UNIT – III
ECOSYSTEM
3.1 CONCEPT OF AN ECOSYSTEM
The term ecosystem was coined in 1935 by the Oxford ecologist Arthur Tansley to
encompass the interactions among biotic and abiotic components of the environment at a
given site. The living and non-living components of an ecosystem are known as biotic and
abiotic components, respectively. Ecosystem was defined in its presently accepted form by
Eugene Odum as, “an unit that includes all the organisms, i.e., the community in a given area
interacting with the physical environment so that a flow of energy leads to clearly defined
trophic structure, biotic diversity and material cycles, i.e. exchange of materials between
living and non-living, within the system”.
Smith (1966) has summarized common characteristics of most of the ecosystems as follows:
1. The ecosystem is a major structural and functional unit of ecology.
2. The structure of an ecosystem is related to its species diversity in the sense that
complex ecosystem have high species diversity.
3. The function of ecosystem is related to energy flow and material cycles within and
outside the system.
4. The relative amount of energy needed to maintain an ecosystem depends on its
structure. Complex ecosystems needed less energy to maintain themselves.
5.
Young ecosystems develop and change from less complex to more complex
ecosystems, through the process called succession.
6. Each ecosystem has its own energy budget, which cannot be exceeded.
7. Adaptation to local environmental conditions is the important feature of the biotic
components of an ecosystem, failing which they might perish.
8. The function of every ecosystem involves a series of cycles, e.g. water cycle, nitrogen
cycle, oxygen cycle, etc. these cycles are driven by energy. A continuation or
existence of ecosystem demands exchange of materials/nutrients to and from the
different components.
3.2
TYPES OF ECOSYSTEM
We can classify ecosystems as follows:
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(a) Natural Ecosystems: These ecosystems are capable of operating and maintaining
themselves without any major interference by man. A classification based on their
habitat can further be made.
1. Terrestrial ecosystems – forest, grassland and desert.
2. Aquatic ecosystems – fresh water ecosystem, viz. pond, lake, river and marine
ecosystems, viz. Ocean, sea or estuary.
(b) Artificial Ecosystem: These are maintained by man. These are manipulated by man for
different purposes, eg. croplands, artificial lakes and reservoirs, townships and cities.
3.3
BASIC STRUCTURE OF AN ECOSYSTEM
Every ecosystem has a non-living (abiotic) and living (biotic) components.
3.3.1 ABIOTIC COMPONENTS
Basic inorganic compounds of an organism, habitat or an area like carbon dioxide, water,
nitrogen, calcium, phosphorus, etc. that are involved in the material cycles are collectively
called as abiotic component. The amount of these inorganic substances present at any given
time, in an ecosystem is called as the standing state or standing quality of an ecosystem.
Whereas, organic components, e.g. proteins, amino acids, carbohydrates and lipids that are
synthesized by the biotic counterpart of an ecosystem make the biochemical structure of the
ecosystem. The physical environment, viz. climatic and weather conditions are also included
in the abiotic structure of the ecosystem.
3.3.2 BIOTIC COMPONENTS
From the trophic (nutritional) point of view, an ecosystem has autotrophic (self nourishing)
and a heterotrophic (other nourishing) components.
(a) Autotrophic component (Producers): this component is mainly constituted by
the green plants, algae and all photosynthetic organisms. Chemosynthetic bacteria,
photosynthetic bacteria, algae, grasses, mosses, shrubs, herbs and trees
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manufacture food from simple inorganic substances by fixing energy and are
therefore called as producers.
(b) Heterotrophic component (Consumers): the members of this component cannot
make their own food. They consume the matter built by the producers and are
therefore called as consumers. They may be herbivores, carnivores or omnivores.
Herbivores are called as primary consumers whereas carnivores and omnivores
are called as secondary consumers. Collectively we can cal them as
macroconsumers.
(c) Decomposers: Heterotrophic organisms chiefly bacteria and fungi that breakdown
the complex compounds of dead protoplasm, absorb some of the products and
release simple substances usable by the producers are called as decomposers or
reducers. Collectively we call them as micro consumers.
3.4
FUNCTIONS OF AN ECOSYSTEM
All ecosystems maintain themselves in a characteristic dynamic state. They are kept going by
the energy that flows through their biotic components and by the circulation of materials like
N, C, H2O within and outside the system. Ecological kinship in the final analysis is energy
oriented. Ultimate source of energy is the sun. Solar energy is trapped by the autotrophs, it
moves to heterotrophs producer-consumer, or producer-herbivore-carnivore relationship. It
means that energy is transferred from one trophic level to the other in succession in the form
of a chain called as food chain.
