Pedogenesis-Hydrosere And Xerosere

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.