Succession is a unidirectional series of changes that brings about a change in community composition. As the succession progresses, it leads to the formation of a stable climax community. The transient stages in the progressive successional series are called the seral stages. Hydrarch succession is a primary type of succession which starts in aquatic environments like ponds and lakes.
Succession in the pond or lake starts with the settlement of a pioneer community like the phytoplankton, and that leads to the formation of the forest. Major examples of hydrarch succession can be observed in the Big Islands of Hawaii as this area is prone to volcanic eruption. As the lava flows in nearby regions, new rocks are formed, modifying the soil profile in the region. This leads to new flora and fauna development.
Primary Succession: The succession occurs in barren or new areas where no species existed before. Example- new volcanic island.
Secondary Succession: It occurs in the region where the developed community has been eradicated by natural forces like flood, fire, or flooding. Example- forest destroyed by fire.
Autogenic Succession: It is a self-powered succession that occurs due to the interaction of organisms with the environment. Example- primary succession.
Allogenic Succession: The succession resulting from the variation in habitat due to external agencies is allogenic succession.
Autotrophic Succession: Succession in which plants are in greater quantity than animals, occuring in an inorganic-rich environment is known as autotrophic succession.
Heterotrophic Succession: In the initial stages, animals and fungi outpower the plant. This occurs in an organic-rich environment.
Progressive Succession: In this kind of succession, simple forms get transformed into complex forms by a series of sequential changes. Example: grassland converted to forest.
Hydrarch succession occurs gradually and includes stages of community formation. The sequence of events in hydrarch succession is explained below:
Phytoplankton Stage: This is the primary stage of hydrarch succession. In this stage, the quality of pond water is inferior in nutrients and does not support larger life forms. Simple organisms like bacteria, algae, phytoplankton, and zooplanktons form the first seral community of the hydrarch succession. They are called pioneer communities.
These organisms proliferate and perform several metabolic activities. In this process populations of simple life forms die, and decomposers begin to multiply bringing about decomposition, and enriching the pond with nutrients. The dead organism settles at the base of the pond to form a layer called a muck.
Submerged Stage: The loose muddy layer at the base of the pond is enriched with nutrients. This layer supports the growth of rooted hydrophytes like Hydrilla, Utricularia, Vallisneria, and Ceratophyllum. These plants emerge at the depth of about 10 feet or more. When these plants die they get buried at the base of the pond, which gradually raises the surface of the pond. This stratification process results in the growth of other plants.
Floating Stage: As the name suggests, the floating plants are the new inhabitants. The depth of the water now reaches about 4 to 8 feet. The broad leaves of floating plants obstruct the light rays to penetrate the water. This leads to the death of submerged plants that colonised the pond earlier. The death of submerged plants along with the evaporation of water and silt deposition from the nearby region makes the pons shallower. Examples of plants found in this stage are Trapa, Pistia, Nymphaea, Limnanthemum, etc.
Reed-Swamp Stage: As the depth of pond water gradually decreases the floating plants get replaced by amphibious plants. The prominent examples of such plants are Typha, Sagittaria, and Phragmites. The leafage of such plants cuts off the sunlight reaching the floating or submerged plants. This leads to death resulting in the deposition, thereby reducing the depth of water. The existing plant species have well-developed root systems and form densely wooded areas.
Sedge Meadow Stage: The stratification process results in the formation of marshy habitat. Plants species like Cyperaceae and Gramineae take over the vegetation. They form dense mat-like vegetation with the help of their creepy rootstock. As the depth gradually decreases, the muddy layer gets well exposed to the air resulting in rampant loss of water. Thus marshy condition gets replaced by mesic vegetation. At the end of this stage, soil loses water and becomes dry. The soil now gets dependent on rainfall and other climatic factors.
Woodland Stage: In this stage, shrubs and, later on, woody trees appear on land. The pioneers of this stage are Salix, Cornus, Alnus, etc. In this stage, the soil gets flourished with humus with abundant microflora. Thus soil gets mineralised, favouring the entrance of new species of plants. This stage leads to the final climax stage.
Climax Stage: Finally, the hydrosere leads to the evolution of climax vegetation. The level of the soil is raised, and the soil gets enriched with humus and other minerals. This habitat favours the growth of a well-adapted, reproductive stable plant community. All types of plants ranging from herbs to tall woody trees, are seen in the climax region. The dominant woody trees control the entire foliage. Thus, a stable, balanced climate-dependent climax community evolves.
The Stages in Hydrosere
The type of succession that begins in drier areas is called xerosere. The two seral stages are:
Lithosere: Succession occurs on barren rocks
Psammosere: Succession occurs in the sandy region
In southern Alaska, Glacier bay is one of the prominent examples of Primary Succession.
Due to global warming glaciers melted, and soon pioneer communities (Epilobium latifolium) and the mountain avens (Dryas octopetala) occupied the region.
These plants fixed nitrogen and modified the soil. Gradually taller shrubs, Sitka spruce (Picea sitchensis), western hemlock (Tsuga heterophylla), and mountain hemlock (T. mertensiana) became the dominant species of the region.
What are the pioneer species of hydrarch and xerarch succession?
Ans: Pioneers species of hydrarch succession are phytoplanktons. They grow independently in sunlight and are referred to as microalgae. In Xerarch Succession the pioneer community is lichen. Lichen is a symbiotic association of algae and fungi.
What are examples of secondary succession?
Ans: Secondary succession is a type where already living forms existed but natural calamities have destroyed the region. The classic example of secondary succession is oak and hickory forests, where wildfires are common. The wildfires destroy the flora and fauna. The nutrient is replenished in the form of ash which is used later for secondary succession.
Max Witynski defines ecological succession as a gradual process that replaces species and habitats.
Hydrarch succession starts in moist places with phytoplankton as pioneer communities.
Xerarch succession starts in drier areas like sandy places or weathered abandoned rocks. The pioneer of this succession is lichens which gradually following various seral stages get transformed into woody foliage.
1. How does ecological succession occur in urban ecosystems?
Primary succession occurs in barren places, while in urban ecosystems where there are human interventions, ecological succession can occur on sites that are abandoned, regions of industrial waste, stone and brick walls, and on side of roads which are not in use.
2. What is an ecological succession in a hydric environment?
Ecological succession that occurs in moist areas like ponds and lakes is called hydrarch succession. As the succession progresses, hydric vegetation is taken over by mesic vegetation.
3. Why is succession unidirectional?
Succession is unidirectional as simpler life forms are the basis of the foundation for complex life forms. The simpler organism can start to grow on lands that are barren and do not have enough nutrients. As they grow and die they lead to the enrichment of regions that now can support complex life forms.