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Hydrarch succession or Hydrosere is a plant succession that takes place in a natural habitat like water or in very wet areas. It starts from shallow water and then culminates in a forest. The successional series ranges from hydric to mesic conditions and therefore leads to medium water conditions which are neither too dry (xeric) nor too wet (hydric). This type of succession does not necessarily head to the development of a land community. Suppose the water body is large and very deep or there is a powerful wave action present where other powerful physical forces are active. In that case, the succession leads to the formation of a stable aquatic community in which any significant change is hardly evident. However, if the succession takes place in relatively small and shallow freshwater bodies such as ponds, lakes, and marshes, the wave action results in the formation of a terrestrial climax community. There are four stages of hydrarch succession as we will explore further.
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Key Terms: Hydrarch Succession, Hydrosere, Succession, Pioneer community, Climax community, Seral stages, Phytoplanktons
What is Hydrarch Succession?
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Hydrarch succession refers to the type of plant succession in wet areas that progresses from hydric conditions to mesic conditions. It eventually culminates into a forest. It is basically a type of plant succession that begins in shallow water. A series of changes that are observed in the hydrarch succession is referred to as hydrosere. This converts water bodies and the communities into land communities.
Pioneer Community
Pioneer Community usually refers to the very first species to colonize a bare area during the primary stage of ecological succession. In the case of Hydrarch succession, Phytoplanktons make up the pioneer community.
Climax Community
The changes occurring during succession eventually lead to the formation of a community that is almost in equilibrium with the environment and is called a Climax community. Forest or grassland is the climax community in a hydrosere.
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Hydrarch Succession Stages
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Any ecological succession is completed through a course of several stages known as the Seral Stages and therefore, hydrosere or hydrarch succession is also completed through a series of different seral stages. Primarily, Hydrosere originates in a shallow water body such as a fresh and virgin pond where the phytoplanktons (pioneer community) undergo a series of various stages under different conditions to give rise to the formation of a stable climax community (Forest or Grassland).
The stages of Hydrarch succession are as follows:
- Phytoplanktons Stage
In this initial stage of hydrarch succession, the pioneer community i.e. the phytoplanktons consisting of microscopic blue-green algae, bacterial life, and diatoms colonize the water body as the algal spores are brought into the water to germinate and multiply to grow in large numbers.
- Phytoplanktons are followed by the free-floating zooplanktons in hydrarch succession.
- All these organisms add a substantial quantity of organic matter and nutrients due to various life activities and ultimately settle at the bottom of the pond to form a layer of muck, post their death.
- Root Submerged Stage
As a result of the death and decomposition of the phytoplanktons in hydrarch succession, the nutrients get assimilated into the soil and water.
- A loose layer of mud is formed at the bottom of the pond due to the silt brought by the rain waters.
- The newly formed habitat is now shallower and richer in nutrients where the penetration of light becomes easy and promotes the growth of some rooted submerged hydrophytes in hydrarch succession such as Hydrilla, Potamogeton, Vallisneria, Ceratophyllum, Chara, etc.
- These plants occur in the form of tangled mass. When these plants die in hydrarch succession, their dead remains are deposited at the bottom of the pond along with other transported materials.
- This further brings up the substratum decreasing the water level and making the body of water even shallower.
- Rooted Floating Stage
As the depth of water reaches up to 4-10 feet, the submerged vegetation starts disappearing from its original spot in the hydrarch succession and eventually decomposes due to which the soil becomes richer in nutrients.
- This further elevates the level of soil at the bottom of the pond in the hydrarch succession.
- These conditions favor the growth of plant species including Nymphaea, Trapa, Monochoria, Nelumbium, etc. which have large leaves floating on the water surface.
- The broad leaves inhibit the penetration of light to the deeper layers of water which results in the total disappearance of the submerged vegetation.
- Many free-floating species such as Azolla, Lemma, Pistia, Salvia, etc. could also become associated with the rooted plants and disappear from the area sooner or later.
