It was Ernst Haeckel who
introduced the term Ecology to us. There were many scientists who were in
interest of giving the theories and putting forth the concepts of ecology in
18th and 19th century. Different types of ecosystem of nature, constituting the
giant ecosystem – the biosphere. These may be artificially categorized as
follows:
Natural ecosystem
These operate by themselves under
natural conditions without any major interference by man. Based upon the
particular kind of habitat, these are further divided as:
1. Terrestrial, as forest, grassland, desert etc.
2. Aquatic (water-open), may be further distinguished as:
(a) Freshwater, which may be
lotic (running- water as spring, stream,
or rivers) or lentic (standing water as lake, pond, pools, puddles, puddles, ditch, swamp etc.).
(b) Marine, such deep bodies as an ocean or shallow ones as a sea
or estuary etc.
Artificial (man-engineered) ecosystems
These are maintained artificially
by man where, by addition of energy and planned manipulations, natural balance
is disturbed regularly. For example croplands like maize, wheat, rice-fields
etc. where man tries to control the biotic community as well as the physiso
–chemical environment, are artificial ecosystem.
In addition to above the rapid
progress made during recent years led to the recognition of some other such
types of ecosystem as space ecosystem etc.
Structure and Function of as Ecosystem
The two major aspects of an
ecosystem are, the structure and function. By structure we mean
(1) The composition of biological
community including species, numbers, biomass, life history and distribution in
space etc.
(2) The quantity and distribution
of the non-living materials, such as nutrients, water etc. and
(3) The range or gradient of
condition of existence, such as temperature, light etc.
By function we mean
(1) The rate of biological energy
flow i.e., the production and respiration rates of the community,
(2) Rate of materials or nutrient
cycles, and
(3) Biological or ecological
regulation of organisms by environment (photoperiodic etc.) and regulation of
environment by the organism,(nitrogen fixing organisms etc.) Thus in any
ecosystem, structure and function are studied together.
STRUCTURE OF ECOSYSTEM
An ecosystem has two major
components:
Abiotic (non-living) component
It includes
1 The
amount of inorganic substances as P,S,C, N, H, etc. involved in material
cycles. The amount of these inorganic substances, present at quality,
2 Amount
and distribution of inorganic chemicals, such as chlorophylls etc. and of
organic materials, as proteins, carbohydrates, lipids etc. present either in
the biomass or in the environment i.e. biochemical structure that link the
biotic and biotic components of the ecosystem,
3 The climate of the given region.
Biotic (living) component
This is needed the trophic
structure of any ecosystem, where living organisms are distinguished on the
basic of their nutritional relationships. From this trophic (nutritional)
standpoint, an ecosystem has two components:
1. Autotrophic component. In which fixations of light energy, use
of simple are inorganic substances and buildup of complex substances
predominate. The component is constituted mainly by green plants, including
photosynthetic bacteria. To some lesser extent, chemosynthetic to the buildup
of organic matter. Members of the autotrophic component are known as Producers.
2. Heterotrophic component. In which utilization, rearrangement and
decomposition of complex materials predominate. The organisms involved are
known as consumers, as they consume the matter built up by the producers
(autographs). The consumers are further categorized as:
(a) Macro consumers- These
are the consumers, which in an order as they occur in a food chain are,
herbivores, carnivores (or omnivores). Herbivores are also known as primary
consumers. Secondary and tertiary consumers, if present, are carnivores or
omnivores. They all are photographs which include chiefly animals that ingest
other organic and particulate organic matter.
(b) Micro consumers-
These are popularly known as decomposers. They are saprotrophs (osmotrophs) and
include chiefly bacteria, actinomycetes and fungi. They breakdown complex
compounds of dead or living protoplasm absorb some of the decomposition or
breakdown products and release inorganic nutrients in environment, making them
available again to autographs.
The biotic component of any ecosystem may be
thought of as the functional kingdom of nature, since they are based on the
type of nutrition and the energy source used. The trophic structure of an
ecosystem is one kind of producer consumer arrangement, which each “food” level
is known as trophic levels or in a component population is known as the
standing crop, a term applicable to both plants as well as animals. The
standing crop may be expressed in terms of
(1) Number of organisms per unit area, or
(2) Biomass i.e. organism mass in unit area, which can be measured
as living weight, dry weight, as-free dry weight, or calories any other
convenient unit suitable for comparative purposes.