Following are the functional aspects of ecosystem:
1. Energy flow in an ecosystem
2. Food chain, food web and ecological pyramids
3. Biogeochemical cycles
4. Ecological succession
3.4.1 ENERGY FLOW IN AN ECOSYSTEM
Ecosystems maintain themselves by cycling energy and nutrients obtained from external
sources. At the first trophic level, primary producers (plants, algae, and some bacteria) use
solar energy to produce organic plant material through photosynthesis. Herbivores, those
animals that feed solely on plants, make up the second trophic level. Predators that eat
herbivores comprise the third trophic level; if larger predators are present, they represent still
higher trophic levels. Organisms that feed at several trophic levels (for example, grizzly bears
that eat berries and salmon) are classified at the highest of the trophic levels at which they
feed. Decomposers, which include bacteria, fungi, molds, worms, and insects, break down
wastes and dead organisms and return nutrients to the soil.
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On average about 10 percent of net energy production at one trophic level is passed on to the
next level. Processes that reduce the energy transferred between trophic levels include
respiration, growth and reproduction, defecation, and non predatory death (organisms that die
but are not eaten by consumers). The nutritional quality of material that is consumed also
influences how efficiently energy is transferred, because consumers can convert high-quality
food sources into new living tissue more efficiently than low-quality food sources
The low rate of energy transfer between trophic levels makes decomposers generally more
important than producers in terms of energy flow. Decomposers process large amounts of
organic material and return nutrients to the ecosystem in inorganic form, which is then taken
up again by primary producers. Energy is not recycled during decomposition, but rather is
released, mostly as heat.
3.4.1.1 Productivity of an ecosystem: The productivity of an ecosystem refers to the rate of
production, i.e., the amount of organic matter accumulated in a unit time interval.
Productivity is of following types:
(a) Primary productivity: It is defined as the rate at which radiant energy is stored
by photosynthetic and chemosynthetic activity of producers. An ecosystem's gross
primary productivity (GPP) is the total amount of organic matter that it
produces through photosynthesis. Net primary productivity (NPP) describes the
amount of energy that remains available for plant growth after subtracting the
fraction that plants use for respiration. Productivity in land ecosystems generally
rises with temperature up to about 30°C, after which it declines, and is positively
correlated with moisture. On land primary productivity thus is highest in warm,
wet zones in the tropics where tropical forest biomes are located. In contrast,
desert scrub ecosystems have the lowest productivity because their climates are
extremely hot and dry.
(b) Secondary productivity: It refers to the consumers or heterotrophs. These are the
rate of energy stored at consumer level. As consumers only use food material in
their respiration, simply changing the food matter to different tissues by an overall
process, secondary productivity is not classified as gross and net amounts.
Secondary productivity actually keeps on moving from one organism to another,
i.e. remains mobile and does not live in situ like the primary productivity.
(c) Net productivity: This refers to the rate of shortage of organic matter not used by
the heterotrophs (consumers), i.e., equivalent to net primary production minus
consumption by the heterotrophs during the unit time, as a season or year etc. SO,
it is the rate of increase of biomass of the primary producers which has been left
over by the consumers.
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Figure 3.1: Energy flow in ecosystem
The simplest way to describe the flux of energy through ecosystems is as a food chain in
which energy passes from one trophic level to the next, without factoring in more
complex relationships between individual species. Some very simple ecosystems may
consist of a food chain with only a few trophic levels.
3.4.1.2 Y-Shaped Model of Energy Flow: We know that energy flow through grazers can be
called as grazing food chain and the energy flow through detritus consumers as detritus food
chain. Partners of these food chains are so intimately associated that sometimes it is difficult
to determine their relative effect on the breakdown of original primary production.
Herbivores
Predators
Plants
Grazing Food Chain
Sunlight
Detritus Food Chain
Detritus Consumers
Figure 3.2: The Y-shaped energy flow model showing linkage between the grazing and
detritus food chains.
As shown in Figure 3.2 one arm represents the herbivore food chain and the other the detritus
food chain. They are sharply separated. However, under natural conditions, they are not
completely isolated from one another. For instance, small dead animals that were once a part
of grazing food chain become incorporated in detritus food chain like the faeces of grazing
animals. This interdependence when represented in the form of figure resemble the letter ‘Y’
therefore, E.P. Odum (1983) called it a Y-shaped model of energy flow.
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Y-shaped model is more realistic and practical working model than single channel models,
because,
 It confirms to the basic stratified structure of ecosystems,
 It separates the grazing and detritus food chains in both time and space, and
 Microconsumers and macroconsumers differ greatly in size metabolism relations.