- Reed-Swamp Stage
This hydrarch succession stage is also known as the Amphibious Stage in the hydrarch succession since the rooted floating plant species are replaced by the plant communities which can survive in both aquatic and terrestrial conditions in this stage.
- The rooted floating stage decreases the water level making it reach up to 2-3 feet, changing the habitat significantly and less suitable for the floating plants.
- Under these conditions, the floating species is replaced by the new amphibious community consisting of plants like Typha, Phragmites, Sagittaria, etc. which have their shoots mostly exposed to the air but are rooted.
- These plants have well-developed rhizomes and form very dense vegetation over the area which restricts the light to enter the lower region.
- This condition restrains the other rooted floating, free-floating, and submerged species from the previous seral stages to disappear entirely.
- Marsh-Meadow Stage
Till now, the water level has subsequently decreased to the point where the soil becomes marshy which is unfitting for the pre-existing reed-swamp community in the hydrarch succession.
- Hence, the habitat begins to be colonized by the species of Cyperaceae and Poaceae.
- A complex interwoven connection of the rhizomes form mat-like vegetation over the top of the soil.
- As a result of an increased rate of transpiration, there is a rapid loss of water due to which the soil is exposed to air for the very first time.
- This exposure to air leads to the oxidation of nutrients like ammonia and sulfides to nitrates and sulfates respectively along with the breakdown of other complex organic components of the soil to simpler ones.
- These conditions collectively turn the marshy habitat into a mesic one with terrestrial soil as the marshy vegetation disappears gradually.
- Woodland Stage
As the marshy vegetation in the hydrarch succession disappears, the soil becomes drier and is now fitting for wet woodland.
- The area is now invaded by terrestrial plants consisting of some shrubs and trees.
- The shedding of leaves from these terrestrial plants results in a large accumulation of humus in the soil.
- The microorganisms like bacteria and fungi present in the topsoil break the organic matter down making it more fertile.
- Forest Stage
This stage marks the climax stage of the Hydrarch succession as forests are the climax community in the hydrarch succession.
- The woodland vegetation is predominated by a variety of large tree forms.
- The canopy of these trees covers the entire area. This leads to a decrease in the population of the shrubs and herbs since the penetration of light is highly reduced.
- The habitat now consists of a nearly stable and uniform plant community including diverse shrubs, herbs, climbers, and woody trees where trees are the dominant species.
- The climate of the region is determined by the type of forest.
Hydrarch Succession Stages
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Things to Remember
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- Hydrarch succession usually originates in a pond.
- The pioneer community is Phytoplanktons.
- The climax community is the forest or grassland.
- The different stages involved in the succession are called seral stages.
- It includes seven different seral stages of plant and animal communities.
- It is a very slow process where conditions change from hydric to mesic.
- The sequence of plants is Small lake plants, Reeds, Grass, Shrubs/Herbs, Trees.
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Sample Questions
Ques. What is Hydrarch succession? (2 marks)
Ans. Hydrarch succession is the succession that takes place in an aquatic environment. This form of plant succession usually initiates in shallow water bodies that eventually turn and develop into a forest. Through a series of several stages, the habitat changes its condition from hydric to mesic.
Ques. What is a climax community? How do sere differ from a seral community? [Delhi 2008 C] (3 marks)
Ans. A climax community can be defined as the community which gets established at the end stage of the ecological succession and are in equilibrium with the environment.
| Sere | Seral Community |
|---|---|
| Sere refers to complete order of communities which switch successively in a given area. | Seral community refers to the individual transitional communities that are established in between the establishment of pioneer and climax communities. |
Ques. In what way does a hydrarch succession progress from hydric to mesic condition? (3 marks)
Ans. Hydrarch succession occurs in wet areas or water, leading to a successional series that progresses from hydric to mesic conditions. In hydrarch succession, the pioneer community is phytoplanktons. These phytoplanktons with time are replaced by free-floating species, followed by rooted hydrophytes, sedges, shrubs, and finally the woody trees which mark the formation of the climax community i.e. forest, leading to a mesic condition.
