Functioning
of an Ecosystem
An ecosystem is driven by the flow of
energy and circulation of matter by biogeochemical cycle between the biosphere,
lithosphere, hydrosphere and atmosphere. The solar radiation is the basic input
of energy entering the ecosystem. In the living organisms or biosphere, the
energy (chemical energy) and matter (food), collectively called as food energy,
are transferred via food chain and food web through various trophic levels.
Thus all the elements of an ecosystem are interdependent and integrated. An
ecosystem itself is integrated with other ecosystems and thus they all become
interdependent. In fact every ecosystem is part of the larger ecosystem, until
the larger ecosystem of the earth-the biosphere – is formed. That is why we see
climatic changes happening in one ecosystem affecting the other ecosystems.
Productivity
of Ecosystem
Only a small fraction of sunlight striking
the earth is concerted to chemical energy by primary producers. The rate of
conversion of solar energy into chemical energy (organic matter) by autotrophs
is called productivity of ecosystem. The productivity of ecosystem depends to
two things:-
(i)
The availability of solar
radiation to autotrophic primary producers. The productivity of ecosystems goes
in decreasing from equator to poles as the amount of solar energy received goes
on decreasing from equator to poles. Therefore, plants in tropical areas will
have higher productivity than plants in temperate or tundra region.
(ii)
The efficiency of plants to
concert solar energy into food or chemical energy which is also called as
primary production. It is measured in two ways:
(a)
Gross primary productivity which is
total amount of chemical energy assimilated by the autotrophs from solar
energy.
(b)
Net Primary Productivity is equal to
Gross Primary Productivity minus the energy lost through respiration.
The Respiration
means energy required to keep the metabolism of an organism going on. So
highly mobile organisms as well as higher life forms such as mammals have
higher respiratory rate. Complex plants such as those in tropical rainforests
have higher respiratory rate than simple plants like algae or grasslands. Net
primary productivity represents the usable amount of energy at the first
trophic level, which is made available to higher trophic levels.
(iii)
Other than sunlight, the
efficiency of the plants is also limited by nutrient availability. Generally the limiting nutrients are
nitrogen or phosphorous but most often water
and temperature, together limit
the productivity of an ecosystem. For example, the Polar Regions having less
sunlight and desert regions having less water have lower productivity. Besides
these there are biological factors
like mutualism, parasitism, predation, etc which increase the productivity of
an ecosystem.
Secondary
Productivity: The rate at which ecosystem’s
consumers convert the chemical energy of the food they eat into their own new
biomass in called the secondary productivity.
Energy
Efficiency: The concept of primary and secondary
productivity a gives the idea of energy efficiency. Not all the energy produced
at one trophic level is available for transfer to next trophic level (as stored
food energy) but majority is lost in respiration.
Energy efficiencies or trophic efficiency
refers to the energy transferred to the next level. Tropic efficiencies
generally range from 5 to 20% e. g. plants transfer around 7 – 10% of total
energy produced by them to the herbivores. Herbivores being mobile have higher
metabolic respiration. As a result relative loss of energy increases at
successively higher trophic levels.
A common way of illustrating ecological
efficiency is via pyramids of productivityThere are a few other terms. Biomass
refers to the quantity or weight of living matter per unit area per unit time.
It is represented in terms of dry weight. Biomass is comprised of plants and
animals and therefore it is referred to as plant biomass or animal biomass.
Total plant biomass including above ground
and subsurface plants is called standing crop.
On the basis, the Productivity of ecosystem
refers to the rate of increase of biomass whereas Production is the amount of
biomass of a given unit area at a given time. Mean net primary productivity for
the whole earth is 320 dry gram/m2/ year. Also the productivity of continental
ecosystems is higher than that of marine ecosystems.
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