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COURSE NAME: ENVIRONMENTAL STUDIES FOR
UNDERGRADUATES FOR ALL COURSES
PAPER TITLE: ENVIRONMENTAL STUDIES FOR
UNDERGRADUATES
UNIT: 3 – ECOSYSTEM
TOPIC: ECOLOGICAL SUCCESSION, FOOD CHAINS, FOOD WEBS
AND ECOLOGICAL PYRAMIDS
OBJECTIVE: By the end of the session, the students will be able to know:Objective – By The end of the session students will be able to 1. Know about concept of Ecological Succession.
2. Learn about causes of Ecological Succession.
3. Understand about types of Ecological Succession.
4. Get information about process of Succession
5. Get details of Hydrosere and Xerosere.
6. Know about Food Chain
7. Know about Food Web
8. Know about Ecological Pyramids
Introduction & Causes of Ecological Succession
Nature is dynamic, an ever changing complex. Many observations of natural
changes long ago have resulted in the concept of Ecological Succession. Bare land
slowly becomes covered with vegetation and its associated animal life. The
vegetation that initially develops in the bare land is gradually replaced in the course
of time by a second type, which crowds out the first. But even the second type of
vegetation is replaced by the third and so on. In the areas where these stages of
replacement occur, it has been found that communities of living organisms (plants
& animals) replace one another in a very systematic, predictable and orderly
sequence. This process of community change is known as Ecological Succession. In
other words it can also be referred as “Development of different communities of
living organisms one after the other over the same area in the course of time.”
The term ‘Ecological Succession’ was first of all used by Hult (1855) during the study
of a community of southern Sweden. A clear cut concept of Succession was
developed by the botanists Warming (1896) and Cowles (1899). In India studies of
Ecological Succession have been made by ecologists like Mishra, Rangnathan,
Champion, Suri, Pandey and Sharma.
Important causes of Ecological Succession
Important causes for succession include Climatic, Topographic and Biotic.
Fluctuating or unfavorable climatic conditions cause partial or total destruction of
the vegetation and as a result bare area develops. Drought, heavy snowfall and
lightening are some of the important factors of climate resulting in destruction of
vegetation. Among the topographic causes erosion of soil and deposition of soil are
important factors.
Sometimes upper fertile soil is removed by the action of wind and water. This is
called soil erosion. As a result of soil erosion bare area is exposed where succession
starts.
Soil deposition results through heavy storms, glaciers, snowfall and landslides. If
deposition of soil takes place over an area covered with vegetation, the plants
occurring there may be destroyed or suppressed. This soil deposition results in a
new bare area on which succession initiates.
Many biological agencies also affect the vegetation in many ways. Grazing, Cutting,
Cultivation, Harvesting and Deforestation, all are caused by biological or living
agencies and directly responsible for either vegetation change or destruction.
Many parasitic plants and animals also destroy the vegetation; as a result bare area
is formed.
Types & Process of Ecological Succession
Types of Ecological Succession
Succession occurring over a bare area which was not previously occupied by
vegetation is called Primary Succession. Succession occurring on rocky surfaces,
sandy layer and in ponds and lakes are common examples of Primary Succession.
However, when succession occurs over an area initially colonized by vegetation but
destroyed due to some reason it is called Secondary Succession. Succession takes
place on abandoned farmland, strip mines, roadsides and landfill areas represent
Secondary Succession.
Succession characterized by early and continued dominance of autotrophic
organisms is called by Autotrophic, but when Heterotrophic organisms are
dominating then it is referred to as Heterotrophic type.
In succession, when organisms react with the environment and change it, causing
its own replacement by new communities it is called Autogenic Succession.
However, when the replacement of existing community is caused by external
conditions and not by the existing community, then it is named as Allogenic.
Process of Succession
The process of Ecological succession involves the sequential stages called
Nudation, Invasion, Competition and reaction and stabilization or climax.
Nudation is the basic process of succession and here formation of bare area takes
place. There are many reasons responsible for bare area formation like landslides,
soil erosion, flooding, long and continuous drought, volcanic eruption, deposition,
fire and diseases.
