In ecological succession on bare rock (xerarch succession), which organisms act as the pioneer species?
Answer and explanation
Correct answer: D. Crustose lichens
- Step 1: Primary ecological succession on dry, barren rock surfaces with no preexisting soil is termed xerarch succession.
- Step 2: The first colonizers capable of adhering to bare rock and surviving extreme desiccation are crustose lichens (such as Rhizocarpon and Lecanora).
- Step 3: Lichens secrete organic acids (such as lichenic acids) that etch and weather the rock minerals, trapping dust particles and initiating the formation of the first microscopic layer of mineral soil, paving the way for foliose lichens and mosses.
Answer
Crustose lichens
Why not the others: 'Perennial hardwood trees' form the late climax community (forest), which requires deep fertile soil; 'Annual grasses' appear later in intermediate seral stages once sufficient topsoil has accumulated; 'Submerged hydrophytes' are pioneer/early seral species in hydrarch (aquatic) succession, not bare rock.
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