UPSC Prelims 2014 Question:

Lichens, which are capable of initiating ecological succession even on a bare rock, are actually a symbiotic association of

Explanation:

Ecological succession is the process by which the structure of a biological community evolves over time. Two different types of succession—primary and secondary—have been distinguished. 

• Primary succession occurs in essentially lifeless areas—regions in which the soil is incapable of sustaining life as a result of such factors as lava flows, newly formed sand dunes, or rocks left from a retreating glacier. Lichens usually initiate primary ecological succession in a region.
  
  
• Secondary succession occurs in areas where a community that previously existed has been removed; it is typified by smaller-scale disturbances that do not eliminate all life and nutrients from the environment.
  
  
• Lichen consists of a symbiotic association of algae (usually green) or cyanobacteria and fungi (mostly ascomycetes and basidiomycetes). Lichens are found worldwide and occur in a variety of environmental conditions. A diverse group of organisms, they can colonize a wide range of surfaces and are frequently found on tree bark, exposed rock, and as a part of biological soil crust.
  
  

Therefore, option (2) is the correct answer.

Subject: Science and Technology | Biology