In this article we will discuss about the primary and secondary ecological succession.
Primary Succession:
Primary succession is the establishment of plants on land that has not been previously vegetated. It begins with colonization and establishment of pioneer species. Primary Succession occurs where no biotic community has previously existed like sand, dry rock, volcanic islands, etc. The area is devoid of any organisms. It may take up to 1000 years before climax community gets established.
Stages in a Primary Succession:
Trees, herbs or shrubs cannot grow on dry rock because of absence of good soil. The first organisms often called pioneer organisms. Lichens are the first organism that grow or develop on the rocky area. Lichens are the combination of algae and fungi. Neither algae nor fungi can grow individually on dry rock.
The fungal component holds and the algae provide food. Firstly crustose lichens grow. The usually cover the rock surface. The erosion of rock, sand, dead and decayed lichens provide sufficient soil for larger plant species to grow. Next foliage lichen, fruticose lichens, mosses, ferns and grasses grow on the rocky surface.
Then the herbs, shrubs and trees replace the earlier vegetation. Ultimately these plants will be succeeded by large seed bearing plants, tall grasses. This would also support number of different kinds of animals’ populations in a biotic community. This final stable self-growing community is called as climax community. The animals also show a succession but these are governed by the plant succession.
Climax Community:
It may take hundreds or sometimes thousands of years for a climax forest community to be established on a sand or dry rock. A climax community is in equilibrium with the environment. A climax community often has one or more dominant species.
It is generally referred to those species which collectively form a greater biomass and are generally larger in size. Climax community supports large number of species. Climax community has a high bio mass and rich organic soil. Climax community provides a wide range of food material. Climax community has a diverse variety of niches for animals.
Types of Climax:
Climax is the final or last vegetation of any well stable community. It depends upon climatic factors, times durations, soil texture, soil pH, soil microbes, soil water etc.
(i) Climatic Climax:
The climax depend upon the climate of community is called as climatic climax. In a single climax the development of climax community is controlled by the climate of the region like temperature, humidity, rain fall etc. it is termed as climatic climax.
Development of Maple-beech climax community over moist soil is the well-known example of climatic climax. Climatic climax is theoretical and develops where physical conditions of the substrate are not so extreme as to modify the effects of the prevailing regional climate.
(ii) Edaphic Climax:
Climax governed by more than one climate is called edaphic climax. When there are more than one climax communities in the region, modified by local conditions of the substrate such as soil moisture, soil nutrients, topography, slope exposure, fire, and animal activity, it is called edaphic climax. Succession ends in an edaphic climax where topography, soil, water, fire, or other disturbances are such that a climatic climax cannot develop.
(iii) Catastrophic Climax:
The establishment of climax vegetation due to wildfire etc.is referred as catastrophic climax. California, chaparral vegetation is the final vegetation and it is the example of catastrophic climax. The wildfire removes the mature vegetation and decomposers. A rapid development of herbaceous vegetation follows until the shrub dominance is re-established. This is known as catastrophic climax.
(iv) Dis-Climax:
When a stable community, which is not the climatic or edaphic climax for the community, is maintained by the disturbance of man or his domestic animals, it is called as dis-climax (disturbance climax) or anthropogenic subclimax (man- generated). Overgrazing by stock may produce a desert community of bushes and cacti where the local climate actually would allow grassland to maintain itself.
(v) Subclimax:
The prolonged stage in succession just preceding the climatic climax is subclimax.
(vi) Pre-Climax and Post-Climax:
In certain areas different climax communities develop under similar climatic conditions. If the community has life forms lower than those in the expected climatic climax, it is called pre-climax; a community that has life forms higher than those in the expected climatic climax is post-climax. Pre-climax strips develop in less moist and hotter areas, whereas Post-climax strands develop in more moist and cooler areas than that of surrounding climate.
Secondary Succession:
Successional dynamics following severe disturbance or removal of a pre-existing community are called secondary succession Secondary succession is the invasion of a habitat by plants on land that was previously vegetated. Pre-exist vegetation may be damaged by natural or human disturbances such as fire, logging, cultivation, or hurricanes.
Dynamics in secondary succession are strongly influenced by pre- disturbance conditions, including soil development, seed banks, remaining organic matter, and residual living organisms. Because of residual fertility and pre-existing organisms, community change in early stages of secondary succession can be relatively rapid.
It occurs when a biotic community has been destroyed and the surface is completely or largely devoid of vegetation. It may be due to earthquake, fire or even clearing of forest by man. As the spores, seeds, rhizomes the organs of vegetative reproduction are present beneath the soil. Often grasses, shrubs, weeds are first to appear. Secondary succession is much more commonly observed and studied than primary succession.
Particularly common types of secondary succession include responses to natural disturbances such as fire, flood, and severe winds, and to human-caused disturbances such as logging and agriculture. The same principle of primary succession applies but it occurs at a much faster.
It has been observed that a destroyed grassland may take 50 – 100 years and a destroyed forest about 200 years. Overgrazing of grasslands is like reversion of the community succession. The introduction of exotic weeds in a cleared forest seriously affects the succession and it may become impossible to regenerate the same old type of forest in a biotic community.
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 backing to a similar state to where it began.
Types of Secondary Succession:
(i) Micro-Succession or Serule:
This type of succession occurs within communities it includes dead trees, animal droppings, etc. Succession of micro-organisms like fungi, bacteria, etc. occurring within a micro-habitat is known as micro-succession or Serule. Microbial communities may also change due to products secreted by the plants, animals and also bacteria.
Changes of pH in a habitat could provide ideal conditions for a new species to inhabit the area. In some cases the new species may compete with the present ones for nutrients leading to the primary species. Changes can also occur by microbial succession with variations in water availability and temperature.
(ii) Allogenic Succession:
Allogenic succession is caused by a change in environmental conditions which in turn influences the composition of the plant community. The deposition of silt into river, lake or any water body may be causing an allogenic succession and leads to from salt marsh to woodland.
Approximate measurements indicate sedimentation rates of about 1 cm or more per year on the mud flats that are found 15 kilometers. Over the last 100 years, this salt marsh has increased its elevation and has extended itself seaward by 800 meters. The adjacent woodland has followed the salt marsh by invading its landward limit.
(iii) Autogenic Succession:
Autogenic succession is a succession where both the plant community and environment change, and this change are caused by the activities of the plants over time.
(iv) Progressive Succession:
Progressive succession is a succession where the community becomes complex and contains more species and biomass over time.
(v) Retrogressive Succession:
Retrogressive succession is a succession where the community becomes simplest and contains minimum or fewer species and less biomass over time. Some retrogressive successions are allogeneic in nature. For example, the introductions of grazing animals result in degenerated rangeland.
Succession begins when an area is made partially or completely devoid of vegetation because of a disturbance. Succession is a directional non-seasonal cumulative change in the types of plant species that occupy a given area through time.
It involves the processes of colonization, establishment, and extinction which act on the participating plant species. Most successions contain a number of stages that can be recognized by the collection of species that dominate at that point in the succession.
Some common mechanisms of disturbance are natural digesters like fires, wind storms, volcanic eruptions, logging, climate change, severe flooding, disease, and pest infestation. Succession stops when species composition changes no longer occur with time, and this community is said to be a climax community.