The term 'Succession' was coined by Hult (1885). Succession is the process of change of the species or community over a time in a specific order.
10.1.    Types of Succession
(1)    Primary Succession
(2)    Secondary Succession
10.1.1.    Primary Succession : When a new bare area that has never previously supported any life or living organism and ecological community, is colonized by plants or animals. This process is called as primary succession. Primary succession takes place on newly exposed rock surfaces, mud banks, sand dunes etc. originated from natural disasters as landslides, earthquake or lava flows.

10.1.2.    Secondary Succession : It is the another type of succession instead of being the result of natural disasters, it is caused by sequential changes in the environment and interactions between the species of one or more communities for nutrition, shade or reproduction. 
10.1.3.    Autotrophic succession :- When the population of an ecological area is replaced by plants (autotrophs) in the place of heterotrophs. This type of succession is called as autotrophic succession.
10.1.4.    Heterotrophic succession :- If the Heterotrophs (eg. Bacteria fungi and some animals etc.) are successive to be placed in an organic environment instead of autotrophs. This succession is called as heterotrophic succession.
10.1.5.    Autogenic succession :- If a community lives in an environmental conditions and the conditions change due to their continue interaction by environment. Due to this environmental changes if another community replaces the old community, this succession is called as autogenic which is responsible to be placed by it's own living community.
10.1.6.    Allogenic succession :- 
If the succession or replacement of community takes place by another reason instead of environmental changes by previously living community, the succession is termed as Allogenic succession.
10.1.7.    Habitat succession :- 
Succession according to their habitats are termed as habitat succession. Types of habitat can be defined as below.
(a)    Hydrach :- If the succession starts from aquatic habitat is termed as 'Hydrach'. When vegetation starts in the Hydrach, it is termed as "Hydrosere" .
(b)    Mesarch :- If the starting habitat of succession is adequate moisture such succession is called as Mesarch succession.
(c)    Halosere :- The succession starting from saline water or soil is known as halosere.
(d)    Xerosere :- If a succession starts from a terrestrial region (eg. sand, rocks etc.). Xerosere succession takes place in the minium amount of moisture. Xeroseres can further be subdivided into following :-
(i)    Psammosere - Succession starts from sand.
(ii)    Lithosere - Succession starts from rocks.
(iii)    Oxylosere - Succession starts from acidic soils.
10.2.    Stages of succession
10.2.1.    Nudation :- 

Nudation is the process of mineralization or fertilization of a bare area to make it nutrient rich for the survival of first living organism with the favorable conditions. Nudation can be the result of some natural disaster also like cyclone, thunder, earthquake etc.
10.2.2.    Pioneer species : 
Pioneer species are the species which starts the beginning of the chain of the whole process of ecological succession. These are the very first species which colonize in a bare area generally. They are lichens or mosses as they are able to take nutrition from rocks through chemical decay of rock.
10.2.3.    Seral species :  
Pioneer species appears during primary succession. After pioneer species, some other communities appear in that area. The second community or species can be plants or animals. If they don't get favorable conditions they migrate to another place. The species which intermediates between the pioneer and final stable species, is termed as seral communities or species. Seral species can be one or more. Seral species can be the result of unfavorable environment conditions or competition. for eg. The seeds of plants are pioneer species. The plants increase their number by reproduction than animals also come there and decrease the number of plants. These animals are called as seral community. Decrease in plant species number also decrease the number of animal species and increase the struggle between species and within species. In order to avoid competition animal migrates to new area. The area becomes favorable for plants again. The plants, again increase their number and one or more animal community again come to balance the number of other organisms or plants. These animals are termed as seral community. Until an equilibrium stage is developed between all communities or species, this process of many seral communities continues. Finally when an equilibrium attained between plants and animal number i.e. animals eat the plants when it increases and animals eat another animals when they increase in number and plants are again grown in the same number which is eaten by the animals. The final species now termed as climax community.
10.2.4.    Competition : 
All seral communities compete with pioneer species or preexisting species. They can compete with their selves also for nutrition or reproduction.
10.2.5.    Climax community : 
When the ecological succession process ends after a pioneer and many seral communities with a final stable community the resulted environment reaches equilibrium such species is termed as climax species. 

10.3.    Causes of Succession :-
(1)    Climatic causes :- The climatic causes include temperature, rainfall, light intensity, gaseous composition, Humidity, wind etc.
(2)    Biotic causes :- In a community, some members can adjust with others but who can not adjust with others, are replaced by another ones.
(3)    Ecesis causes :- The soil condition also changes and many other environmental conditions changes by migration and other interactions of species.
(4)    Stabilising cause :- Every species wants to survive and stabilise in a particular area for reproduction, nutrition and shade or for many things which is essential for their life. So this reason causes them to make competition from another species and than succession takes place.
10.4.    Characters of succession :-
(1)    It is an ordered process of a new community development.
(2)    It results to a static ecosystem.
(3)    Succession varies nutritional conditions for the adjustment and diversity of species.
(4)    It includes the modification of physical and biological environment of the community.

