Knight (1965) has defined ecological succession as an orderly sequence of different communities over a period of time in some particular area. According to Smith (1966) ecological succession is an orderly and progressive replacement of one community by another till the development of a stable community in that area.
Benton and Warner (1959) have proposed another definition of ecological succession according to which succession is the occurrence of gradual, orderly and predictable changes in the composition of communities towards the climax type.
From the above definitions it can be inferred that the development of community in an ecosystem begins with pioneer stages which are replaced by a series of more mature communities until a relatively stable community is formed which is in equilibrium with the local conditions. The whole series of communities which develop in a given area is called sere, the relatively transitory communities are called seral stages or pioneer stages and the final stabilized community is called the climax community.
Patterns of Succession
Depending upon the types of habitat and varying amount of moisture, the successions are variously designated. The chief patterns are hydrosere or hydrarch (in water), xeroses or xearch (in dry conditions) and mesarch or mesosere (an intermediate type with adequate moisture.
(A) Xerosere — One of the best examples of xerosere is the succession which starts on bare rock, wind blown sand, rocky talus slopes any such place where there is extreme deficiency of water. The various stages in xerosere can be enumerated as follows –
1. Lichen stage : Due to great exposure to sun and extreme deficiency of water, the first pioneers on the bare rock area are a few simple organisms. The most successful of such organisms are crustose lichens. These are able withstand extreme desiccation due to excessive dryness. During rainy
son they absorb large quantities of water and flourish rapidly. Migration to distant rocks takes place either by spores or soredia by wind. The common species of crustose lichens are Rhizocarpon. Rinsodina etc. These begin the slow process of rock disintegration Rock particles and dead organic matter of lichens accumulate to provide conditions possible for the growth of higher forms of lichens.
2. Moss stage: With the accumulation of dust and humus in small entities, the environment is altered enough to allow the establishment of secondary communities in a rather definite sequence. Scattered patches of mosses such as Tortula, Grimmia, Beryum and Barfula etc. begin to invade environment that had so far been dominated by lichens. Later on, mosses ice Funaria, Sphagnum and polytrichnm make their appearance.
Among the animals, mites become more varied ; some small speders md spring tails as well as tardigrades become associated with this secondary community.
3. Herbaceous stage : As the mats of mosses become more extensive more soil accumulates; much of the soil is blown in from surrounding areas during windy periods. More mineral material is added to the soil as acids leach out from the overlying vegetation and increase the depth of the mineral layer. Many annual weeds develop which are, later on, followed by biennial and finally perennial grasses, Andropogon, commonly known as “, sedge, becomes a dominant grass in many areas. With the influx of grasses, the fauna (animals) also becomes varied. Nematodes and larval insects, collembolan, ants and mites appear in the gradually altered environment.
4. Shrub stage : Further modification of the environment provides conditions for the germination and growth of shrubs and perennial woody plants such as Acacia, Prosopis, Capparis, Zizyphus etc.
With the approach of shrubs, the animals also become vivid and numerous and join hands with the vegetation in altering the environment.
5. Climax forest: With the establishment of shrubs, more and more soil is formed and environment becomes increasingly humid. This favours the growth of woody trees. In the beginning trees show stunted growth and are sparsely placed. Finally a climax forest community is established and a number of terrestrial vertebrate make their appearance. The climax community is the last aggregation in the successional series. If the climax conditions do not change and no catastrophic event alters the area, the community maintains itself indefinitely.
(B) Hydrosere : Hydrosere or Hydrarch succession starts in water. A freshly built pond can be taken us a most suitable example of hydrarch succession. The various stages of hydroserecan be enumerated as follows
1. Submerged stage: In initial stages water is poor in nutrients and devoid of life. The pioneers in an aquatic habitat are plankton. The phytoplankton grow floating or suspended in water and multiply. With the death of phytoplankton and zooplankton, the substratum is enriched with organic matter. As water becomes rich in organic and mineral substances, certain rooted submerged hydrophytes make their appearance. Prominent among them are Ceratophyllum, Potamogeton, Vallisneria and Utrioularja etc. By the death and decay of these plants there is further enrichment of the medium. With the increase in nutrients, the level of the pond is raised and it becomes shallow.
2. Floating stage: When the water level in the pond remains only 6 to 8 ft. deep, floating plants begin to appear. These plants include Nymphaea, Nelumbiuni Trapa and Monocharia, etc. These have their roots rooted in the mud and their levels freely floating at the surface. Later on deep floating, plants like Lemna, Azolla and Wolffia grow profusely to cover the water surface.
3. Reed-Swamp stage : As the water body becomes shallow by silting and removal of water by transpiration, the environment becomes less suitable for the free floating and submerged plants. Now reed-swamp plants like Typha, Rumex, and Sagittaria invade the area. The reed swamp plants build up the shores by retaining the sediment and accumulation of plant remains. Beavers, muskrats and other animals carry material into the pond, deciduous vegetation blows in from the shore, and silt is carried in from the surrounding land. Rafts of vegetations from the pond margin drift offshore, strand, and take root, thus establishing islets that grow in size until they meet also and join the shore.
4. March-meadow stage : As the free water is changed to swampy and, the water plants give way to swampy plants such as sedges and rushes. As succession continues, marshy meadow becomes too dry for swampy plants and these are subsequently replaced by herbs and shrubs.
5. Woodland stage : As succession continues, the soil is further built up, so that it becomes drier and is also changed chemically. In time certain mailer species of trees invade the area, taking the place of the shrubs, and eventually full sized forest trees will dominate the scene. At the same time when the vegetation is undergoing these profound changes in the hydrarch succession, the animal life of the community is correspondingly altered. Fish, weavers and muskrats are gradually excluded and land vertebrates make their appearance.