Question

In hydrarch succession, reed swamp stage is preceded by
A. Forest community
B. Scrub stage
C. Sedge meadow stage
D. Floating stage

Answer 1

The process through which the structure of the biological community evolves with the processing of time is called ecological succession. The time limit can be of some decades to millions of years. The ecological succession is considered to be the advanced theory used in ecology.

Complete Answer:
Hydrarch succession usually occurs in the wetter conditions. It is the plant succession which usually starts on relatively shallow water bodies such ponds and lakes and culminates into mature forest. In comparison, the search succession takes place on bare ground or huge rocks and culminates into a climax mature forest. The lichens and mosses usually called pioneer species cause the weathering of rocks and bare ground which transform it by trapping the dust molecules and forming a thin layer of soil for growth of small plants.

Stages in hydrarch succession:
The succession in aquatic habitats like freshly formed ponds is hydrosere. The successive series progress from hydric to mesic conditions.
1. Pioneer Community: It is formed by the phytoplankton i.e., the minute microscopic autotrophic organisms like diatoms, unicellular, colonial or filamentous green algae and blue-green algae. The spores of these organisms reach the newly formed pond through wind or animals. They multiply rapidly due to which it becomes a suitable habitat for zooplanktons which feed upon them. The organic matter formed by death and decay of planktons, mixes with clay and silt at the bottom of the pond to form soft mud. The habitat becomes suitable for the growth of the next stage.

2. Transitional Communities :
(I) Submerged plant stage: With their roots they are anchored in the mud at the bottom of the water body. e.g., Myriophyllum, Hydrilla, Vallisneria, Potamogeton etc.
(II) Submerged free-floating plant stage: Due to the accumulation of dead and decaying remains of submerged plant bottom level is raised as well as ponds become rich in minerals (nutrients) which become suitable for free-floating plants. e.g., Azolla, Wolffia, Pistia etc.
(III) Reed-swamp stage: More shallowing of plants occur due to continued siltation which paves way for the growth of rooted emergent plants i.e., reeds (amphibious plants). e.g. Typha, Sagittaria, Phragmites etc.
(IV) Marsh-meadow stage: Reed-swamp stage is invaded by marshy plants. With increased settling of silt and deposition of dead organic matter derived from floating and rooted species, the pond becomes shallower until it gets transformed into terrestrial habitat. e.g. Carex, Juncas, Cyperus.
(V) Scrub stage: Marsh-meadow stage is replaced by shrubs. e.g. Salix, Populus, Alnus.

3. Climax community: The scrub stage is replaced by trees which grow to greater heights. The nature of the climax community depends on the climate of that area. e.g. Forest.

Hence the correct answer is option D.

Note: Biotic community is seldom static, its composition and structure changes with time due to interactions between biotic and abiotic components, this change is orderly and sequential, parallel with the changes in the physical environment. These changes lead finally to a community that is in near equilibrium with the environment and is called climax community. During succession, some species colonise an area and their populations become more numerous, whereas populations of other species decline and even disappear.