3.4.2 FOOD CHAIN, FOOD WEB AND ECOLOGICAL PYRAMIDS
3.4.2.1 Food Chains: A food chain is a series of populations through which food and the
energy contained in it passes in an ecosystem. A food chain is simple if it has only one
trophic level besides the decomposers, eg. Eichhornia in eutrophic pond. A complex food
chain has both producer and consumer trophic levels. Trophic levels are various steps in the
passage of food. There are two main types of food chain:
TYPES OF FOOD CHAINS
Predator or grazing food chain
(i)
Saprophytic or detritus food chain.
Predator or grazing food chain: The grazing food chain begins with the
photosynthetic fixation of light, carbon dioxide, and water by plants (primary
producers) that produce sugars and other organic molecules. Once produced, these
compounds can be used to create the various types of plant tissues. Primary
consumers or herbivores form the second link in the grazing food chain. They gain
their energy by consuming primary producers. Secondary consumers or primary
carnivores, the third link in the chain, gain their energy by consuming herbivores.
Tertiary consumers or secondary carnivores are animals that receive their organic
energy by consuming primary carnivores.
Eg. i) Grass  Cattle  Man
ii) Grass  Rabbit  Fox  Wolf  Tiger
(ii)
Detritus food chain: The detritus food chain differs from the grazing food
chain in several ways:

The organisms making it up are generally smaller (like algae, bacteria, fungi,
insects, & centipedes)

The functional roles of the different organisms do not fall as neatly into
categories like the grazing food chain's trophic levels.

Detrivores live in environments (like the soil) rich in scattered food particles.
As a result, decomposers are less motile than herbivores or carnivores.
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
Decomposers process large amounts of organic matter, converting it back into
its inorganic nutrient form.
Eg. A common terrestrial detritus food chain is:
Detritus  Earthworm  Sparrow  Falcon
Figure 3.3: Grazing food chain
Figure 3.4: Detritus food chain
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3.4.2.2 Food Web: Under natural conditions, the linear arrangement of food chains hardly
occurs & these remains connected interconnected with each other through different types of
organisms. Interlocking pattern of several interlinked food chains is termed as FOOD WEB.
Figure 3.5: Food web in grassland ecosystem
Food web illustrates several alternative pathways. Food webs are very useful in maintaining
stability of an ecosystem. If the number of rabbits in an area decreases, owls are expected to
die of starvation. But due to decrease in the number of rabbits, more grass is left out that
helps to increase the population of rats. Owls now feed upon rats and allow the rabbits to
increase in number. Thus the ecosystem does not get permanently disturbed when food
operates.
The complexity of any food web depends on the diversity of organisms in the system.
Accordingly, it would depend on two main points:
(i) Length of the food chain: Diversity in the organisms based on their food habits
would determine the length of food chain. More diverse the organisms in food habits,
longer would be the food chain.
(ii) Alternatives at different points of consumers in food chain: more the alternatives
more would be the interlocking pattern. In deep oceans, seas etc, where we find
different types of organisms, the food webs are much complex.
3.4.2.3 Ecological Pyramids: The quantitative and the easiest method for studying the
relationship between organisms in an ecosystem and for showing them diagrammatically, is
the ecological pyramid, given by Elton (1927). In these pyramids the lower most trophic
level is formed by the producers, while the topmost trophic level is that of carnivores.
Generally, three types of pyramids are considered:
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ECOLOGICAL PYRAMIDS
Pyramid of Number
Pyramid of Biomass
Pyramid of Energy
(i) Pyramid of numbers: This pyramid illustrates the relationship between the number of
producers, herbivores and carnivores. The organisms of an area are first counted and then
grouped into their trophic levels. We have studied three common ecosystems, viz. forest
ecosystem, grassland ecosystem and pond ecosystem.
 In forest ecosystem, the shape of pyramid is rhomboidal. The producers are
represented by an angle large tree, on which depend several fruit eating birds etc.
Therefore, the number of primary consumers is more than the number of producers.
Thereafter, the number of secondary and tertiary consumers decreases progressively.
 In grassland ecosystem, grasses are producers. The number of consumers decreases
towards the top of the pyramid. The number of primary consumers or herbivores like
rats, rabbits etc, is lesser than the number of grasses. The number of secondary
consumers like lizards, snakes etc. is lesser than the number of primary consumers.
The number of last or tertiary consumers is still less than the number of secondary
consumers. So, we see that the number of organisms falls progressively from the first
trophic level to the last trophic level. Therefore, pyramid of number in grassland is
straight or upright.
 In pond ecosystem, the number of organism decreases progressively from the first
trophic level to the last trophic level. Therefore, the pyramid of number in pond
ecosystem is straight upright.
