Ecological succession
A given area has different populations. The sum total of all the populations in a given habitat is called community. The composition and structure of community change as the environmental conditions change. In fact this is a characteristic feature of any community. This change is sequential, in a definite order and lead finally to be a climax community which is in near equilibrium with the environment.
This gradual (and predictable) change in the species composition of a given area is called ecological succession. The ecological succession may replace one population of species by another leading to change in physical environment also.
There are two types of successions; primary and secondary.
Primary succession
The primary succession starts in the area where no living organisms ever existed, such as bare rock, newly formed pond or newly cooled lava, etc.
Secondary succession
The secondary succession starts in an area which has lost all the the living organisms once existed, for example, an abandoned crop field, burned or cut forest, land affected by floods, etc.
Primary succession is very slow process while secondary succession is comparatively faster.
The developmental stage of ecological succession are called seral stage. The organism of the first seral stage are called pioneers. The pioneers (mostly autotrophs) face the adverse conditions and establish themselves, reproduce and survive in the area. Gradually the pioneers modify the physical environment that enables new species to arrive in the given area. It is followed by arrival of heterotrophs in the area. Finally a stable community establishes in the area, which is known as the climax community.
The amount of water determines the pattern of ecological succession in the given area. Accordingly, there are two types: hydrarch succession (hydrosere) and xerarch succession (xerosere).
Hydrarch succession (hydrosere)
Hydrarch succession occurs in the areas, where water is present in abundance. It begins with small phytoplanktons followed by submerged and free floating plants and then rooted hydrophytes, sedges, grasses and finally the trees. The transformation takes place from a pool of water to swamp then marsh and them mesic conditions.
Xerarch succession (xerosere)
The desert areas show xerarch succession (xerosere). It begins with growth of lichens which produce acids to dissolve rocks and bring about weathering of rocks that results in soil formation. Then small plants like mosses can inhabit followed by herbs, shrubs and then trees. Ultimetaly a stable climax forest community evolves. Both hydrarch and xerarch successions lead to to mesic conditions (neither too dry nor too wet),