The whole process of biotic succession is actually completed through a number of sequential steps, which follow one another. Ihese steps in sequence are as follows:
(i) Nudation : Formation of bare area without any form of life is called nudation. The causes of nudation may be climatic (e.g wind, erosion, floods, drought, storm, frost), topographic (e.g., landslide, volcanic eruptions) or biotic (e.g., epidemic, human activities, etc)
(i) Invasion : Successful establishment of a species in this bare area is called invasion. It is completed in three steps: migr
(dispersal), ecesis (establishment) and aggregation.
The transfer of seeds, spores or other propagules of the first settler species to the bare area is called migration. It is
generally brought about by the agency of wind and water The species which invade the bare area must grow, establish and reproduce for successful succession. This depends largely on
the characteristics of substratum, climatic and other environmental factors of the area. Those species which get adjusted to
this situation ultimately get established. As a result of ecesis, individuals of the species become established in the are
After ecesis, the individuals of the species increase in number by reproduction and they get close to each other. This process
s called aggregation
(ii) Competition and coaction: Initially, many species invade the bare area simultaneously. However, those whi
However, those which find environment
favourable ultimately establish and get aggregated in the limited available area. This results in competition, mainly for space and nutrition Competition may be interspecific or intraspecific. Various kinds of interspecific and intraspecific interactions affect the individuals of species in various ways called coaction. The species which are unable to compete are ultimately eliminated
(iv) Reaction The living organisms reciprocally influence and get influenced by the environment. This is known as reactio As a result of reaction, changes take place in soil, water, light conditions, temperature etc., of the area. Thus, the environment of the area gets modified, becoming unsuitable for the existing community, which in course of time, gets replaced by anocommunity (seral community)
(v) Stabilisation : Finally, the terminal or climax community becomes stabilised for a longer period of time as it can maintain itself in equilibrium with the prevailing conditions. The cdimatic causes determine the nature of the climatic climax, ie, the end point of succession. They have a profound effect in determining the population from the beginning to end, the number and ki
of stages, as temperature regions, the plant populations are often quite different. This is due to the cimat well as the reactions of the successive stages. While the process of succession in the tropics is similar to that in
TYPES OF ECOLOGICAL SUCCESSION
us types of succession have been grouped in different ways, on the basis of different aspects. Some basic types of succession are, however, as follows:
Primary succession is a succession unes, ig biotic succession which occurs on a previously bare or unoccupied area, e.g., new exposed rock area, sand
neous rocks, deltas, newly created pond or reservoir. The establishment of a new biotic community is generally slow
t group of organisms establishing on such bare or unoccupied areas are known
as the pioneers, primary community or
f ecological succession, changes in vegetation occur, which in turn, affect foo
primary colonisers. As a result various types of animals.
Based on the nature of the habitat, succession of plants is of two types-hydrarch (or hydrosere) and xerarch (or xerosere) Hydrosere starts in regions where water is in plenty, and progresses from hydric to mesic conditions (adequate moisture conditions) On the other hand, xerosere starts in regions where moisture is present in minimal amounts, such as dry deserts, rocks etc., and it progresses from xeric to mesic conditions. Both hydrarch and xerach successions lead to medium water conditions (mesic) neither too dry nor too wet.
Secondary succession
This is the succession of communities in previously inhabited areas, which have been naturally or
artificially disturbed, and where soil and some organisms are already present. Secondary succession is a biotic succession that occurs in an area from which a community has been removed and
where nutrients and conditions for existence are present, e.g, cut over forest, abandoned crop land
ploughed fields and lands that have been flooded
Natural phenomena, such as avalanches (massive snow-sides, landsides, volcanic eruptions, earthquakes loods, prolonged severe droughts and forest fires set by lightning, destroy existing communities and pave the way for secondary succession. An abandoned tield, heavily overgrazed pasture, deforested tract also
provide sites for secondary succession
Secondary succession is much faster than the primary succession, hecause the soil and certain organisms are already present Surviving seeds, underground stems, persisting and new invading species rapidly grow on the return of favourable conditions. This re-establishes the biotic community through necessary seral changes, in due course of time.
In an abandoned field, grasses and annuai weeds quickly move in and cover the soil with a carpet of low vegetation. in course of time some taller perennial plants move in, which shade the ground, and their long roots monopolise the soil water. The seedlings of the pioneer species now find it difficuit to grow. The tall herbs are decrived of sunlight and water by the shrubs, which get the same treatment from the trees Finalily, a forest of dlimax species is established to persist till itis not disturbed. A destroyed grassland may take 50- 100 years and
a damaged forest over 200 years to recover fuly as Autogenic succession
The succession resuiting from changes brought about by the organisms themselves is caled autogenic succession. In autogenic
succession the principai force of change comes from within the community. It occurs on time scales commensurate with the life
of organisms in the community. An abandoned agricultural field changing into a mature forest over long span of time is an example of autogenic succession.
Allogenic succession The succession resulting from changes brought about by factors external to the community is called allogenic succession. Su external forces may include climate change, massive disturbance, or changes in temperature and other environmental factors
Autotrophic succession This is characterised environment and the energy flow is maintained indefinitely. The primary and secondary successi succession and are widespread in nature
by early and continued dominance of autotrophic organisms like green plants it starts in a predominantly inorganic
ions, both come under autotrophic Heterotrophic succession This is characterised by early dominance of heterotrophs such as bacteria, fungi and actinomycetes. It begins in a predominantly organic
environment e g, a stream heavily polluted with sewage. In this type of succession, energy is maximum at the beginning followed by
a progressive decline in the energy content. The end point is utilisation of al the energy, and dispersian of the community Induced succession
This occurs due to extensive external interference. Here the initial community has high productivity which gradually decreases.
