Succession, Types of succession, Mechanism of succession, Model of succession and hydrarch and xerarch succession

1. Unit-9 (Plant community)
Core course-9
Succession
Succession is a universal process of directional change in the community.
Its composition and structure constantly change in response to environmental condition.
It is orderly and sequential parallel changes with the physical environment.
These changes lead finally to the equilibrium of the community with the environment
called a climax community. Climate community marks the endpoint of succession.
Ecological succession is a gradual and fairly predictable change in the species
composition of a community in a given area.
An intermediate stage in the successional process towards the climax community is
known as seral community and the temporary stage is called the seral stage. More than
one seral stage involves until climate community is attained.
Types of succession
Primary and secondary succession
Section occurring in bare areas such as new volcanic Island, where no community existed
before is called primary succession. In primary succession the unoccupied terrestrial site
is first colonized by a few pioneer species which are often microbes, lichens and mosses.
Soil develops gradually as rocks weather and organic matter accumulates from the
decomposed remains of the early colonizers. Pioneers that initiate primary succession
must be able to establish and grow on substrates that are nutrient to poor and have
unfavourable moisture conditions. Colonization may be limited to cyanobacteria, lichens
and bryophytes, with no further vegetation development. Sites rich in mineral nutrients
are often colonized by herbaceous species and grasses with faster growth rate.
Secondary succession occurs at a site where already developed community has been
destroyed by some natural disturbance such as fire or flooding; or by human activity and

2. a series of community subsequently developed at the site. Hence, secondary succession is
the development of biotic community after the complete or partial destruction of the
existing community. For example, the forest destroyed by fire reoccupied by herbs in the
initial stages. The reappearance and establishment of communities in such area is
secondary succession.
Secondary succession starts on a well developed soil already formed at the site. Hence
this form of succession is relatively faster as compared to primary succession which we
often require hundreds of years. Secondary succession is more common type of
succession as compared to primary succession.
Autogenic and allogenic succession
Autogenic succession is self driven, resulting from the interaction between organisms and
their environment. In this case successional changes are brought about by the organism
themselves.
Allogenic succession occurs as a result of the changes brought about in the habitat by
external agencies and not by the existing community itself.
Autotrophic and heterotrophic succession
The succession where initially the green plants are much greater in quantity than the
animals is known as autotrophic succession. Such succession takes place in a rich
inorganic substances medium. In heterotrophic succession, the population of
heterotrophic organisms like animals and fungi are present in greater quantity in the
initial stages. It begins, prominently in an organic environment and there is a progressive
decline in the energy content.
Progressive and retrogressive succession
The kind of succession takes place from single and few forms of complex and several
kinds of forms are known as progressive succession. A change from grassland
community to forest community is an example of progressive succession. In some cases,
the reverse situation is seen, that is called retrogressive. In retrogressive succession the

3. community become simplistic and contents fewer species and less biomass over time. A
forest changing into a grassland community is an example of retrogressive succession.
Mechanism of succession
According to Clements succession is a process involving several steps. These steps or processes
are: nudation; invasion or migration; ecesis; aggregation; competition, reaction and stabilization-
climax.
Nudation
Succession begins with the development of a bare area without any form of life.
Invasion or migration
Many species invade or reach a new site from any other area. The seeds, spores or other
propagules of the plant species reach the bare area. This process is known as migration
and is generally brought about by air, water and various other agents.
Ecesis
After reaching the new area the process of successful establishment of the species as a
result of adjustment with the condition prevailing there is known as ecesis. Ecesis is
considered to be complete, if the plant is able to sexually reproduce in that particular area.
Aggregation
After successful establishment of a species as a result of reproduction, the individuals of
the species increase in number. So, as compared to earlier stages, there are a larger
number of individuals of a species that have aggregated in the given area.
Competition
The aggregation of individual in an area leads to interspecific and intraspecific
competition. The competition is usually for water, particularly when there is a shortage of
water nutrients or if the plant grows the shade of another, shortage of carbon dioxide,
oxygen and space takes place.

