Monday, April 30, 2012

7. Ecosystem

An ecosystem is a functional unit of nature, where living organisms interact among themselves and also with the surrounding physical environment.
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.
-    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.
-    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).
-    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.
-    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.
-    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 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
-    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.
-    The cost of climate regulation and habitat for wildlife are about 6 % each.

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