Series of biotic communities that develop in a newly
•
formed pond or lake is called hydrosere.
The various stages together with their chief components of
•
plant species during primary succession in water are : Phytoplankton stage :
• This stage constitute the pioneer community. Some blue-green algae, green algae, diatoms and bacteria etc. are the first organisms to colonize the primitive medium of the pond.
Rooted submerged stage :
• As a result of death and
decomposition of phytoplanktons, and their mixing with the silt, there develops a soft mud at the bottom of pond.
This new habitat which tends to be a bit shallower and
•
where light penetration may now occur easily becomes suitable for the growth of rooted submerged hydrophytes. E.g., Hydrilla, Ceratophyllum, Potamogeton etc.
Floating stage :
• The roots of the plants of this stage are anchored in the floor and their leaves float on the surface of water. E.g., Nymphia, Nelumbium.
Some free floating plants also arrive.
• E.g., Lemna, Pistia,
Azolla, Eichhornia, Salvinia etc. The amount of water decreases very quickly. Soil is quickly built - up. In a short time, water becomes very shallow.
Reed-swamp stage :
• This stage is also known as
amphibious stage as the plants of community are rooted but most parts of their shoots (assimilatory organs) remain exposed to air. E.g., Typha, Polygonum, Ipomea, Marsilia.
They possess well-developed rhizomes and form a very dense vegetation.
Marsh or sedge - meadow stage :
• Because of the
successive decrease in water level and further changes in the substratum, species of Carex, Juncus, Cyperus, Eleocharis, etc. colonize this area.
With the help of their much branched rhizomatous systems
•
they form a mat-like vegetation towards the centre of the pond. As a result of high rate of transpiration, there is much rapid loss of water, and sooner or later the mud is exposed to air due to which nutrients like ammonia, sulphides, etc. become oxidized to nitrates and sulphates.
These
• mesic conditions approach the area and marshy vegetation disappear gradually providing the way for the next stage.
Woodland stage :
• Due to disappearance of marshy sedge-meadow stage, soil becomes drier for most time of the year. This area is now invaded by terrestrial plants, which are some shrubs and trees.
By this time there is much accumulation of humus with
•
rich flora of microorganisms. Thus, mineralization of soil
favours the outcome of new trees in the area. Climax forest stage :
• Forest stage is the climax
community. In tropical climates with heavy rainfall, there grows mixed forest of Acer, Quercus, etc.
But in regions of moderate rainfall, there develops •
deciduous forests.
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MTBIOLOGY TODAY
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FEBRUARY ’13 31Secondary succession T
• his is the succession of communities in previously inhabited areas which have been naturally or artificially disturbed and where soil and some organisms are already present. Secondary succession is a biotic succession that occurs in
•
an area from which a community has been removed and where nutrients and conditions for existence are present, e.g., cut over forest, abandoned crop land, ploughed fields and lands that have been flooded.
Natural phenomena, such as
• avalanches (massive snow-
slides), landslides, volcanic eruptions, earthquake, floods, prolonged severe dought and forest fires set by lightning or human activity, destroy existing communities and pave the way for secondary succession.
An
• abandoned field, heavily overgrazed pasture, deforested tract also provide a site for secondary succession.
Secondary succession is much faster than the primary •
succession because the soil and certain organisms are already present.
Surviving seeds, underground stems, persisting and new
•
invading species rapidly grow on the return of favourable conditions.
This
• re-establishes the biotic community through necessary seral changes in due course of time.
In an abandoned field, grasses and annual weeds quickly
•
move in and cover the soil with a carpet of low vegetation. Some taller perennial plants move in. These new plants
•
shade the ground and their long roots monopolize the soil water.
The seedlings of the pioneer species now find difficult to
•
grow. The tall herbs are deprived of sunlight and water by the shrubs, which get the same treatment from the trees.
Finally, a forest of climax species is established to persist
•
till it is disturbed.
A destroyed grassland may take 50 – 100 years and a •
damaged forest over 200 years to recover fully.
Fugitive species :
• The species which occur in an area for a short period only during ecological succession are known as fugitive species.
Table : Differences between primary succession and secondary succession
Primary succession Secondary succession 1. It occurs in an area which
has been bare from the beginning.
Secondary succession occurs in an area which has been denuded recently. 2. Soil is absent in the
beginning of primary succession.
Soil is present in the area where secondary succession begins. 3. There is no humus in the
beginning. Humus is present very beginning. from the 4. Reproductive structures of
any previous community are absent.
Reproductive structures of the previous occupants are present in the area. 5. Pioneer community comes
from outside. Pioneer develops partly from community previous occupants and partly from migrants.
6. Seral communities are
many. Seral communities are a few.
7. Primary succession takes a long time for completion,
1000 years or more.
Secondary succession takes less time for completion,
50-200 years.