Figure 3.6: Upright Pyramid of Numbers (A) Grassland Ecosystem (B) Pond Ecosystem
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Figure 3.7: Pyramid of number in Forest Ecosystem
(ii) Pyramid of Biomass: The total mass of organisms is called biomass. It can be
determined in terms of net mass, dry mass or ash free dry weight. The biomass at the time of
sampling is called standing biomass or standing crop biomass.
In forest ecosystem and grassland ecosystem, the pyramid of biomass is upright. The
amount of biomass continues to decrease progressively from the first trophic level of
producers to the last trophic level of carnivores.
In pond ecosystem, the number of producers is large, but their biomass is the least of all,
being very small in size. The amount of biomass continues to increase progressively with
primary, secondary and tertiary trophic levels. Therefore the pyramid of biomass in pond
ecosystem is inverted.
Figure 3.8: Inverted Pyramid of Biomass in Aquatic Ecosystem
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Figure 3.9: Upright Pyramid of Biomass for Grassland Ecosystem
(iii) Pyramid of Energy: The most ideal and fundamental method of representing the
relationships between organisms in different trophic levels is the pyramid of energy.
We know that in every ecosystem, only producers possess the capacity to use the energy from
the sun and convert it into food. The energy in the form of food gets transferred from one
trophic level to another. Therefore the flow of energy is always uni-directional. The amount
of energy that reaches the net trophic level is lesser than it was present in the earlier trophic
level. Thus, the amount of energy decreases with each successive higher trophic level.
Therefore, in all types of ecosystem, such a pyramid would be upright.
Figure 3.10: Upright Pyramid of Energy
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3.4.3 BIOGEOCHEMICAL CYCLING
The transport and transformation of substances in the environment, through life, air, sea, land,
and ice, are known collectively as biogeochemical cycles. These global cycles include the
circulation of certain elements, or nutrients, upon which life and the earth's climate depend.
3.4.3.1 Carbon Cycle - the movement of carbon, in its many forms, between the biosphere,
atmosphere, oceans, and geosphere.

Plants obtain carbon dioxide from the air and, through photosynthesis, incorporate
carbon into their tissues

Producers & consumers - transform part of the carbon in their food back into
carbon dioxide via respiration

Decomposers - release the carbon tied up in dead plants & animals into the
atmosphere

Another major exchange of carbon dioxide occurs between the oceans and the
atmosphere. The dissolved CO2 in the oceans is used by marine biota in
photosynthesis.

Two other important processes are fossil fuel burning and changing land use. In
fossil fuel burning, coal, oil, natural gas, and gasoline are consumed by industry,
power plants, and automobiles. Changing land use is a broad term which
encompasses a host of essentially human activities, including agriculture,
deforestation, and reforestation.
Figure 3.11: Carbon Cycle
The global carbon cycle is out of balance, making rapid global climate change more
likely. Atmospheric CO2 levels are rising rapidly -- currently, they are 25% above where
they stood before the industrial revolution. Carbon dioxide forms when the carbon in
biomass oxidizes as it burns or decays. Many biological processes set in motion by people
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release carbon dioxide. These include burning fossil fuels (coal, oil, & natural gas), slashand-burn agriculture, clearing land for permanent pasture, cropland, or human
settlements, accidental and intentional forest burning, and unsustainable logging and fuel
wood collection. Clearing vegetation cover from a forested hectare releases much of the
carbon in the vegetation to the atmosphere, as well as some of the carbon lodged in the
soil. Logging or sustainable fuel wood collection can also degrade vegetation cover and
result in a net release of carbon.
3.4.3.2 Nitrogen Cycle - Almost all of the nitrogen found in terrestrial ecosystems
originally comes from the atmosphere. Small proportions enter the soil in rainfall or
through the effects of lightning. Most, however, is biochemically fixed within the soil by
specialized micro-organisms like bacteria. Members of the bean family (legumes) and
some other kinds of plants form mutualistic symbiotic relationships with nitrogen fixing
bacterial. In exchange for some nitrogen, the bacteria receive from the plants
carbohydrates and special structures (nodules) in roots where they can exist in a moist
environment. Scientist estimates that biological fixation globally adds approximately 140
million metric tons of nitrogen to ecosystems every year.
Figure 3.12: Nitrogen Cycle
3.4.3.3 Phosphorus Cycle – Phosphorus is the key to energy in living organisms, for it is
phosphorus that moves energy from ATP to another molecule, driving an enzymatic
reaction, or cellular transport. Phosphorus is also the glue that holds DNA together,
binding deoxyribose sugars together, forming the backbone of the DNA molecule.
Phosphorus does the same job in RNA.