Ecological pyramids
Trophic structure, i.e. the
interaction of food chain and the size metabolism relationship between the
linearly arranged various biotic components of an ecosystem is characteristic
of each type of ecosystem. The trophic
structure and function at successive trophic levels, i.e.
produces-herbivores-carnivores, may be shown graphically by means of ecological
pyramids where the first or producer level constitutes the base of the pyramid
and the successive levels, the tiers making the apex. Ecological pyramids are of three general types-
(1) Pyramid of numbers, showing
the number of individual organisms at each level
(2) Pyramid of biomass, showing
the total dry weight and other suitable measure of the total amount of living
matter, and
(3) Pyramid of energy, showing the rate of
energy flow and/or productivity at successive tropic levels. The pyramids of
numbers and biomass may be upright or inverted depending upon the nature of the
food chain in the particular ecosystem, whereas pyramids of energy are always
upright.
Pyramids of numbers- They
show the relationship between producers, herbivores and carnivores and
carnivores at successive tropic levels in terms of their number. The pyramids
of numbers in three different kinds of ecosystems are shown in. This number then shows a decrease towards
apex, as the primary consumers (herbivores) like rabbits, mice etc. are lesser
in number than grasses; the secondary consumers, snakes and lizards are lesser
in number than the rabbits and mice. Finally, the top (tertiary) consumer’s
hawks or other birds are least in number. Thus, the pyramid becomes
upright. Similary, in a pond ecosystem
the pyramid is upright. Here the
producers, which are mainly the phyto-planktons as algae, bacteria etc. are
maximum in number; the herbivores, which are smaller fish; rotifers etc lesser
in number than the producers; and the secondary consumers (carnivores), such as
small fish eating each other, water beetles etc, are lesser in number than the
herbivores. Finally, the top (tertiary) consumers, the bigger fish are lease in
number. In a forest ecosystem, however the pyramid of numbers is somewhat
different in shape. The producers, which are mainly large- sized stress, are
lesser in number, and form the base of the pyramid. The herbivores which are
the fruit- eating birds, elephants, deer’s, etc. are more in number the
producers. Then there is a gradual decrease in the number of successive
carnivores, thus making the pyramid again upright. However, in a parasitic food
chain the pyramids are always inverted.
This is due to the fact that a single plant may support the growth of
many herbivores and each herbivore in turn may provide nutrition to several
parasites, which support many hyper parasites. Thus, from the producer towards
consumers, there is a reverse position, i.e. the number of organisms gradually
shows an increase, making the pyramid inverted inverted in shape.
Actually the pyramids of numbers
do not give a true picture of the food chain as they are not very
functional. They do not indicate the
relative effects of the geometric food chain and size factors of the organisms.
They generally vary with different communities with different types of food
chains in the same environment. It becomes sometimes very difficult to
represent the whole community on the same numerical scale (as in forests).
Pyramids of biomass: They
are comparatively more fundamental as they, instead of geometric factor, show
the quantitative relationships of the standing crops. The pyramids of biomass
in different types of ecosystem are shown in Figure. In grassland and forest there is generally a
gradual decrease in biomass of organisms at successive levels from the
producers to the top carnivores. Thus pyramids are upright. However, In a pond as the producers are small
organism, their biomass is least, and this value gradually shows an increase
towards the apex of the pyramid, thus making the pyramid inverted in shape.
Pyramids of energy: Of the three types of ecological pyramids,
the energy give the best picture of overall nature of the ecosystem. Here,
number and weight of organisms at any level depends not on the amount of fixed
energy present at any one times in the level just below but rather on the rate
at which food is being produced. In contrast with the pyramids of numbers and
biomass, which are pictures of the standing situations (organisms present at
any moment), the pyramids of energy is a picture of the rates of passage of
food mass through the food chain. In shape
it is always upright, as in most of the cases there is always a gradual
decrease in the energy content at successive tropic levels from the producers
to various consumers.
Functional of Ecosystem
The foregoing account presents
the gross structure of an ecosystem of an ecosystem. But for a fuller understanding of nature
functions (particularly rate functions) must also be investigated. Both,
structure and function are best studied together and now we shall consider a
brief outline of gross function of an ecosystem i.e. how an ecosystem works or
operates under natural conditions?
From the operational viewpoint
the living and nonliving and non-living components of ecosystem are so
interwoven into the fabric of nature that their separation from each other
becomes practically very much difficult.
The mode of movement of materials and energy in an ecosystem is shown in
simple model presented in Figure. The
producers, green plants, fix radiant energy and with the help of minerals (C,
H, N, P, L, Ca, Mg, Zn, Fe, etc.) taken from their soil and aerial environment
(nutrient pool) they build up complex organic matter (carbohydrates, fats,
proteins, nucleic acids etc.). Some ecologists prefer to call the green plants
as converters or transducers, since they feel that the term ‘producer’ from
energy viewpoint is somewhat misleading.
