Wednesday, August 5, 2020

Ecosystem - Notes | Class 12 | Part 5: Ecological Succession

14. ECOSYSTEM

ECOLOGICAL SUCCESSION

It is a gradual, slow and predictable change in the species composition of an area leading to a climax community (community that is in equilibrium with the environment).

In this, some species colonize an area and increase in number, whereas other species decline and disappear.

The entire sequences of communities that successively change in an area are called sere. Individual transitional communities are termed seral stages (seral communities).

The species invading a bare area are called pioneer species.

During succession, there is a change in species diversity, increase in number of species and organisms and an increase in total biomass.

Present-day communities are due to succession of millions of years. Succession and evolution were parallel processes.

Succession is 2 types:
  • Primary: The succession taking place in areas where no living organisms ever existed. E.g. newly cooled lava, bare rock, newly created pond or reservoir.
To establish a biotic community, fertile soil must be formed. So primary succession is a very slow process.
  • Secondary: The succession taking place in an area after the existed organisms are lost. E.g. abandoned farm lands, burned or cut forests, lands that are flooded.
Since some soil or sediment is present, succession is faster than primary succession.
The species that invade depend on the nature of the soil, availability of water etc.

In succession, changes in vegetation affect food & shelter of animals. Thus, succession leads to change in number and types of animals & decomposers.

Natural or human induced disturbances (deforestation, fire etc.) convert a particular seral stage to an earlier stage. They create new conditions that encourage some species and discourage or eliminate other species.

Succession of Plants 

Based on the nature of the habitat, succession of plants is 2 types: hydrarch and xerarch.
  • Hydrarch succession: It takes place in wetter areas. It progresses from hydric to mesic conditions.
  • Xerarch succession: It takes place in dry areas. It progresses from xeric to mesic conditions.
Hence, both hydrarch & xerarch successions lead to medium water conditions (mesic, the climax community).

Primary succession on rocks (xerophytic habitat):

Lichens (pioneer species. They secrete acids to dissolve rock, helping in weathering & soil formation) → small plants like bryophytes (they need only small amount of soil) → bigger plantsforest (mesophytic).

The climax community (forest) remains stable if the environment remains unchanged.

Primary succession in water:

Phytoplankton (pioneers) → rooted-submerged plants → rooted-floating angiosperms → free-floating plants → reed-swamp → marsh-meadow → scrub → trees (climax community is a forest).

With time, the water body is converted into land. 
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