Again, the keystones of getting phosphorus into trophic systems are plants. Plants absorb
phosphorous from water and soil into their tissues, tying them to organic molecules. Once
taken up by plants, phosphorus is available for animals when they consume the plants.
When plants and animals die, bacteria decompose their bodies, releasing some of the
phosphorus back into the soil. Once in the soil, phosphorous can be moved 100s to 1,000s
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of miles from were they were released by riding through streams and rivers. So the water
cycle plays a key role of moving phosphorus from ecosystem to ecosystem.
Figure 3.13: Phosphorus Cycle
3.4.4 ECOLOGICAL SUCCESSION
The gradual and continuous replacement of plant and animal species by other species until
eventually the community, as a whole, is replaced by another type of community. It is a
gradual change, and it is the organisms present which bring about this change. It involves the
processes of colonization, establishment, and extinction which act on the participating
species. It occurs in stages, called seral stages that can be recognized by the collection of
species that dominate at that point in the succession.
Succession begins when an area is made partially or completely devoid of vegetation because
of a disturbance. Some common mechanisms of disturbance are fires, wind storms, volcanic
eruptions, logging, climate change, severe flooding, disease, and pest infestation. It stops
when species composition no longer changes with time, and this community is called the
climax community.
3.4.4.1 Types of Succession: The various types of succession have been grouped in different
ways on the basis of different aspects. Some basic types of succession are, however, as
follows:
1. Primary succession: It occurs on an area of newly exposed rock or sand or lava or any
area that has not been occupied previously by a living (biotic) community.
2. Secondary succession – It takes place where a community has been removed, e.g., in a
plowed field or a clear cut forest.
3. Autogenic Succession – After the succession has begun, in most of the cases, it is the
community itself, which as a result of its reactions with the environment modifies its own
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environment and thus causing its own replacement by new communities. This course of
succession is known as autogenic succession.
4. Allogenic Succession – In some cases, however, the replacement of the existing
community is caused largely by any other external condition and not by the existing
organisms. Such a course is referred to as allogenic succession.
On the basis of successive changes in nutritional and energy contents, successions are
sometimes classified as:
1. Autotrophic Succession – It is characterized by early and continued dominance of
autotrophic organisms like green plants. It begins in a predominantly inorganic
environment and the energy flow is maintained indefinitely. There is gradual increase in
the organic matter content supported by energy flow.
2. Heterotrophic Succession – It is characterized by early dominance of heterotrophs,
such as bacteria, actinomycetes, fungi and animals. It begins in a predominantly organic
environment, and there is a progressive decline in the energy content.
3.4.4.2 Ecological Succession Based on Habitat: The following types of succession are
known which are based on the type of habitat:
(i) Hydrosere or hydrarch: This type of succession occurs in water bodies like ponds, lakes,
streams etc.
Succession that occurs in water bodies is called hydrosere. It is a succession occurring in the
aquatic environment. It starts with the colonization of phytoplankton and finally terminates
into a forest. There are about seven stages of hydrosere.
1) Phytoplankton Stage: It is pioneer stages of hydrosere. In this stage many organisms
like bacteria, algae and aquatic plants occur. All these organisms add a large amount of
organic matter death and decay.
2) Submerged Stage: It comes after phytoplankton stage, when a loose layer of mud is
formed on the bottom of pond. Some rooted submerged plants develop.
3) Floating Stage: As water depth reduces the submerged plants give way to a new form
of aquatic vegetation. This may be a cause for disappearance of submerged plants. Later
rapid soil building process reduces the water depth to such n extent that it becomes too
shallow for survival of the floating plants.
4) Amphibious Stage: Due to rapid soil formation ponds and lakes become too shallow
so the habitat is unfit for floating plants. Under these conditions the amphibious plants
appear. These plants live in aquatic as well as in aerial environment.
5) Sedge-Meadow Stage (Marginal mats): Soil formation takes place and this result in
marshy soil, which may be too dry. Important plants of this stage are the member of
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cyperaceae and gramineae. These dry habitats may be totally unfit for hydrophytic plants
and gradually shrubs and small size trees starts appearing.
6) Woodland Stage: In this stage large amount of human, bacteria, fungi and other
accumulate in the soil. All this favors the entry of many trees in the vegetation leading to
the climax Stage.
7) Climax Stage: Hydrosere may change in to climax forest, vegetation. In this stage
herbs and trees are most common. The nature of the climax is dependent upon the climate
of the region. It is a very slow process and requires many years to reach the climax stage.
Figure 3.14: Process of Hydrosere
(ii) Xerosere or xerarch: This type of succession occurs in terrestrial areas with low
moisture, eg. rock, sand etc.
