Ecosystem

Primary succession begins in areas where no living organisms ever existed, such as bare rock, newly cooled lava, or newly created ponds, where soil is absent. The first organisms to colonise such inhospitable surfaces are the pioneer species, and on bare rock these are typically lichens. Lichens secrete acids that dissolve the rock, initiating soil formation and paving the way for the establishment of small plants like bryophytes and later larger plants. Tall trees cannot be pioneers because they require deep, developed soil and represent a much later, climax stage of succession. Shrubs similarly need established soil and appear in intermediate seral stages. Grasses with deep roots also require some soil and follow the mosses, so they are not the first colonisers on bare rock. As described in NCERT Class 12, Chapter 14 (Ecosystem), the species that invade a bare area are called pioneer species, and succession proceeds through a series of seral stages to a climax community. This contrasts with secondary succession, which begins in areas that lost their existing vegetation but retained soil, and therefore proceeds much faster because the soil already contains nutrients and dormant seeds. Plausibility check: since soil must form before vascular plants can root, only soil-independent lichens fit the pioneer role, confirming the answer.