Hydrosere or Hydrarch
·
It
is succession in water like newly dug pond.
·
The
succession from open water to climax woodland takes centuries or millennia.
·
Hydrosere
is the primary succession sequence which develops in aquatic environments such
as lakes and ponds. It results in conversion of water body and its community
into a land community.
·
A
pond or lake is a suitable model to study hydrosere. Successive stages of
development in a water body can be identified, which are as follows-
1. Phytoplankton stage-
Microscopic plants such
as green algae, blue- green algae and diatoms are the pioneer species of a bare
water body such as pond. Their spores are carried by air to the pond. The phytoplankton
are followed by zooplankton. They settle down to the bottom of the pond after
death and decay into humus that mixes with silt and clay particles brought into
the basin by run off water and wave action and form soil. As soil builds up,
the pond becomes shallower and further environmental changes follow.
2. Submerged stage-
When phytoplankton
die, they are mixed with the silt brought from the surrounding land by rain
waters and by wave action of pond water. It results into the development of a
soft mud at the bottom of the pond. Since light can reach the bottom of the
pond, rooted submerged hydrophytes like Myriophyllum, Vallisneria, Elodea, Hydrilla,
Ultricularia and Ceratophyllum start growing. Their growth decreases water
level making the pond shallower. These changes in the habitat replace these
plants with those having floating levels.
3. Floating stage-
The floating plants
are rooted in the mud but some or all their leaves float on the surface of the
water. These include species like Azolla, Lomna, Wolffia, Selvinia, Pistia etc.
The large and broad leaves of floating plants shade the water surface and
conditions become unsuitable for growth of submerged species which start
disappearing. The plants decay to form organic mud which makes the pond more
shallow yet (1-3 ft.).
4. Reed swamp stage-
The pond is now
invaded by emergent plants sch as Phragmites (reed grasses), Typha (cattail)
and Zizania (wild rice) to form a reed swamp. These plants have creeping
rhizomes which knit the mud together to produce large quantities of leaf
litter. This litter is resistant to decay and reed peat builds up, accelerating
the autogenic change. The surface of the pond is converted into water- saturated
marshy land.
5. Sedge- meadow stage-
Successive
decrease in water level and further changes in the substratum help members of
Cyperaceae and Graminae in the colonization of species like Carex and Juncus. These
species form a mat like vegetation at the centre of the pond. Due to high rate
of transpiration, there is much rapid loss of water. As soon as the mud is
exposed to air, nutrients like ammonia, sulphides etc. are oxidized to nitrates
and sulphates. Marshy vegetation starts disappearing gradually.
6. Woodland stage-
With the loss of
marshy vegetation, soil tends to be dry. The area is now invaded by shrubs like
Salix, and trees like Populus and Almus. Soil accumulates humus with rich flora
of micro-organisms. Mineralization of soil favors growth of other species.
7. Forest stage (Climax stage)-
Finally,
a self- perpetuating climax community develops. It may be a forest if the climate
is humid, grassland in case of sub-humid environment, or a desert in arid and
semi-arid conditions. A forest is characterized by presence of all types of
vegetation including herbs, shrubs, mosses, shade-loving plants and trees. Decomposers
are frequent in climax vegetation.
Appearance of animal community in Hydrosere
- Animal life in water also keeps on changing with the age of the pond. A rich planktonic growth attracts a variety of fish and insects. In the next submerged stage, the insects like caddisflies are replaced by other animals that can creep over the submerged vegetation. Thus dragon flies, may flies and some crustaceans such as Daphnia, Cyclops, Gammarus, Cypris etc. inhabit the pond.
- At the floating stage, the animal life of a pond comprises Hydra, Snails, frogs, salamenders, turtles and snakes.
- At the reed-swamp stage, the pond becomes shallower and the bottom surface is exposed gradually. This brings changes in the community structure. Gill breathing snails are replaced by lung breathers such as Lymnea, Physa, Gyraulus. Insects are replaced by water scorpion, giant water bug and scavenger beetles. The bottom of the pond is inhabited by annelids. Musk rats and beavers also occupy the area.
- At the sedge-meadow stage, the snails become common. This stage leads to final woodland stage that supports various species of terrestrial animals.
- During succession of a pond, major structural and functional changes take place. Change in the species composition and diversity leads to a change in biomass and community metabolism. These changes attribute to ecological subility of a pond.
Lithosere and Xerosere or Xerarch
In the
succession from bare rock, there must first be the long process of rebuilding
the soil. Various stages are described below-
1. Crustose lichen stage-
Substratum is
very poor in moisture and organic matter when subjected to extreme
temperatures. However, lichens like Rhizocarpon, Rinodina and Lecanora manage
to survive. They cause weathering of rocks.
2. Foliose lichen stage-
They appear on
substratum built by the crustose lichens. Major species of this community are Parmelia,
Dermatocarpon that possess large leaf like thalli. They can absorb and retain
more water. They can accumulate dust particles that further help in making the
substratum. The process allows the formation of humus. Weathering of rocks and
mixing with humus results in the formation of soil.
3. Moss stage-
Development of the soil
favors the growth of xerophytic mosses. They compete and outnumber the lichens.
Due to the death and decay of lichens, soil becomes rich in organic matter.
 |
| Lichens and mosses |
4. Herbs stage-
The formation of soil
and change in the habitat favors the growth of herbs which are chiefly the
annuals. They are followed by biennials and perennials. They die and decompose
making much more soil to accumulate and further favor rock weathering. The habitat
by now is occupied by rooted grasses that are gradually replaced by shrubs.
 |
| Herbaceous Plants |
5. Shrub stage-
shrubs soon outnumber
the herbs. The soil is enriched by their growth. Later, the shrubs are replaced
by trees that make a climax community.
 |
| Shrubs |
6. Forest stage (climax stage)-
Some
xerophytic plants invade the area. Further weathering of rocks and formation of
humus in the soil favor the growth of trees. Xerophytic population changes into
mesophytic vegetation, thus forming a forest community.
 |
| Trees |
Appearance of animal community in Xerosere
Mites form
the pioneer community of xerosere. A few spiders and ants can be seen in the
cracks and crevices of rocks. With the development of grasses, qualitative and
quantitative changes in animal populations occur. Nematodes, insects and mites
appear in the new environment. Several species of invertebrates and vertebrates
occupy the habitat and thus a climax community becomes rich in flora and fauna.