Ques. Explain the various stages in the hydrosere. (5 marks)
Ans. The various stages in hydrosere are :
- Phytoplankton stage: This constitutes the pioneer community.
- Rooted submerged stage: The death and decomposition of phytoplankton and the wave action of pond water lead to the development of a layer of soft mud at the bottom of the pond.
- Rooted floating stage: This stage includes hydrophytes that are rooted in the soil with their broad leaves floating on the water surface.
- Reed swamp stage: This stage is also called the amphibious stage. The plant species have well-developed rhizomes and form very dense vegetation.
- Marsh-meadow stage: In this step, the mesic conditions approach the area. The marshy vegetation gradually disappears.
- Woodland stage: As the marshy vegetation disappears with time, the soil becomes dry for most of the remaining time in the year. Terrestrial plants including trees invade the area.
- Forest stage: Several woody trees rapidly invade the woodland community. This forms a stable and uniform climax community.
Ques. All the successions result in a stable climax community having mesic conditions. Explain this statement. (5 marks)
Ans. Any ecological succession involves a series of gradual changes in the species composition of a given area. There are types of plant successions based on the nature of the habitat.
- The succession taking place in dry regions i.e. xeric conditions is called Xerarch succession.
- The succession which takes place in very wet or Hydric conditions is called Hydrarch succession.
Each succession undergoes various stages which are called the seral stages.
In Xerarch Succession:
- The pioneer communities in the Xerarch Succession are Lichens.
- Lichens secrete a few acids which dissolve the rocks and help in the process of weathering and soil formation.
- Following this, the bryophytes invade the region and hold the soil.
- Bryophytes are then replaced by herbs, shrubs, and eventually big trees.
- As a result, a stable climax forest community is formed.
- Lastly, a xerarch condition is turned into a mesic one.
In Hydrarch succession:
- The pioneer community ie. phytoplanktons present in the pond are replaced with time by free-floating species followed by the rooted hydrophytes, marshes, shrubs, and finally the trees.
- At last, a stable and uniform climax forest community is formed.
- In this way, an aquatic habitat is converted into a mesic habitat.
Hence, all the communities eventually terminate to a stable climax forest community i.e. mesic conditions.
Ques. What are the differences between primary succession and secondary succession? (5 marks)
Ans. The difference between primary and secondary succession is:
| Primary succession | Secondary succession |
|---|---|
| It usually occurs in a barren or lifeless area. | It usually occurs in areas that were previously inhabited and are recently destroyed. |
| The rate of completion is very slow i.e. about 1000 years or more. | The rate of completion is relatively faster i.e. 50-200 years or more. |
| The soil is absent at the beginning of the succession and the environment is unsuitable for inhabiting any life forms. | Soil is present at the beginning of the succession along with a few organisms. |
| A various number of seral communities are involved in this type of succession. | The number of seral communities involved is very small. |
| E.g.- Formation of forest on barren land. | E.g.- Establishment of forest on burnt land. |
Ques. Taking an example in a Hydrarch succession, explain what Biotic succession is. (5 marks)
Ans. One of the main characteristics of a community of organisms existing in a given region is that their composition continuously undergoes changes that are sequential and parallel to the environment. This alteration in the physical environment gives rise to another set of organisms to form a stable community. Hence, this process of biologically controlled modification in a particular area is termed biotic succession or simply ecological succession.
There are various seral stages involved in a hydrosere:
- Phytoplankton stage
- Root submerged stage
- Rooted floating stage
- Reed-swamp stage
- Marsh meadow stage
- Woodland stage
- Forest stage
Through a sequence of these steps and many changes in habitat conditions, phytoplanktons such as blue-green algae are replaced by the submerged hydrophytes followed by plant species like rooted floating, amphibious species, sedge marshes, shrubs, and herbs, and eventually terminate to the forest which is the climax community. As the stages progress further, the water level keeps on decreasing which changes the physical conditions for the preexisting community that in turn gets replaced by another. In this way, ecological succession takes place in a pond to form a stable forest community.
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