Plant Ecology and Environmental Biology Prof.(Dr.) Punam Jeswal Head M.Sc semester III Botany Department

HYDROSERE OR HYDRARCH The ecological succession can be broadly classified into two kinds on the basis of the nature of habitat :- 1) Hydrarch (Hydrosere) 2) Xerarch (Xerosere) Hydrosere A hydrosere is a plant succession which occur in a area of fresh water body such as pond, lakes and marshes. Hydrosere is the primary succession which develops in aquatic environment such as lakes and ponds. It results in conversion of water body and its community into a land community. In time, area of open freshwater will naturally dry out, ultimately becoming woodland. During this change, a range of different land types such as swamp and marsh will succeed each other. Such a succession does not necessarily lead the aquatic communities towards the development of land communities. If the water body is large and very deep or strong wave action and other powerful physical forces are at work, the succession results in a stable aquatic community in which any considerable further change is hardly recognizable. If the hydrarch succession starts from a comparatively small water body such as pond, lakes etc there is always a high probability for the formation of a terrestrial climax community. Important Characteristics of Hydrosere are :- 1) Hydrosere usually starts from a pond. 2) Phytoplankton's are the pioneer community. 3) A forest or a grassland will be the climax community. 4) It includes several seral stages of plant and animal communities. 5) Seral communities of plants are more obvious than animals. Stages of Hydrosere The various stages in a hydrosere are well studied in ponds, pools or lakes. Hydrosere, originating in a pond, starts with the colonisation by the pioneers like phytoplanktons and finally terminates into a forest, which is a climax community. Successive changes in hydrosere takes place both in plants as well as animals life. But the changes are more obvious in plants than animal, it looks as it is a succession of plant only. Hydrosere completed through a sequence of several seral stages (seral communities). The various stages with their chief components of plant species of a hydrosere are as follows:- Seral Stages of Hydrosere - (1) Phyto-plankton Stage. (2) Rooted Submerged Stage. (3) Rooted Floating Stage. (4) Reed-swamp Stage. (5) Sedge-Meadow Stage. (6) Woodland Stage. (7) Forest Stage. Phyto-plankton Stage :- In this initial, the pond water is poor in nutrient and devoid of much life. At this stage, phytoplanktons consisting of microscopic blue green algae, green algae, diatoms and bacteria etc are the first organism to colonise the primitive medium of the pond. They constitute the pioneer community. Phytoplankton are followed by zooplankton. All these organisms add large amount of organic matter and nutrients due to their various life activities and after their death, they settle at the bottom of pond to form a layer of muck. Root Submerged Stage :- As a result of death and decomposition of phytoplanktons, and their mixing with slits, brought from the surrounding land by rain waters, a soft mud at the bottom of pond develops. The new habitat which is now shallower and richer in nutrients, and where light is available upto a certain depth becomes suitable for the growth of rooted submerged hydrophytes like, Hydrilla, Potamogeton, Vallisneria, Utricularia and Ceratophyllum. These plants bring about further build up of the substratum as a result of their death and decay. The water level also decrease making the pond more shallower. Rooted Floating Stage :- Now the water depth is almost 2-5 feet. At this stage the pond is colonised by the plant species which are rooted in mud with their large leaves floating on the water surface. These are species of Nymphaea, Trapa, Monochoria etc. Some free floating species as Azolla, Lemma, Wolffia, Pistia, Salvia etc, also become associated with the rooted plants. The water level by now becomes very much decreased, making the pond more swallower. The decomposing organic matter formed due to death of these plants brings further build up of the substratum. Floating species sooner or later disappear from the area. Reed-Swamp Stage:-This stage is also called the "Amphibious Stage", as the plants of the community are rooted but most parts of their shoots remain exposed to air. Species of Typha, Sagittaria and Phragmites etc, are the chief plants of this stage. Plants in this stage have well- developed rhizomes and form dense vegetation over the area and this will prevent the light penetration to the lower portion. At this condition, the remaining rooted floating or free floating or submerged plants of the previous seral stage disappears. Sedge Marsh or Meadow Stage:- Due to successive decrease in water level and further changes in the substratum, species of some Cyperaceae and Gramineae, such as Carex, Juncus, Cyperus and Eleocharis colonize the area. They form a mat like vegetation with the help of their much branched rhizomatous system. 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. Once the soil is exposed to the air, the nutrients such as ammonia, sulphides etc, become oxidized to nitrates and sulphates. The condition in the area gradually changes from marshy to mesic and marshy vegetation disappears gradually and gradually. Woodland stage :- Disappearance of marshy vegetation, soil becomes drier for most of the time and become suitable for development of wet woodland. This area is now invaded by the terrestrial plants, which are some shrubs (Salix, Cornus) and tress (Populus, Alnus). By this time of succession, there is much accumulation of humus with rich flora of micro- organism like bacteria, fungi and other. Thus mineralization of the soil favours the arrival of new tree species in the area leading to the climax stages. Forest Stage (Climax Stage) :- Forest stage is the climax community in hydrarch succession. The woodland community is invaded by a variety of large tree forms which soon develops into climax community. The nature of the climax community (forest) is dependent upon the climate of the region. In tropical climates with heavy rainfall, tropical rain forest develop, whereas in temperate regions mixed forests of Ulmus, Acer and Quercus develop. In region of moderate rainfall, there develop tropical deciduous forest or monsoon forest.

Successive Changes In Animal Life During Hydrosere. There are also changes in the animal life , but these are not be as obvious as those in the plant communities. Thus with the ageing of a pond and the development of the marshes, the animal life also undergoes changes. These are as fellow : - The protozoans like Paramecium, Amoeba, Euglena etc are the pioneers. When the planktonic growth forms are very rich, other animal life form as blue gill fish, sun fish, largemouth bass etc., start appearing. Some caddis flies are also found. In the next, submerged stage the caddis flies are replaced by other animals that may creep over the submerged vegetation. Thus dragon flies, mayflies and some crustaceans inhibit the pond at this stage. At the floating stage, the animal life is chiefly represented by Hydra spp., gill breathing snails, frog, salamander diving beetles and other insects. Some turtles and snakes also appear. At the reed swap stages, the pond becomes shallower, and the bottom starts to be exposed. The floating animals are replaced by different species of mayflies and dragon flies, whose nymphs remain attached to submerged parts of the vegetation, and adults present on the surface of emergent parts. Gill breathing animals like snails are replaced by lung breathers as Lymnea, Physa etc . Among insects water scorpion, giant water bugs, scavengers beetles etc., are present at this stage. Finally, at the woodland stage, under terrestrial condition most of the terrestrial forms of animal life appear in the area.

Fig :- Scheme of Succession In A Pond

Fig :- Diagram Showing Different Plant Communities Appearing At Different Stages of A Hydrosere Origination In A Pond.

Fig :- Diagrammatic Representation of Stages of Hydrarch Succession.