It takes place on the surface, which is extremely dry characterized by deficiency of water and
available nutrients. It starts on a base rock. In such extreme dry environment only those
plants can survive which can resist the extreme dry environment only. The various stages of
Xerosere have been described as follows:
1) Crustose Lichen Stage: The rocks are completely devoid of moisture and nutrients. It
is the pioneers in Xerosere. The important crustose lichens are Rhizocarpus. The Lichens
secrete carbonic acid which helps to corrode and decompose the rock supplementing the
other factors of waethering.
2) Foliose Lichen Stage: Weathering of the rocks and decaying of the crustose lichens
forms the first layer of the soil on the rock surface. Gradually the conditions become
favourable for the existing foliose and fructicose lichens.
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3) Moss Stage: The foliose and fructicose lichens stage is followed by moss stage. As soil
formation takes place on rocks surface, Xerophytic masses grow and become dominant.
Common examples of Xerophytic mosses are Polytrichum, Tortula, Grimmia etc.. This
mat of moss is formed on the soil. As the mat become thicker, it increases the water
holding capacity of soil. Now moss stage is replaced by new herbaceous stage.
4) Herbaceous Stage: Initially certain annual herbs migrate and germinate. The humans
of the soil increases year after year because of the death and decay of annual herb. Slowly
biennials and perennial herbs grow. The more organic matter and nutrients accumulate in
the soil. This makes the habitat more suitable for woody plants.
5) Shrub Stage: More and more soil is formed in the herbaceous stage for the woody
shrubs. The herbs are shaded by the over growing shrubs, decaying herbs and leaves,
twigs of shrubs. These also enrich the soil with humus. The humidity is increased over
such areas. All this favors the growth of large mesophytic trees.
6) Climax Stage: This stage is occupied by large number of trees. The first trees growing
in such areas are relatively small with the increase in water holding capacity of the soil,
these trees disappear and large mesophytic trees develop.
Figure 3.15: Process of Xerarch
(iii) Lithosere: This type of succession starts on a bare rock.
(iv) Halosere: This type succession starts on saline water or soil.
(v) Psammosere: This type of succession starts on a sandy area.
3.4.4.3 Process of Ecological Succession: Every primary succession, irrespective of the bare
area from which it initiates, exhibits the following five steps which follow in succession
stages.
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(i) Nudation: The step involves the development of bare area which may be due to soil
erosion, deposition etc.
(ii) Invasion: The step involves the successful establishment of a species in a bare area.
The species reaches this area from some other region.
(iii) Competition and Co-action: The species occupied new area develops intra and interspecific competition for food and space. The completion between already existing species
and those which have just entered the area, results in the destruction of one of them which
is unsuitable.
(iv) Reaction: The species or the community that has established itself in a new area
affects the environment by modifying light, water, soil etc. This results in the elimination
of the community which then makes way for another community for which the modified
environment is more suitable. The different communities or stages represented by
combination of mosses, herbs, shrubs and trees replacing one another during succession
are known as seral stages, seral communities or developmental stages.
(v) Stabilization: This is the final stage, during the course of succession when a
community attains equilibrium with the climate of an area and becomes comparatively
stable. This final community is known as climax community.
3.5 STRUCTURE OF DIFFERENT ECOSYSTEMS
3.5.1 FOREST ECOSYSTEM (TERRESTRIAL ECOSYSTEM)
A forest is an area with a high density of trees. World’s total land area is 13,076 million
hectares, of which total forests account for about 31% of the world’s land area. In India, the
forest cover is roughly 19% of the total land area. The forest ecosystems are of great concern
from the environmental point of view. It provides numerous environmental services.
Biotic components
The various biotic components, representatives from the three functional groups, of a forest
ecosystem are:
1) Producer Organisms: In a forest, the producers are mainly trees. Trees are of different
kinds depending upon the type of forest developed in that climate. Dominant species of trees
in major types of forest ecosystems are Tectona grandis, Acer, Betula, Picea, Pine, Cedrus.
2) Consumers: In a forest, consumers are of three main types:
a) Primary Consumers - These are Herbivores which feed directly on producers. Eg.
Ants, Beetles, Bugs, spiders etc. feeding on tree leaves. Larger animals such as Elephants,
Deer, giraffe etc. grazing on shoots and/or fruits of trees.
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b) Secondary Consumers - These are carnivores and feed on primary consumers. Eg:
Birds, Lizards, Frogs, Snakes and Foxes.
c) Tertiary Consumers - These are secondary carnivores and feed on secondary
consumers. These include top carnivores like Lion, Tiger.
3) Decomposers: These include wide variety of saprotrophic micro- organism like; Bacteria
(Bacillus Sp., Clostridium sp., pseudomonas), Fungi (Aspergillus sp., Ganoderma sp.,
Fusarium) and Actinomycetes (Streptomyces). They attack the dead or decayed bodies of
organisms & thus decomposition takes place. Therefore, nutrients are released for reuse.