4. Reaction
The mechanism of modification of the environment, through the influence of living
organism on it is known as a reaction. As a result of the reaction, changes takes place in
soil, water, light condition, temperature and many other factors of the environment. The
environment thus get modified and become unsuitable for the existing community and it
is eventually replaced by another community. The old occupants are ousted and fresh
migrants establish themselves.
Stabilization-Climax
The whole process of succession results in stabilization of the vegetation which is now in
complete harmony with the environment of the place. And it is likely to persist as long as
the environment climate and physiographic condition remains unchanged. The soil is
fully occupied by plants and the community is closed. This final community is not
replaced and is known as climax community and the stage as climate stage. It was
Clements who used the term climax to describe the idealized endpoint of succession. The
climax community is in equilibrium with its environment and represents a steady state of
species composition, community structure and energy flow.
Three major theories have been proposed to define, identify and interpreter the climax
communities. These theories are Monoclimax theory, polyclimax theory and climax
pattern theory.
Monoclimax theory
• Monoclimax theory is also known as climatic climax theory. It was proposed by Frederic
Clements. This theory recognizes only one climax, no matter how great the variety of
environmental condition is at the start. Each climax is a direct expression of its climate.
In order to account for a variety of more or less stable communities but different from
regional climatic climax, Clement proposed following terms:

5. Preclimax
• In certain area different climax communities developed under similar climatic conditions.
If the community has a life forms lower than those in the expected climatic climax is
known as preclimax.
Postclimax
• Community that has life forms higher than those in the expected climatic condition of
climatic climax is postclimax.
Subclimax
• The stage in succession just preceding the climatic climax is subclimax.
Disclimax
• When a stable community which is not the climate climax for the given site is maintained
by man for his domestic animals, it is designated as disclimax.
Polyclimax theory
It was proposed by Tansley. The theory considers that the climate vegetation of a reason consists
of not just one type, but a mosaic of vegetational climax controlled by several factors such as soil
moisture soil nutrients, topography slope exposure, fire and animal activities. Tansley recognized
following climax types:
Climatic climax
• Climax community, which is determined by the general climate of the region.
Edaphic climax
• Climate communities, which are determined by the soil properties such as soil moisture,
soil nutrients.
Topographic climax
• Climax community, which is determined by the topography of the region.

6. Fire climax
• Climax resulted by repeated fire is called fire climax.
Climax pattern theory
A variation of the polyclimax was proposed by R.H. Whittaker (1953). The natural community is
adapted to the whole pattern of environmental factors in which it exists. In addition to the
climatic factors, e.g. wind, there are a whole range of edaphic factors, fire and biotic factors
including grazing, disease and mutualistic and competitive interactions. Whereas monoclimax
theory allows one climatic climax in a region and the polyclimax theory allows several climaxes
in the same area, the climax pattern hypothesis allows a community of climax types, varying
gradually along environment gradients and not neatly package into discrete climax types.
Model of succession
There are three models to explain the ecological
processes of community change during succession.
These models are facilitation model, tolerance
model and inhibition model.
Facilitation model
• The classical model that explains the mechanism of succession is the facilitation model
according to this model certain Pioneer species with quality ideal for early succession can
colonize the newly exposed landforms after and ecological disturbance.
• This initial species modify the site, making it more suitable for invasion by other species,
for example, by carrying out the earliest stages of soil development.
• Once established, the latter sectional species eliminate the pioneers through competition.
• This ecological dramatic proceed through a progression of stages in which early species
are eliminated by later species, until the climate stage is reached.

7. • This model seems to be most appropriate in explaining changes in many primary
successes, but less so for secondary succession.
Tolerance model
• According to tolerance model, new Pioneer species neither inhibit nor facilitate the
growth and success of other species.
• All species in the succession are capable to establish on the newly distributed site,
although with exhausting successors in turn of the rapid attainment of a large population
size and biomass.
• In contrast with the facilitation model the early occupations of the site do not change
environmental condition in ways that favour the subsequent invasion of later successional
species rather with increasing time the various species short them self out through their
difference is tolerance of the successional increasing intensity of biological stresses
associated with competition.
• In the tolerance model competition tolerant intolerant species are relatively successful in
early successional stages when site conditions are characterized by a free availability of
resources.
• However these species are eliminated later on because they are not as competitive as later
species which eventually develop a climate climax community.
Inhibition model
• In third suggested mechanism of succession is the innovation model.
• As with the tolerance model, both early and the late successional species can establish
populations soon after distribution disturbance.
• However some early species make the site less suitable for the development of other
species.