During invasion, plants and animals from surrounding areas reach the bare area,
establish there and aggregate by the way of reproduction. The seeds, spores and
other Propogules of organisms reach the bare area by agency of air, water and
animals and the whole process is called Migration or Dispersal. Migrated plant
species try to establish in new area by the way of germination of seed, seedling
growth, vegetative growth, flowering and fruiting. Plant species that are successful
in fruiting establish there, while unsuccessful species disappear.
The successful establishment of species in bare areas is called as Ecesis. Ecesis
involves germination, growth and reproduction. After successful establishment,
species increase their number by reproduction forming large population and the
phenomenon is called Aggregation.
When population increases in limited bare area, the competition or struggle for
nutrients and space starts among them. The struggle may be intra or inter species.
Due to struggle and other type of abiotic and biotic interactions, the environment is
modified and gradually becomes unsuitable to the existing community, which
sooner or later starts getting replaced by a new community. In modified
environmental conditions more suitable species enter the area and compete with
the previously present species. Due to the new suitable species, previously present
species either become sub-dominant or are eliminated.
The addition of organic matter, moisture and minerals by small plants in the area,
again makes it suitable for large plant species. Increased availability of food
material allows various carnivore animals and birds to join the community. In this
way various communities develop successively, called ‘serial communities’, till a
final stable community is developed, known as climax community. Ecologists are of
the opinions that climax community is relatively more stable.
The rate of succession changes is rapid or fast initially and gradually becomes slow.
The process and early appearing species have a short life cycle while species of
later stages have long life span.
Ecological Succession: Hydrosere
Types of Succession:
A. Hydrosere: Succession in the Hydrosere has the following stages:
(i)
Phyto-plankton Stage: Depth of water is 15-20 inches. Algae like
chlormydomorous and spirogyra are common.
(ii)
Submerged Stage: Depth of water is almost 10 inches. Common plants are
hydrilla, vallisneria are to be seen. When they die the depth decreases due
to their debris getting deposited at the bottom.
(iii)
Floating Stage: Depth of water is 2-5 inches. Plants like pistia and
eichhorata grow here. On their death and deposition at the bottom of the
lake, the depth decreases, making way for another stage, i.e. reed swamp
stage or amphibious stage
(iv)
Reed Swamp or Ampbibious Stage: In the reed swamp or amphibious
stage, depth of water is upto 2 inches. Common plants are typha,
phragmites. When they die they are succeeded by marsh meadow.
(v)
Marsh Meadow Stage: Marsh meadow stage occurs when water is
hardly1 to 2 inches deep. Here the soil is marshy and is invaded by many
plant species like Juncus, Carex, Polygonum and Eleocharis. All these plants
react with the habitat and make it suitable for forthcoming Woodland
stage.
(vi)
Woodland Stage: Woodland stage has shrubs and trees. Those plants that
can tolerate water logged condition invade here and survive. Examples of
plant species present at this stage are Salix, Cornus, Cephalanthus, Alnus
and Populus. These plants by their reaction make the soil unsuitable for
woodland plants themselves and suitable for forest trees and shrubs.
(vii) Climax Stage: The moist soil of the woodland stage, enriched with
Bacteria, Fungi and other Organisms, is invaded by forest trees and shrubs,
forming the Climax stage. After a few generations only the most tolerant
species survive and gradually pure forest is established. Thus, the area
once covered by deep water is now changed into a forest.
During the Hydrosere the changing plant stages also see simultaneous changes in
animal types. Protozoans like Paramaecium, Euglenas and Amoeba form the
pioneer stage. Abundant plankton growth support Blue gill fish (Lepomis) & sun
fish (Micropterus).
With submerged vegetation dragonflies and few crustaceans inhabit the plants.
Hydra, gill breathing snails, frogs, diving beetles, whirling beetles and other insects
which utilize the undersurface of leaves, associate with floating stage. Different
kinds of Turtles and snakes are also found with this stage.
Animals that can live with emergent plants appear with Reed swamp stage.