Abiotic components
These include basic inorganic & organic compounds present in the soil & atmosphere. In
addition dead organic debris is also found littered in forests.
3.5.2 GRASSLAND ECOSYSTEM (TERRESTRIAL ECOSYSTEM)
Grasslands (also called Greenswards) are areas where the vegetation is dominated by grasses
and other herbaceous (non-woody) plants. Grasslands occupy about 24% of the earth’s
surface. They occur in regions too dry for forests and too moist for deserts. The annual
rainfall ranges between 25- 75 cm, usually seasonal. The principal grasslands includes
Prairies (Canada, USA), Pampas (South America),Steppes (Europe & Asia), Veldts (Africa).
The highest abundance & greatest diversity of large mammals are found in these ecosystems.
The dominant animal species include wild horses, asses & antelope of Eurasia, herds of Bison
of America; and the antelope & other large herbivores of Africa.
Biotic components
1) Producer Organisms: In grassland, producers are mainly grasses; though, a few herbs &
shrubs also contribute to primary production of biomass. Some of the most common species
of grasses are: Brachiaria sp., Cynodon sp., Desmodium sp., Digitaria sp.
2) Consumers: In grassland, consumers are of three main types;
a) Primary Consumers - The primary consumers are herbivores feeding directly on
grasses. These are grazing animals such as Cows, Buffaloes, Sheep, Goats, Deer, Rabbits
etc. Besides them, numerous species of insects, termites, etc are also present.
b) Secondary Consumers - These are carnivores that feed on primary consumers
(Herbivores). These include;-Frogs, Snakes, Lizards, Birds, Foxes, Jackals etc.
c) Tertiary Consumers - These include hawks etc. which feed on secondary consumers.
3) Decomposers: These include wide variety of saprotrophic micro- organism like: Bacteria;
Fungi; Actinomycetes.
Abiotic components
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These include basic inorganic & organic compounds present in the soil & aerial environment.
The essential elements like C, H, N, O, P, S etc. are supplied by water, nitrogen, nitrates,
sulphates, phosphates present in soil & atmosphere.
3.5.3 DESERT ECOSYSTEM (TERRESTRIAL ECOSYSTEM)
A desert is a landscape or region that receives almost no precipitation. Deserts are defined as
areas with an average annual precipitation of less than 250 millimetres per year. It occupies
about 17% of the earth’s surface. Deserts are characterized by hot days & cold nights. The
deserts of the world are mainly located in the South- western United States, Mexico, North
America, Asia (Thar, Gobi, Tibet) & west Asia. Deserts are characterized by scanty flora &
fauna. Soils of deserts often have abundant nutrients but little or no organic matter.
Biotic components
1) Producer Organisms: In a desert, producers are mainly shrubs/bushes; some grasses & a
few trees. Dominant plant species include: Succulents (water - retaining plants adapted to arid
climate or soil conditions) & hardy grasses. Besides some lower plants such as lichens &
xerophytic mosses are also present.
2) Consumer Organisms: These include animals such as insects, reptiles which are capable
of living in xeric conditions. Besides some nocturnal rodents, birds & some mammalians like
camel etc are also found.
3) Decomposers: Due to poor vegetation with very low amount of dead organic matter,
decomposers are poor in desert ecosystem. The common decomposers are some bacteria &
fungi, most of which are thermophillic.
Abiotic components
Due to high temperature & very low rainfall, the organic substances are poorly present in the
soil.
3.5.4 AQUATIC ECOSYSTEMS
Aquatic ecosystems deal with biotic community present in water bodies. In terrestrial
ecosystem, carbon dioxide & oxygen are present in gaseous form whereas in aquatic
ecosystem, these are available in dissolved state. Depending upon the quality and nature of
water, the aquatic ecosystem is categorized into:
i.
Freshwater Ecosystem and
ii.
Marine Ecosystem.
3.5.4.1 FRESHWATER ECOSYSTEMS:
Freshwater ecosystems cover 0.8% of the Earth's surface and contain 0.009% of its total
water. They contain 41% of the world's known fish species. Aquatic ecosystems perform
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many important environmental functions. For example: They recycle nutrients, purify water,
attenuate floods, recharge ground water and provide habitats for wildlife. Aquatic ecosystems
are also used for human recreation, and are very important to the tourism industry, especially
in coastal region. There are three basic types of freshwater ecosystems:
i.
Lentic: slow-moving water, including Pools, Ponds, and Lakes.
ii.
