Ecological Succession in plants - Practice Questions & MCQ

• An important characteristic of all communities is that composition and structure constantly change in response to the changing environmental conditions. 
• 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 that is called a climax community. 
• The gradual and fairly predictable change in the species composition of a given area is called ecological succession. 
• During succession some species colonise an area and their populations become more numerous, whereas populations of other species decline and even disappear.
• The entire sequence of communities that successively change in a given area are called sere(s). 
• The individual transitional communities are termed seral stages or seral communities.
• In the successive seral stages there is a change in the diversity of species of organisms, increase in the number of species and organisms as well as an increase in the total biomass.
• Succession can be of two types depending on the areas that are colonised:
  • Primary succession: process that starts where no living organisms are there – these could be areas where no living organisms ever existed, say bare rock.
  • Secondary succession: in areas that somehow, lost all the living organisms that existed there.

Primary succession:

• Examples of areas where primary succession occurs are newly cooled lava, bare rock, newly created pond or reservoir. 
• The establishment of a new biotic community is generally slow. 
• Before a biotic community of diverse organisms can become established, there must be soil. 
• Depending mostly on the climate, it takes natural processes several hundred to several thousand years to produce fertile soil on bare rock.

Secondary succession:

• Secondary succession begins in areas where natural biotic communities have been destroyed such as in abandoned farm lands, burned or cut forests, lands that have been flooded. 
• Since some soil or sediment is present, succession is faster than primary succession.
   

• Based on the nature of the habitat – whether it is water (or very wet areas) or it is on very dry areas – succession of plants can be:
  • Hydrarch succession takes place in wetter areas and the successive series progress from hydric to the mesic conditions. 
  • Xerarch succession takes place in dry areas and the series progress from xeric to mesic conditions.
• Hence, both hydrarch and xerarch succession lead to medium water conditions (mesic) – neither too dry (xeric) nor too wet (hydric)

Hydrarch Succession:

• In primary succession in water, the pioneers are the small phytoplanktons, they are replaced with time by free-floating angiosperms, then by rooted hydrophytes, sedges, grasses and finally the trees. 
• The climax would be a forest. With time the water body is converted into land.
• Hence, it consists of following stages:
  • Submerged stage: a number of submerged aquatic plants, such as Hydrilla, Elodea, Potamogeton, Ceratophyllum, Najas, Vallisneria, Utricularia, Ranunculus and several algae
  • Floating stage: characterized by plants like Nymphaea, Polygonum, Limnanthemum and Castalia etc. These plants are rooted in the mud, and their broad leaves float on the surface of the water
  • Reed-Swamp stage: by amphibious plants like Scirpus, Typha, Phragmites etc. These plants remain only partly submerged in water.
  • Sedge meadow stage: characterized by plants like Carex, Juncus and Eleocharis.
  • Woodland stage: dominated by shrubs and small trees 
  • Climax forest stage: large trees

Xerarch Succession:

• In primary succession on rocks these are usually lichens which are able to secrete acids to dissolve rock, helping in weathering and soil formation.
• These later pave way to some very small plants like bryophytes, which are able to take hold in the small amount of soil. 
• They are, with time succeeded by bigger plants, and after several more stages, ultimately a stable climax forest community is formed. 
• The climax community remains stable as long as the environment remains unchanged.
• With time the xerophytic habitat gets converted into a mesophytic one.