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.
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.
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