Monday, 11 May 2015

Ecological succession and its various stages .

Ecological succession is the observed process of change in the species structure of an ecological community over time. The time scale can be decades (for example, after a wildfire), or even millions of years after a mass extinction.  The community begins with relatively few pioneering plants and animals and develops through increasing complexity until it becomes stable or self-perpetuating as a climax community. The ╩║engine╩║ of succession, the cause of ecosystem change, is the impact of established species upon their own environments. A consequence of living is the sometimes subtle and sometimes overt alteration of one's own environment.  It is a phenomenon or process by which an ecological community undergoes more or less orderly and predictable changes following a disturbance or the initial colonization of a new habitat. Succession may be initiated either by formation of new, unoccupied habitat, such as from a lava flow or a severe landslide, or by some form of disturbance of a community, such as from a fire, severe windthrow, or logging. Succession that begins in new habitats, uninfluenced by pre-existing communities is called primary succession, whereas succession that follows disruption of a pre-existing community is called secondary succession.  Succession was among the first theories advanced in ecology. The study of succession remains at the core of ecological science. Ecological succession was first documented in the Indiana Dunes of Northwest Indiana which led to efforts to preserve the Indiana Dunes. Exhibits on ecological succession are displayed in the Hour Glass, a museum in Ogden Dunes.
Ecological succession is the gradual process by which ecosystems change and develop over time. Nothing remains the same and habitats are constantly changing.There are two main types of succession, primary and secondary.

Primary succession is the series of community changes which occur on an entirely new habitat which has never been colonized before. For example, a newly quarried rock face or sand dunes.

Secondary succession is the series of community changes which take place on a previously colonized, but disturbed or damaged habitat. For example, after felling trees in a woodland, land clearance or a fire.

The Major Points:
  • The species living in a particular place gradually change over time as does the physical and chemical environment within that area.
  • Succession takes place because through the processes of living, growing and reproducing, organisms interact with and affect the environment within an area, gradually changing it.
  • Each species is adapted to thrive and compete best against other species under a very specific set of environmental conditions. If these conditions change, then the existing species will be outcompeted by a different set of species which are better adapted to the new conditions.
  • The most often quoted examples of succession deal with plant succession. It is worth remembering that as plant communities change, so will the associated micro-organism, fungus and animal species. Succession involves the whole community, not just the plants.
  • Change in the plant species present in an area is one of the driving forces behind changes in animal species. This is because each plant species will have associated animal species which feed on it. The presence of these herbivore species will then dictate which particular carnivores are present.
  • The structure or 'architecture' of the plant communities will also influence the animal species which can live in the microhabitats provided by the plants.
  • Changes in plant species also alter the fungal species present because many fungi are associated with particular plants. 
  • Succession is directional. Different stages in a particular habitat succession can usually be accurately predicted.
  • These stages, characterised by the presence of different communities, are known as 'seres'.
  • Communities change gradually from one sere to another. The seres are not totally distinct from each other and one will tend to merge gradually into another, finally ending up with a 'climax' community.
  • Succession will not go any further than the climax community. This is the final stage.

    This does not however, imply that there will be no further change. When large organisms in the climax community, such as trees, die and fall down, then new openings are created in which secondary succession will occur.
  • Many thousands of different species might be involved in the community changes taking place over the course of a succession. For example, in the succession from freshwater to climax woodland.
  • The actual species involved in a succession in a particular area are controlled by such factors as the geology and history of the area, the climate, microclimate, weather, soil type and other environmental factors.

    For example, the species involved in a succession from open freshwater to climax woodland in Central Africa, would be quite different to those which have been quoted in these pages as occurring in Britain. However, the processes involved would be the same.
Succession occurs on many different timescales, ranging from a few days to hundreds of years.

  • It may take hundreds of years for a climax woodland to develop, while the succession of invertebrates and fungi within a single cow pat (cow dung), may be over within as little as 3 months.

    By this time, the dung has been transformed into humus and nutrients and has been recycled back into the soil. The holes clearly visible in the cow pat (right) have been made by the animals which have colonized it.

    An example of Secondary Succession by stages: 1. A stable deciduous forest community 2. A disturbance, such as a wild fire, destroys the forest 3. The fire burns the forest to the ground 4. The fire leaves behind empty, but not destroyed, soil 5. Grasses and other herbaceous plants grow back first 6. Small bushes and trees begin to colonize the area 7. Fast growing evergreen trees develop to their fullest, while shade-tolerant trees develop in the understory 8. The short-lived and shade intolerant evergreen trees die as the larger deciduous trees overtop them. The ecosystem is now back to a similar state to where it began.

    Stages of Ecological Succession

    When talking about the types of ecological succession it is important to remember that the “types” occur within the stages, but they may not necessarily be unique to that stage. What determines the stage that an ecosystem is in is dependent on its energy balance – which is discussed in the next section. There are four main types of ecological succession:
    • Pioneer – pioneer types are the new lifeforms that enter into a primary succession and begin to take hold. This can be anything from a seed to a bacteria to an insect or to an animal wandering into a new area and bedding down to make it their home. The pioneer has no connection to the environment, but it does find enough present in the new ecosystem to begin to establish its life.
    • Establishing – the establishing type can be hard to pinpoint because it crosses into the pioneer and sustaining. Establishing is the process in which lifeforms identify elements in an ecosystem that can sustain their basic needs – such as food, water and safe habitat.
    • Sustaining – Sustaining type means that life in the ecosystem has begun to enter into a pattern that allows for a cycle of life to continue. This means that birth and death are occurring, and there is little migration outside of the ecosystem – this is most common in the climax succession.
    • Producing – the producing type occurs during the secondary succession. This is when lifeforms are breeding and growing, but there is migration because what is produced is also not capable of being supported within the ecosystem. There are also more areas of overgrowth or overpopulation due to seed levels.
    Pioneer species are the ones that thrive the new habitat at the beginning of ecological succession. Pioneer species are ‘r-selected’ species that are fast growing and well-dispersed. Early succession is therefore dominated by so called ‘r-selected’ species. As succession continues, more species enter the community and begin to alter the environment. These are called ‘k-selected’ species. They are more competitive and fight for resource and space. The species that are better suited for the modified habitat then begin to succeed the other species. These are superseded by newer set of species. This goes on till the stage of climax or equilibrium is achieved.
    When succession reaches a climax, where community is dominated by stable and small number ofprominent species and no other species can be admitted, that is called the  state of equilibrium or the climax community.