Lotic: rapidly-moving water, for example Streams and Rivers.
iii.
Wetlands: areas where the soil is saturated with water or inundated for at least part of
the time
Lakes & Pond Ecosystem: A pond is a place where living organisms not only live but
interact with biotic & abiotic components. Ponds are often exposed to tremendous
anthropogenic pressure which significantly affects the system. Lakes are usually big standing
freshwater bodies. They have a shallow water zone called Littoral zone; an open water zone
where effective penetration of solar light takes place, called limnetic zone and a deep water
zone where light penetration is negligible, called Profoundal zone.
Biotic components
1) Producer Organisms: It includes submerged, free floating and amphibious macrophytes
(like; Hydrilla, Utricularia, Wolfia, Azolla, Typha etc.) and minute floating and suspended
lower phytoplanktons (like; Ulothrix, Spirogyra, Oedogonium etc.)
2) Consumer Organisms
a) Primary consumers - These are zooplanktons (ciliates, flagellates, other protozoan,
and small crustaceans) and benthos.
b) Secondary consumers - These are carnivores like insects and fishes feeding on
herbivores
c) Tertiary consumers - These are the large fishes feeding on small fishes.
3) Decomposers: Micro – organisms like bacteria, fungi and actinomyctes.
Abiotic component
These are the inorganic as well as organic substances present in the bottom soil or dissolved
in water. In addition, to the minerals, some dead organic matter is also present.
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Figure 3.16: Zonation in a lake ecosystem
3.5.4.2 MARINE ECOSYSTEMS:
Marine ecosystems are among the Earth's aquatic ecosystems. They include: oceans, estuaries
and lagoons, mangroves and coral reefs, the deep sea and the sea floor. These are the gigantic
reservoirs of water covering approximately 71% of the Earth's surface (an area of some 361
million square kilometers). These ecosystems are different from freshwater ecosystem mainly
because of its salty water. The salt concentration in an open sea is usually 3.5% (35 parts per
thousand (ppt)). Dominant ions are sodium & chloride. Average temperature of Marine
ecosystem is 2-3 degree centigrade, devoid of light.
Biotic components
1) Producers: It includes phytoplanktons (diatoms, dinoflagillates), large seaweeds (mainly
algae like chlorophyceae, phaeophyceae & rhodophyceae; angiosperms like Ruppia, Zostera,
posidonia ), and mangrove vegetation (like Rhizophora, Carapa etc.)
2) Consumers:
a) Primary consumers - These are herbivores and feed directly on producers
(Crustaceans, Mollusks, fish etc.)
b) Secondary consumers - These are carnivorous fishes (Herring, Sahd and Mackerel)
c) Tertiary consumers - These are top carnivorous fishes (Cod, Haddock, etc.)
3) Decomposers These are micro – organisms like bacteria, fungi
Abiotic components
High Na, Ca, Mg and K salt concentration, variable dissolved oxygen content, light &
temperature make a unique physiochemical conditions in marine water.
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Figure 3.17: Zonation in an oceanic ecosystem
Case study: Threats to wetlands in Assam
Almost 40% of all wetlands in Assam are under threat. A survey conducted by the
Assam Remote Sensing Application Centre (ARSAC), Guwahati, and the Space
Research Centre, Ahmadabad, has revealed that 1367 out of 3513 wetlands in Assam
are under severe threat due to the invasion of aquatic weeds and several developmental
activities. The wetlands of Assam form the greatest potential source of income for the
state in terms of fisheries and tourism. Through the wetlands of Assam have the
capacity of producing 5,000 t/ha/yr of fish, around 20,000 t of fish have to be imported
to meet local demands. This is primarily due to poor wetland management.
QUENTIONS
1.
Define an ecosystem.
2.
What is the structure of an ecosystem?
3.
Explain the function of the ecosystem.
4.
Explain the following:
(a) Producers
(b) Consumers
(c) Decomposers
(d) Food Chain
(e) Food Web
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(f) Ecological Pyramids
5.
What is ecological succession? Explain.
6.
Discuss energy flow in an ecosystem.
7.
What is ecosystem energetic? Describe the energy flow in a typical ecosystem.
8.
Mention the differences between grazing and detritus food chains.
9.
Discuss Y-shaped energy flow model.
10. Describe features and structure of the following ecosystem:
(a) Grassland ecosystem
(b) Pond ecosystem
(c) Desert ecosystem
(d) Ocean ecosystem
(e) Forest ecosystem
11. What is ‘ecological succession’? Describe the causes and basic types of ecological
succession.
12. Define food web. Depict diagrammatically a terrestrial food web and oceanic food
web.
13. ‘The flow of energy is one way and continuous in an ecosystem’. Jutify.
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