Primary Succession:
According to Clarke (1954), ecological succession that begins on a bare area where no life has existed, or where the previous flora and fauna have been completely destroyed, is known as the primary succession.
If succession proceeds from an area devoid of organisms, or from an area which has not been changed physically by organisms, it is called primary succession (Benton and Werner, 1976).
The diverse places that may be available for primary succession (i.e., sand bars, glacial moraines, recently formed ponds) may be classified as xeric, mesic or hydric according to whether the initial moisture conditions are dry, intermediate or wet. Seres starting from these types of situations represent xerarch, mesarch, and hydrarch succession respectively.
A striking example of primary succession is the hydrarch succession in which a pond or lake and its community is converted to dry land with completely different community. Such environments may become dry by two major processes.
They may fill up with materials washed into the basin and with the remains of dead organisms which once lived in the lake or the outlet may cut down its stream bed so that eventually it becomes level with the bottom of the lake.
The pond or lake may be colonized by aquatic plants and animals. Microscopic organisms will find their way to deep open water. In shallow areas, where light can penetrate near the bottom, rooted submerged aquatic vascular plants will invade. In more shallow water, rooted aquatic plants with floating leaves may become established.
In a few centimeter deep water emergent’s (plants having most of their stems and leaves in the air) will be found. One shore, plants which can tolerate saturated soil will dominate. Each of these types of plants is said to form a community, since there are several species of animals and plants associated with each major plant type mentioned. In all the communities, organic matter and silt are constantly accumulating, decreasing the depth of the water. With this change of environment each community invades the one next to it in deeper water, and is in turn invaded by the one behind it in shallower water.
Thus, as the filling process occurs, the submerged aquatic plants will be displaced by the floating rooted aquatics, and these by the emergent aquatics. As succession continues, the emergent’s will be replaced by plants of the saturated soil region, and these in turn by ordinary terrestrial plants. Gradually the terrestrial communities will move in where once there was open water, until finally a terrestrial climax community occupies the site.
Another situation in which primary succession may occur is in dry environment. In such case the starting point may be bare rock, which would probably be invaded first by lichens (called the pioneers). Thereafter, other organisms such as mosses might appear, followed by other herbaceous plants, then by shrubs, and finally by trees. Sometimes trees or shrubs can be pioneers. In some studies it has been found that conifers were pioneers and herbaceous species were not a prerequisite for invasion of an area laid bare by volcanic activity.
Secondary Succession:
The term secondary succession refers to a community development on sites previously occupied by well developed communities, or it is the succession on sites where nutrients and conditions of existence are already favorable, such as abandoned croplands, plowed grasslands, and cut-over forests.
Secondary succession may start when the principal species of the community have been destroyed by fire, disease, and flood or by human activities such as farming or lumbering. In some instances of secondary succession a community is established that is essentially the same as a stage in the previous primary succession. In secondary succession the rate of change is much more rapid, and the time required for the completion of the final stage is much shorter. The end point for both types of succession will be a climax community.
In a study of secondary succession on landslides it has been found that although areas of bare rock were still mostly uncovered after 72 years; herbs, shrubs and trees became established within 9 years in areas where residual forest soil remained or collected between the rocks. After 72 years, the herb layer was virtually similar to the surrounding forest, and the tree species composition was approaching the climax.
Autotrophic Succession:
Succession that begins in a predominantly inorganic environment and is characterized by early and continued dominance by autotrophic organisms is called the autotrophic succession. The primary and secondary successions both come under the autotrophic succession and are widespread in nature. A model for autotrophic type of ecological succession is given in Table 4.2.
Heterotrophic Succession:
This succession is characterized by early dominance of heterotrophy. It occurs in the special and rare case where the environment is predominantly organic as, for example, in a stream heavily polluted with sewage. Other examples of heterotrophic succession include the successions involved in rotting of dead trees, reduction of animal dung, invasion and destruction of plant galls. In these cases green plants are usually not involved. Instead, the organisms in heterotrophic successions use up the energy of the dead organic matter.
In this type of succession, energy is maximum at the beginning and gradually declines as succession proceeds until additional organic matter is added from outside. In contrast, energy flow does not necessarily decline in the autotrophic type of succession but is usually maintained or increased. In heterotrophic succession, a series of different organisms (animals, fungi, bacteria and other forms) are involved and replace each other but the end point is utilization of all the energy and dispersion of the community.
Autogenic Succession:
Succession that results from changes brought about by the organisms themselves is called autogenic succession. The change from an abandoned agricultural field to a mature forest is an example of autogenic succession. In autogenic succession, the principal force of change comes from within the community. Therefore, autogenic succession occurs on time scales commensurate with the life span of the organisms in the community (Ricklefs and Miller, 1999).
Allogenic Succession:
Succession that results from factors external to the community is called allogenic succession. In allogenic succession, the principal force of change comes primarily from outside the community. Such external forces may include climate change, changes in temperature and other environmental factors, or other types of massive disturbances. Allogenic succession occurs on a time scale which is in accordance or proportionate with the time scale of the disturbance. Allogenic succession resulting from climate change may occur over thousands of years. Seasonal changes in temperature and light intensity in freshwater lakes are responsible for succession of their phytoplankton communities.