Their viewpoint is that green plants produce carbohydrates and not
energy and since they convert to transducer radiant energy into chemical form,
they must be better called the converters or transducers. However, the term
‘producer’ is so widely used that it is preferred to retain it as such. The two
ecological processes of energy flow and mineral cycling, involving interaction
between the psycho-chemical environment and the biotic communities, may be
thought as the “heart” of the ecosystem dynamics. According to the model shown
in Figure energy flows in non-cyclic manner (unidirectional) from sun to the
decomposers via producers and macro consumers( herbivores and carnivores),
whereas the minerals keep on moving in a cyclic manner. The cycling of the
minerals is accomplished by different biogeochemical cycles super-imposed upon
the unidirectional energy flow through the biotic component of the
ecosystem. It would become, however, clear
later that energy not only flows unidirectional but also lost from the system
in several in ways and that minerals too similarly show a net loss in several
ways.
Productivity of ecosystem
The productivity of an ecosystem
refers to the rate of production i.e. the amount of organic matter accumulated
in any unit time. Productivity is of the
following types:
Primary productivity It is associated with the producers which are
autotrophic, most of which are photosynthetic, and to a much lesser extent the
chemosynthetic microorganisms. These are
the green plants, higher macrophytes as well as lower forms, the phytoplankton
and some photosynthetic bacteria. Primary productivity id defined as “the rate
at which radiant energy is stored by photosynthetic and chemosynthetic activity
of activity of producers.” Primary
productivity further distinguished as:
(a) Gross primary productivity It is the total rate of
photosynthesis including the organic matter used up in respiration during the
measurement period. This is also
sometimes referred to as total (gross) photosynthesis or total
assimilation. It depends on the
chlorophyll content. The rates of
primary productivity are estimated in terms of either chlorophyll contents as,
Chl/g dry/weight/unit area, or photosynthetic number i.e. amount of Co2 fixed/g
Chl/hour.
(b)Net primary productivity It is the rate of storage of organic
matter in plant tissues in excess of the respiratory utilization by plants
during the measurement period. This is
thus the rate of increase of biomass and is also known as apparent
photosynthesis or net assimilation.
Thus, net primary productivity refers to balance between gross
photosynthesis and respiration and other plant losses as death etc.
Secondary productivity It refers to the consumers or heterotrophs.
These are the rates of energy storage at consumer’s level. Since consumers only utilize food materials
(already produced) in their respiration, simply converting the food matter to
different tissues by an overall process, secondary productivity is not divided
into ‘gross’ and ‘net’ amounts. Thus,
some ecologists as Odum (1971) prefer to use the term assimilation rather than
production at this level the consumer’s level.
Secondary productivity actually remains mobile (i.e. keeps on moving
from one organism to another) and does not live in situ like the primary
productivity.
Net productivity it refers
to the rate of storage of organic matter not used by the heterotrophs
(consumers) i.e. equivalent to net production minus consumption by the
heterotrophs during the unit period, as a reason or year etc. It is thus the rate of increase of biomass of
the primary producers which has been lift over by the consumers. Net
productivity is generally is expressed as production of Cg/m2/day which may
then be consolidated on month, season or year basis.
ECOLOGICAL FOOD CHAIN
The transfer of food energy from
the producers, through a series of organisms (herbivores to carnivores to
decomposers) with repeated eating and being eaten, is known as a food
chain. Producers utilize the radiant
energy of sun which is transformed to chemical form, ATP during
photosynthesis. Thus green plants
occupy, in any food chain, the first trophic (nutrition) level- the producer’s
level, and are called the primary producers.
The energy, as stored in food matter manufactured by green plants, is
then utilized by the plant eaters- the herbivores which constitute the second
trophic level- the primary consumers level, and are called the primary
consumers(herbivores). Herbivores in turn are eaten by the carnivores, which
constitute the third trophic level the secondary consumers level, and are
called the secondary consumers (carnivores).
These turn may be eaten still by other carnivores at tertiary consumers level
i.e. by the tertiary consumers (carnivores).
Some organisms are omnivores eating the organisms the producers as well
as the carnivores at their lower level in thee food chain. Such organisms thus may occupy more than one
trophic levels in the food chain. This
classification of all the living organisms of any ecosystem is one of their
functions and not of species. Species
that are taxonomically widely different from each other may occupy the same
tropic levels as all have a common function in the food chain. Typha, Nymphaea,
Chara, Volvex, Nostoc, photosynthetic
Vaibhav Rajdeep
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