There are various types of successions

Succession is the process of establishment of communities over an ecological time scale. It is unidirectional and includes a series of changes to the ecosystem aiming for a stable community. The final community thus established is called climax community.

The individual stages of the succession are called seral stages. These seral stages are not discrete but a continuum of individual events. These stages are characterized by different species of organisms which are unique to the seral stage. Succession is usually preceded by disturbance.

Evolution can be considered as a combination of large number of such successions over geologic time scale. Succession is limited to small time scale. The process of succession usually initiates the formation of a single new local community at the uninhabited site. This is called as primary succession.

Causes of succession

Three major causes have been identified for the process of succession.

a. Initiating causes - These include biotic and climatic factors which destroy the existing populations of the area. Climatic factors include wind, fire, natural disasters, erosion etc. The biotic factors include activities of other organisms.

b. Continuing causes - This is also called as ecesis. These processes are continuous such as aggregation, competition, migration etc. This results in a series of changes in the soil structure of the area. Common changes include change of soil nutrients, accumulation of organic matter in litter or humic layer, change in soil pH etc.

c. Stabilizing causes. These include the climatic factors resulting in the stabilization of community.

Types of succession

There are mainly two types; primary and secondary succession

Primary succession is the process by which a single new community gets established in a bare area. The species is usually microbes which act as the pioneer species of the succession. These activities are on newly exposed landforms. Examples include lava flown areas, new sand dunes etc.

Secondary succession is the process of establishment of communities at sites where other species originally existed and got destroyed by some activities either partially or completely.

Autogenic succession is caused by changes in soil structure and composition. Plant structure can also change the community structure. In a forest ecosystem, the larger trees tend to produce broader leaves providing shade to the forest floor area. This in turn causes the exclusion of small shrubs and herbs which require more light and supports the shade tolerant species alone. Autgenic succession is commonly found where the established community in response to environmental conditions or other factors changes its habitat eventually destroying itself. It is a kind of self destruction which is triggered by the external factors.

Allogenic succession is caused by changes in environmental conditions influencing the population. For example the soil structure changes due to natural processes such as erosion, leaching etc and this alter the nutrient value of the soil. This can lead to changes in water dependent relations of the ecosystem. Allogenic succession also owes heavily to animals since they act as pollinators and seed dispersers in addition to herbivory. These activities can change the soil pattern and nutrient content of the soil affecting the other species. This type of succession can occur in areas where there was high disturbance or erosion. The areas are also characterized by influx of nutrients from outside the system modifying the original habitat and altering the existing community.

Autotrophic succession is marked by the presence of floral species and their dominance through the early seral stages of succession. This kind of succession is usually found in inorganic habitats and the process helps to maintain the energy balance of the area.4 Sequential Steps involves in the Process of a Primary Autotrophic Ecological SuccessionbyPuja MondalEnvironmentThecompleteprocess of a primaryautotrophicecological succession involvesthefollowingsequential steps, which follow one another:1. Nudation:Theprocessof succession begins with the formation of a bare area or nudation by severalreasons, such as oleanic eruption, landslide, flooding, erosion, deposition, fire, disease, or other catastrophic agency.New lifeless bare areas are also created by man, for example, walls, stone quarrying, burning, digging, flooding large land areas under reservoirs, etc.2. Invasion:The invasion is the arrival of the reproductive bodies or propagules of various organisms and their settlement in the new or bare area. Hants are the first invaders (pioneers) in any area because the animals depend on them forfood. The invasion includes the following three steps:(1) Dispersal or migration:The seeds, spores or other propagules of the species reach the bare area through the agency of air, water or animals. The process starting from the time a propagule leaves the parent plant to the time it arrives the bare area is called migration.(2) Ecesis:This is the successful establishment of migrated plant species into the new area. It includes germination of seeds or propagules, growth of seedlings and starting of reproduction by adultplantsonly a few immigrant propagules are capable of doing this under primitive hard conditions, and thus most of them disappear.(3) Aggregation:This final stage of invasion, the successful immigrant individuals of a species increase their number by reproduction and aggregate in a large population in the area and in consequence individuals of the species come close to each other.3. Competition and reaction:As the numbers of individuals of a species increase due to multiplication and all aggregate at the limited place, thecompetitionfor space and nutrition is started among them (intraspecific competition). They also compete with individuals of other species that may enter the area (interspecific competition).All individuals also interact with theenvironment. Consequently due to increased intra-and interspecific competitions and other type of biotic and a biotic interactions, the environment is modified and progressively becoming unsuitable for the existing community which sooner or later is replaced by new invaders or another community (seral community).Finding the modified environmentmoresuitable more species enter the area and compete with the previous occupants. This results in a balance among the species in which the former species is brought down to a subdominant status or is completely eliminated. The addition of organic matter, nutrients and more moisture in the substratum by small plants make it suitable for larger ones.Increase availability of food allows variouskinds of animalsto join the community and the resulting interactions further modify the environment and pave the way for fresh invasion by other species of plants and animals to move on the process ofsuccession.4. Stabilization or climax:Eventually a stage is reached when the final terminal community becomes more or less stabilized for a longer period of time and it can maintain itself in the equilibrium or steady state with the climate of that area. Theoretically at least, this last serai stage is mature, self-maintaining, self reproducing through development stages and relatively permanent. The vegetation is tolerant of the environmental conditions it imposed upon itself.This terminal community is characterized be equilibrium between grossprimaryproduction and total respiration, between theenergycaptured from sunlight and energy released by decomposition, between the uptake of nutrients and the return of nutrients by litter fall.It has a wide diversity of species, a well-developed spatial structure, and complex food chains; and its living biomass is in a steady state. This final stable community of the sere is the climax community, and the vegetation supporting it is the climax vegetation

