Ecological Succession

Ecological Succession
Succession is an orderly and progressive change in a community structure of an ecosystem over a long period of time. Such changes mostly take place due to change in environment or due to some disturbance such as fire, volcanic eruption, flood, movement of glaciers etc. These disturbances remove the existing community and the disturbed area may be colonized by a variety of new species. The new species, in turn may gradually be replaced by still other species over a period of time. The replacement process goes on till a relatively stable community is developed. Thus during succession the community passes through many predictable transitional stages called seres. The first established community is called pioneers or primary community and the final relatively stable and mature community is known as the climax community. The word succession was used by Hult for the first time for orderly changes in community. Two main types of successions are
1. Primary Succession
2. Secondary Succession
1. Primary Succession
If the succession begins in a lifeless area where no living organism is present and where soil has not yet been formed, it is called primary succession. In other words the community is formed from a scratch. For example succession on a new volcanic island or on newly formed sand done, is a primary succession. Primary succession require thousands of venus to form a climax community.
2. Secondary Succession
It occurs in places where an existing community has been renowned by some disturbance such as fire or other climatic factors. In such a place there may not be any living organisms but the soil remains intact. As a result the secondary succession is more rapid than primary succession. The new communities developed are different from the pressure ones. Depending on the habitat, where the succession begins, the succession is divided into
i. Hydrosere: Succession occurring in water
ii. Xerosere: Succession occurring on dry soil or dry terrestrial habitats such as rocks, sand dunes, etc.
iii. Mesosere: Succession on land but where adequate moisture is available.
i. Hydrosere OR Hydrach
Primary succession starting in open water such as ponds, lakes etc. which are ultimately converted to land community is called hydrosere. There are several seral stages based on dominant plant types. The seral stages of hydrosere in a pond are
a. Phytoplankton Stage
The first invaders are phytoplanktons such as green algae (spirogyra), blue green algae (eynobacteria), diatoms etc. Since the pioneer community is phytoplankton, this seral stage is called the phytoplankton stage. Later on Protozoans such as Amoeba, Euglena etc also invade the aren.
b. Submerged Stage
The death and decay of planktons make the soil suitable for growth of rooted hydrophytes such as Hydrilla, Vallisnaria etc. Animal life at this stage is blue fish, sun fish.
c. Floating Stage
The death and decay of submerged hydrophytes and accumulation of sediments from surrounding area decrease the water level. Therefore pond becomes suitable for growth of rooted plants with floating leaves. These plants include Nymphea, Nelumbium etc. Some free floating plants are also seen. These plants lose water through transpiration and consequently the water level is very much reduced. Animal life is represented by Hydra, snails, frogs.
d. Reed Swamp Stage
This stage is also called amphibious stage. due to death and decay of plants, pond becomes more and more shallow. Plants are rooted but part of their bodies is above water level. Examples of such plants are Typha, Sagittaria. Animals are insects, water spiders, giant bugs.
e. Sedge Meadow Stage
The water level continuously decreases and the substratum becomes more suitable for growth. The stem is usually in the form of rhizome. Transpiration decreases the water level more and more and finally the marshy soil is exposed. The oxygen of the air oxidises the nutrients of the wet soil to form nitrates and sulphates. Thus the soil fertility is increased plants of this stage are Juneus.
f. Woodland Stage
The water of the soil evaporates rapidly and the soil becomes more and more drier. The marshy vegetation is replaced by land plants such as Salix, a shrub and populs, a tree. Animals are terrestrial like Zebra, Deer etc.
g. Climax Stage
The climax community is represented by trees and thus forests are formed. If there is heavy rainfall in the area, as in tropical regions, rain forests are developed. In areas with moderate rain fall, mixed forests are developed.
ii. Xerosere
This type of succession starts on a bare rock on land where there is deficiency of water and organic matter. The serial stages xerosere are
a. Crustose Lichen Stage
A bare rock does not possess moisture and organic matter, but some special types of lichens the crustose lichens, can survive on such bare surfaces and form the pioneer species. These lichens can produces acids which help in breaking the rocks and therefore start formation of the soil on other species. The lichens of this stage are Rhizo-carpon etc.
b. Foliage Lichen Stage
These lichens are large and leaf like in structure. They have ability to absorb moisture from the air and retain it. They can bind the loose soil particles together and thus the soil becomes more rich. Lichen of this stage are Pasmelia etc.
c. Mass Stage
Due to death and decay of the lichens, the organic matter is deposited in the soil which becomes richer in nutrients and can hold water also. As result plants like mosses develop. They can absorb water from the soil through rhizoids. Plants of this stage are polytrichum, selaginella etc.
d. Herb Stage
Due to more availability of moisture, humus and better anchorage, small herbaccous plant develop. In the beginning annual herbs are formed but later on perennial herbs develop. Some larger animals such as nematodes, ants and other insects also invade the area.
e. Shrub Stage
The conditions become suitable for growth of larger plants (shrubs). They shadow the herbs which die off adding more humus to the soil. Large number of animals such as snails, millipedes, some amphibians, reptiles and mammals like shrews, squirrels , foxes some birds are also found.
f. Climax Stage
More and more humus is deposited in the soil due to death and decay of plants and animals. This allows formation of woody trees. Thus final climax community is formed. The type of forest will depend on the climate conditions.