This document discusses ecological succession, which is the process of change in species composition of a community over time. It defines primary and secondary succession, and describes different types of succession including autogenic, allogenic, autotrophic, heterotrophic, progressive, and retrogressive succession. It also discusses Clements' model of succession involving nudation, invasion, ecesis, aggregation, competition, reaction, and stabilization. Finally, it outlines three major theories of climax communities - monoclimax, polyclimax, and climax pattern theory - and three models of succession - facilitation, tolerance, and inhibition.

1. Succession
Dr. Emasushan Minj
Assistant Professor
Department of Botany

2. Introduction
• 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.

3. Types of succession
Primary succession 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.
• Secondary succession occurs after
the destruction of existing
community by some natural
disturbance such as fire or flooding;
or by human activity.
• 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.

4. • 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.
• 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.

5. • 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 retrogressive succession the 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.

6. Mechanism of succession
• According to Clements succession is a process involving several steps.
These steps or processes are:
▫ Nudation- Succession begins in 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.
▫ 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 and able to reproduce
sexually in the particular area.
▫ Aggregation- After successful establishment of a species as a result
of reproduction, the individuals of the species increase in number.

7. • 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.
• 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.
• 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, where the 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.

8. Three major theories have been proposed to define, identify and
interpreter the climax communities. These theories are
Monoclimax theory,
Polyclimax theory
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.
• Preclimax- 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.

9. • 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.
▫ Fire climax- Climax resulted by repeated fire is called fire climax.

10. • 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.

11. Model of succession
• There are three models to explain the ecological processes of
community change during succession.
These models are-
• Facilitation
model
• Tolerance
model
• Inhibition
model.

12. 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.
• This model seems to be most appropriate in explaining changes in many
primary successes, but less so for secondary succession.

13. 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.

14. • Inhibition model
• 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.
• 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.
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.

15. Thank you