8. • For example some plants are known to secret toxic biochemical into the soil these are
called aloe chemicals which inhibits the establishment and growth of other species.
• Eventually however the inhibitory species die and this creates opportunities that later
successional species can exploit.
• This gradual change eventually culminates in the development of the climax community.
• All three of these models facilitation tolerance and inhibition can be supported by
selected evidence from many ecological studies.
• All these models differ significantly in their predictions about the organizing principles of
successional dynamics it appears that none of them is correct all of the time.
Hydrarch and Xerarch succession
The ecological succession can be broadly classified into two kinds on the basis of the nature of
habitat from which primary succession starts hydrarch and Xerarch.
Hydrarch
Section initiated with the establishment of Pioneer communities in a wet area is termed as
hydrarch succession and the succession null stages are hydrosere and helosere.
Hydrosere- when succession starts in freshwater habitats like pond, pools, lakes and marshes.
Halosere- when it starts in saline water habitat example of mangroves coral reefs and estuaries.
Xerarch
Succession initiated with the establishment of Pioneer communities in dried area termed as
Xerarch taxation and the succession stages is called xerosere. It is further divided as
Lithosere- when succession takes place on bare rocks
Psammosere-when succession takes place in a sandy area like sand dunes.

9. Hydrarch
The succession in a freshwater ecosystem is also referred to as hydrosere. Succession in the pond
begins by colonization by the Pioneer like the phytoplankton and finally terminated into a forest,
which is a climax community. The whole process of succession of the hydrosere is further
subdivided into a number of substages depending on the kinds of organisms dominating the stage.
Stages
• Phytoplankton stage
In this initial stage the pond water is poor in nutrients and divide of much life full stop at this
stage the water is incapable of supporting larger lifeforms. In such situation phytoplanktons
consisting microscopic algae begins to multiply and they quickly become the pioneer colonizers.
As the phytoplanktons and the dependent animal population die, decomposers organisms
increase in number and bring about decomposition, which results in the release of minerals and
enrichment of aquatic habitats.
• Submerged stage
The habitat which is now shallower and richer in nutrients and where light is available up to a
certain depth becomes suitable for the growth of rooted submerged hydrophytes like hydrilla,
potamogeton, vallisneria, utricularia and ceratophyllum. These grow at various depth, mostly
rooted in the muddy or sandy bottom depending on the species and also on the clearness or
turbidity of water.
• Floating stage
The pond is now colonized by plant species which are rooted in mud but their leaves reach water
surface and float. Due to the growth, death and decay of these organisms, the water level by now
becomes very much decreased making the pond much shallower. Evaporation of water along
with the addition of slit from the adjacent area also contributes the in making the pond shallower.

10. • Reed swamp stage
The stages also known as amphibious stage, as the plants of the community are rooted but most
part of their shoots remain exposed to air. Species of Typha, Sagittaria and Phragmites are some
examples of this stage. These plants have well developed root system and the form dense patches
of vegetation.
• Sadge meadow stage
Species of Cyperaceae and Gramineae such as Carex, Juncus, Cyperus and Eleocharis colonize
the area. The forms mat-like vegetation with their much branched rhizomatous system. All these
reach up on the habitat by binding water-carried and wind-borne soil, accumulating plant debris
and transferring enormous quantities of water. There is rapid loss of water and sooner or later the
mud is exposed to air. The conditions in area gradually changed from marshy to mesic and the
marshy vegetation shows a decline. Up to the end of sadge-meadow stage, the climate of the
region has no control over the succession because water content of soil is high irrespective of
rainfall and climate of the region. At the end of the stage, the soil becomes dry and its water
content dependents on the rainfall and climate of the region.
• Woodland stage
When the lowland has been built up to an extent where the soil is saturated perhaps only in
spring and early summer, certain Species of shrubs and trees may appear. Those can tolerate
waterlogged soil around their roots will be the pioneers. Where species of Salix Cornus,
Cephalanthus, Alnus and Poplus is may form dense thickets. By this time of succession is much
accumulates humus with rich flora of microorganisms like bacteria fungi and others. This
mineralization of the soil favors the arrival of new trees species in the area leading to the climax
stage.
• Climax stage
A variety of trees invade the woodland community which soon develops into the climax
community. The nature of a climax is dependent upon the climate of the region.