10.5.    Model of Succession :-
Connell and slatyer (1977) proposed 3 models 
(1)    Facilitation     (2)    Inhibition    (3)    Tolerance
10.5.1.    Facilitation model / Relay floristics model :-
These initial species modify the site or their own environment by ecological disturbance to make more suitable for succeeding. Stage and less suitable for themselves. So facilitation assures that each existing species creates a more favourable habitat for succeeding species.

(i)    Glacier retreat at Glacier Bay. Alaska

(a)    The first species to colonize the bare earth following the retreat of the glaciers are small species such as cyanobacteria, moss and lichens.
(b)    Mountain avens, Dryas drummondii is a flower common in the dryas seral stage.
(c)    Solid nitrogen and litterfall increase rapidly as alder, Alnus sinuata, invade. Note also the appearance f a few spruce trees higher up the valley.
(d)    Spruce, P. sitchensis and hemlock, T. Suga sppitrees in a climax spruce-hemlock forest at glacier bay, with moss carpeting the ground two hundred years ago, glaciers occupied this spot.
(e)    Spruce and hemlock trees in a climax forest results facilitation in the form of increased soil nitrogen, soil depth. Fuel's succession in this community.
10.5.2.    Inhibition model:-
Inhibition implies that early colonists prevent/inhibit the establishment and growth of other species. Eventually, however, the inhibitory species die and this creates opportunities that later successional species exploit. Species replacement is not necessarily any new colonists.

Here  0   indicate  →  no effect   –   indicate  →  inhibition
Example:- Some plants are secreted a toxic chemical into the soil which inhibits the establishment and growth of other species. Such as in subtidal habitats, bryozoans can prevent the establishment of tunicates and sponges.

Wayne Sousa 1979, Created an environment in the marine intertidal zone for testing how succession works in the intertidal zone by scraping rock faces clean of all algae. The first colonists of these areas were green algae (Ulva), much later, these were replaced by several species of red algae including the large alga chondracanthus canaliculantus. By removing Ulva from the substrate, Sousa showed that chondracanthus was able to colonize more quickly.
The results of Sousa's study indicate that early colonists can inhibit rather than facilitate the invasion of subsequent colonists.
10.5.3.    Tolerance model:-
The early colonists neither facilitation nor inhibit later colonists. They are unaffected by previous colonists.
In this process, any species can start the succession, but the eventual climax species community is reached in a somewhat orderly fashion, early species neither facilitate nor inhibit the later colonists. Species replacement is unaffected by previous colonists. Species that are more tolerant of limited resources, replace the other species. The tolerance model is similar to the facilitation model, in that later species may be facilitated by earlier species, but later species can also invade in the absence of earlier ones.

(1)    Some old field successions.
(2)    Oak and hickory seedlings can establish in shade. Thus these broadleaf species are the climax species which can regenerate in the climax negotiation.
Plants have different niches and can tolerate different levels of light, nutrients etc. Later species are also able to tolerate lower resource levels due to competition and can grow to mutually in the presence of early species, eventually outcompeting them.
10.6.    Shelford's low of tolerance:-
There are upper and lower threshold values, on the gradient beyond which the species can not survive. Tolerance ranges differ for one species to another. The whole range over which the species is able to survive is the range of tolerance.

10.7.    The major step of primary autotrophic succession:-
(1)    Nudation:- Succession begins with the development of a base area without any form of life.
(2)    Migration:- Dispersal of spores, seeds to the bare area by water, air.
(3)    Ecesis:- Successful germination of seeds and spores and its establishment in the bare area.
(4)    Aggregation:- Increase in the number of species as a result of reproduction.

Invasion = Migration + Ecesis + Aggregation.

(5)    Competition and coaction:- Inter and intraspecific competition for space, food and nutrients result in elimination of unsuitable plants individuals of species affect each other is life in a various way, called coaction.
(6)    Reaction:- Mechanism of modification of the environment. through the influence of living organisms change to take place in soil, temperature, the light condition of the environment. The environment becomes modified and unsuitable for the existing community and this community replace by the next incoming community.
(7)    Stabilization:- Ultimately the final climax community become more stable and in equilibrium with its environment and represent a steady state of species composition, community structure and energy flow.

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