Ecological Succession is the gradual process by which ecosystems change and develop over time. It is the observed process of change in the species structure of an ecological community over time. Within any community, some species may become less abundant over time or even vanish from the ecosystem altogether. The change observed over time in ‘what is living’ in a particular ecosystem is Ecological Succession. Ecological Succession may also occur when the conditions of an environment suddenly and drastically change. Forest fires, wind storms, and human activities like agriculture all greatly alter the conditions of an environment. These massive forces may also destroy species and thus alter the dynamics of the ecological community triggering a scramble for dominance among the species still present. The rise and fall of numerous species within various communities illustrate two types of motive forces of succession: the impact of an established species to change a site’s environmental conditions, and the impact of large external forces to alter the nature of a site suddenly. The “engine” of succession, the cause of ecosystem change, is the impact of established species on their own environments. The original environment may have been optimal for the first species of plant or animal, but the newly altered environment is often optimal for some other species of plant or animal. Under the changed conditions of the environment, the previously dominant species may fail and another species may rise.
Stages of Ecological Succession
Ecological Succession proceeds through various stages, from the Pioneer Community to the Climax Stage. Each such stage is called sere or seral community. A seral community is an intermediate stage found in ecological succession in an ecosystem advancing towards its climax community. In many cases, more than one seral stage evolves until climax conditions are attained. A pri-sere is a collection of seres making up the development of an area from non-vegetated surfaces to a climax community. A seral community is the name given to each group of plants within the succession.
Ecological succession breaks down into three fundamental phases:
(1) Primary Succession, (2) Secondary Succession, and (3) Climax Community.
Primary Succession: It occurs when organisms colonize an area devoid of life, usually after a catastrophic natural event that leaves the land barren. Often the first organisms to take hold are algae, fungi, and simple plants such as lichens and mosses. Over time a thin layer of soil builds up so that more advanced plants, such as grasses and ferns, can take root. Along with the successful colonization of plants come animals, insects, birds, and small invertebrates.
An example of primary succession is the pioneer communities that begin to inhabit a newly created lava bed, where life cannot exist until the rock surface cools to a moderate temperature.
Pioneer Community: It is a group of organisms that invade a new area in the process of Ecological Succession. These organisms are typically plants, animals, and fungi. They are the first to move into a new habitat and often make the habitat more hospitable for future inhabitants. The pioneers through their death and decay leave patches of organic matter in which small animals can live. Organic debris accumulates in pockets and crevices, providing soil in which seeds can become lodged and grow. As the community of organisms continues to develop, it becomes more diverse, and competition increases, but at the same time new niche opportunities develop. The pioneer species disappear as the habitat conditions change and the invasion of new species progresses, leading to the replacement of the preceding community.
Climax community: The culminating stage in the succession is the establishment of a stable community in the area, which is known as the climax community. That is, the final stage of ecological succession is the formation of the climax community. Climax communities are relatively stable and can vary widely in a given region. Some of the features of the climax community are: the vegetation of this region is tolerant to the environmental conditions. The species diversity is large and the food chains of these species are complex and with spatial structure. It is a balanced ecosystem. There is also an equilibrium between the nutrients taken in from the soil and the return of the nutrients to the soil by litter fall. The individual organisms in the climax ecosystem are replaced by other organisms of the same kind. Thus it maintains species equilibrium.
Secondary Succession: It is when an established community is replaced by a different set of plants and animals. Most ecological changes occur as secondary succession. Most biological communities are in a continual state of secondary succession. Secondary succession is gradual, always moving toward the climax community. It occurs when an area previously had an ecological community and the community is disturbed or changed such that the original community was destroyed and a new community moves in.
Such successions are comparatively more rapid. This is more common than primary succession and is often the result of natural disasters such as fires, floods, and winds, as well as human interfaces such as logging and clear-cutting. It starts from previously built-up substrate with already existing living matter. This type of succession starts in a given area where the conditions for life are favorable because the area was occupied earlier by a well-developed community. E.g.- Succession in an abandoned crop field.
Other prominent types of Ecological Succession:
Autotrophic Succession: It is characterized by the early and continued dominance of autotrophic organisms like green plants. It begins in a predominantly inorganic environment, indefinitely maintaining the energy flow.
Allogenic Succession: Allergenic Succession, the replacement of the existing community is caused largely by any other external condition and not by the existing organisms.
Autogenic Succession: Autogenic Succession refers to that type when the community itself, as a result of its reactions to the environment, modifies its environment thus causing its replacement by new communities
References
NCERT Biology
IGNOU Material
Shankar IAS Environment book
Recitals Magazine
Allen notes Biology
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