Different species of many flies and dragonflies spend their nymphal stage on
submerged plant stem and climb to the surface when they are ready to emerge as
adult. Gill breathing organisms are replaced by lung breathing organisms. Among
insects, water scorpion, giant water bug and scavenger beetles are also present.
Amnelid worms colonize the bottom. Some birds also become common to the area.
In marsh meadow stage the pond becomes a temporary pond which may dry up in
summer. So, only those animals are present which can withstand drying in summer
and freezing in winter. Like snails, beavers and flashes are gradually excluded.
Plants of marsh meadow stage are replaced by woody forest and accordingly there
is a change in animal life also. Terrestrial forms now occupy the habitat.
Thus, a lake gets converted to forest through succession. That is why there is a
popular saying, “A lake begins to die the moment it is born”.
Ecological Succession: Xerosere
B. Xerosere
When succession begins on a dry place where there is extreme shortage of water it
is called xerarch and with different stages of development it is referred as Xerosere.
Xerosere is completed with pioneer lichen, foliose lichen, moss, herbaceous plants,
shrub and climax stages.
Over rocky surface, where most conditions are unsuitable for the growth of most
plants, few blue green algae and crustose lichens inhabit forming Pioneer stage.
Blue green algae like Scytonema are found to adhere to the rocky surface by their
mucilaginous sheath. The algae tolerate all extreme conditions and utilize
atmospheric Nitrogen. Usually at high altitude where climate is cooler, crustose
lichens like Rhizocarpon and Rinodina are common pioneers.
Lichens produce some acids which cause weathering of rocks. Weathering of rocks
form small particles, which mix with dead remains of algae and lichen, forming a
thin moist layer on the rocky surface. Over this moist layer foliose lichens can grow
successfully constituting second stage. As soon as little soil is accumulated on rocky
surface some foliose lichens like Dermatocarpon, Parmelia and Umbilicaria grow
there. Foliose lichens slowly replace the crustose lichens by making an overshadow
over crustose lichen by their thalli.
By death & decay of Thalli of foliose lichens, weathering of rock and collection of
dust particles by wind, more soil is formed over which moss stage occurs. Mostly
Xerophytic mosses like Polytrichum, Tortula and Grimmia make appearance.
Sometimes foliose lichen and mosses exist together but later on mosses completely
replace the foliose lichens. Mosses also increase the amount of soil, and by their
death and decay a mat like cover is formed over rocky surface. This cover can hold
sufficient amount of water and with soil particles it is suitable for growth and
development of herbaceous plants. Under Herbaceous stage, annual, biennial and
perennial herbs make their appearance like Poa and Festuca.
Process of rock disintegration and accumulation of humus and nutrients are greatly
accelerated. Herbaceous plants, now shaded, tend to disappear and these plants
are replaced by shrubs plants of shrub stage. These shrubs may start from seeds or
invade from surrounding area by rhizomes. These make the conditions unsuitable
for herbs and over shadow them.
Shade of shrubby plants also controls the evaporation of water from soil and wind
movement. Humus and Humidity increases, which favors the growth of tree
seedlings that form climax stage. First some xeric species of tree appear which later
on give place to mesophytic tree species. These mesophytic tree species increase
both in number and vigour and ultimately a forest is formed.
Food Chains & Food Webs
The Ecosystem consists of two partsA- Abiotic or Non Living Part – Including Soil, Atmosphere, Climate, Water and
Natural forces.
B- Biotic-Part or Living Part – Including Organisms – Plants and Animals
In the living part of ecosystem all organisms have different feeding status,
known as Trophic Levels. There are five trophic-levels.
Trophic level 1 – It includes all green plants.
Trophic level 2 – Includes all types of Herbivores.
Trophic level 3 –Includes carnivores which eat herbivores
Trophic level 4 –Carnivores which eat carnivores of lower tropic level, hence
they are called top carnivores
Trophic level 5 – All decomposers constitute this level
Arrangement of all trophic levels is represented by Food Chains, Food Web,
and Ecological Pyramids.
Food Chain:Food chain is the linear arrangement of Trophic-levels, where a series of organisms
pass energy from one to another, as each provides food to the next level.
