, maintain biodiversity, pollinate crops,
provide storage site for carbon and provide aesthetic, cultural & spiritual
values.
- Robert Constanza and his colleagues have
tried to put price tags on nature’s life-support services.
- Researchers have put an average price tag
of US $ 33 trillion a year on these fundamental ecosystems services. This is
nearly twice the value of the global gross national product GNP which is (US $
18 trillion).
- Out of the total cost of various ecosystem
services, the soil formation accounts for about 50%.
- Contributions of other services like
recreation & nutrient cycling are less than 10% each.