Heterotrophic succession on the other hand is marked by dominance of heterotrophic species. The pioneer species usually include bacteria, fungi, actinomycetes and small animal species. This is found in organic habitats.

Seasonal and cyclic succession is not dependent of disturbance to the ecosystem for initiation. These are regular in nature which results mostly from recurring events. The change of vegetation as well as the animal species is hence more or less predictable. They can also result from any changes in intra or interspecific relations. The succession model is able to incorporate observational fluctuations in the climax concept.

Geographical succession assumes that the communities are more or less similar based on their latitudes, altitudes and climax community. The changes are hence more predictable and precise in this model of succession.

Aquatic succession is the primary succession in aquatic bodies. All inland water sources are considered to be temporary habitats. Here, a terrestrial environment is needed to support the climax community.

Temporal dynamics collectively represents the succession and similar processes which affect the developmental properties of ecological systems.

1. Primary Succession:In any of thebasicenvironments (freshwater, marine, terrestrial), one type of succession is primary succession, whichstartsin a given area where the conditions oflifeare not favourable in the beginning. The first group of organisms establishing the known as the pioneers,primarycommunity or primary colonisers, e.g., succession in sand on exposed rock surfaces etc.In the primarysuccession-taking place on the exposedrocksurface, usually, lichens their appearance first. Lichens followed by the mosses start the initial stage of succession, lichens and mosses change the physical environment so that the new species of autotrophs a in the area and establish themselves. Consequently, heterotrophs also arrive in the area. In way succession continues in the area, finally leading to the establishment of a stable community in the particular ecosystem.2. Secondary Succession:It starts from previously built-up substrata with already exists living matter. Thistypeof succession starts in a given area where the conditions of life favourable because of the fact that the area was occupied earlier by well-developed community e.g., succession in an abandoned crop field. Suchsuccessionsare comparativelymorerapidThe development stages of ecological succession are called stages. Organisms (usuallyplants), which start the initial stage of succession, are called pioneers.The pioneers established themselves in the given area, reproduce and survive in the area. Gradually, the physical environment this changed environment enables new species to arrive in the give area and establish themselves in the area. Depending on the types of autotrophs in heterotrophs (animals) also arrive in the area. In this way, the succession continues in the one community following the other.The culminating stage in the succession is the establishment of a stable community in the area, and this is known as the climax community. Thus, the final stage of ecological succession is the formation of the climax community. The chain of stag from the pioneers to the climax community is called the sere.In the abandoned crop-field, the weeds (autotrophs) are pioneers. The weeds modify physical environment so that more and moreherbsinvade the area in course of time. Later the shrubs enter the area and finally the trees and establish themselves forming the climax community3.Autotrophic Succession: It is characterised by early and continued dominance o autotrophic organisms like green plants. It begins in a predominantly inorganic environment and theenergyflow is maintained indefinitely.4.Allogenic Succession: In some cases, however, the replacement of the existing community is caused largely by any another external condition and not by the existing organisms! Such a course is referred to as allogenic succession.5.Autogenic Succession: After the succession has begun, in most of the cases, it is the community itself, which, as a result of its reactions with the environment, modifies its own1environment and thus causing its own replacement by new communities.6.Micro Succession: This involves the succession of microorganisms. It begins in a predominantly organic environment and there is a progressive decline in the energy content.The succession of protozoa in the hay infusion is called micro-succession. The time taken for this succession is about 30 days. Dried hay is boiled inwaterand if the solution is kept for a few days, heterotrophic bacteria develop. If some pond water, containing various microscopic organisms, is added to this solution, the succession of protozoa can be observed in this culture. First small flagellates appear in the infusion and this is followed by ciliates. At this stage paramecia can be seen in plenty.This may be followed by the appearance of amoeba and rotifiers. This represents the climax.