Food Chains are classified into two typesHerbivorous grazing chain & Detritus grazing chain
A. Herbivorous grazing Chain – It always starts with green plants where whole plant
or plant part is used as food. Different types of habitats have different types of
organisms, so the food chain is also of various types. Forest, Crop-lands, Grasslands, Deserts, Rivers, Lakes and Oceans, all have different types of food chains.
Some of the common examples areForest –
Ground Plants feed the Deer, which in turn are food for the Fox. The foxes are
eaten by Lions.
Ground Plants are eaten by Rabbits. Rabbits are food for the Jackal. The jackal may
be eaten by Lions.
Croplands –
Crop Plants are consumed by Rodents, which are eaten by Snakes. In turn, snakes
may be eaten by various kinds of birds like a Peacock, a Kite or the Hawk.
Crop Plants are eaten by Insects, which, in turn, are food for the Frog. The frog is
eaten by the Snake which in turn is eaten by Birds.
Grass- Lands
Grass Plants are eaten by Grasshoppers, which may be eaten by a bird.
Grass Plants are eaten by Grasshoppers, which may be eaten by a Lizard, and the
lizard may be food for the Hawk.
Grass Plants are eaten by Rodents. Rodents are eaten by Snakes. Snakes may be
eaten by the Peafowl.
Grass Plants are eaten by Rabbits, which, in turn, may be eaten by a Jackal or
aWolf.
Rivers and Streams
Smaller Water-Plants are eaten by Herbivorous Fish. The fish are eaten by
Carnivorous Fish. Carnivorous fish are food for the Duck.
B. Detritus Grazing Chain – The Detritus grazing chain always starts with dead
organic matter or decaying matter.
Dead organic matter is consumed by Earthworms, which, in turn, are eaten by
Frogs. The frogs are food for the Snake, which, in turn, is food for the Pea Fowl.
Food-Web
In the ecosystem, food chains are interconnected with each other forming
interlocking patterns. Food Web is a net or Web-like trophic inter-relationship.
Food web provides stability to the Ecosystem as the number of organisms is
controlled through a network in which one organism consumes the other.
Ecological Pyramids
Ecological pyramid is the graphic or diagrammatic representation of different
trophic levels with autotrophs or green plants at the base and herbivores and
carnivores being placed above them. Ecological pyramid is represented in terms of
numbers, biomass and energy. Hence the various types of Ecological Pyramids are:
01. Pyramid of Numbers
02. Pyramid of Biomass
03. Pyramid of Energy
01. Pyramid of Numbers – The pyramid of numbers shows the relationship between
the primary producers, Herbivores and Carnivores at successive trophic-levels in
terms of their numbers. These pyramids are upright OR Inverted.
In grasslands there are maximum numbers of grass plants but successively
numbers of herbivores and carnivores decreases, hence pyramid is upright. In
Forest the pyramid of numbers is rhomboid shaped, but in tree ecosystem 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 provides nutrition to several
parasites. For example, a tree provides nutrition to several parrots and each parrot,
in turn, provides nutrition to lice or other parasites.
02. Pyramids of Biomass – Pyramids of biomass are more fundamental, showing a
quantitative relationship of the Biomass in producers, herbivores and carnivores. In
forest and grasslands there is a gradual decrease in biomass at successive levels
from producers to top carnivores. So, in forest and grasslands pyramids are upright.
In case of pond, producers are very small organisms with lower value of biomass,
but this value gradually increases making the pyramid inverted.
03. Pyramid of Energy – The pyramid of energy is always upright because there is
gradual decrease in energy content at successive trophic levels from the producers
to consumers. The pyramid of energy represents the total quantity of energy used
by different trophic levels of organisms of an ecosystem per unit area over a set
period of time.
Conclusion:
In this way, ecological succession takes place in the ecology and food chains, food
webs and ecological pyramids play their role in maintaining a balance in the
ecosystem. Each organism is interdependent with the other, thus forming an
essential link in the natural cycle. The absence of a single link in the cycle will
destroy the cycle and thus the ecology, leading to total chaos and destruction of
the ecology. It is, therefore, essential to understand the important role that each
organism plays and respect them for the role that they play in the entire natural
cycle.