UPSC Prelims 2021 Q20: Life Without Soil — Pioneer Species Explained

In the nature, which of the following is/are most likely to be found surviving on a surface without soil?

1. Fern

2. Lichen

3. Moss

4. Mushroom

Select the correct answer using the code given below.

  1. 1 and 4 only
  2. 2 only
  3. 2 and 3
  4. 1, 3 and 4

Answer: (c) 2 and 3

Statement-wise explanation

  • 1: incorrect — A fern is a vascular plant with true roots, stems and leaves. Most ferns grow in moist, shaded soil or leaf litter. A few ferns grow on tree bark or in rock crevices, but those spots already hold trapped humus and moisture. Ferns are not typical colonisers of a bare surface, so a fern is not 'most likely' to be found there.
  • 2: correct — A lichen is a partnership between a fungus and an alga or cyanobacterium. The photosynthetic partner makes food, and the fungus holds water and fixes the lichen to the surface, so it needs no soil. Lichens grow on bare rock, tree bark, walls and roofs. They are the classic pioneer species of primary succession on rock, and the acids they release help break rock down into the first soil.
  • 3: correct — Mosses are bryophytes. They have no true roots, only rhizoids that anchor them, and they absorb water and minerals through their whole surface. They commonly grow on rocks, walls and bark. In succession on bare rock, mosses usually follow lichens and grow in the thin layer of material the lichens leave behind.
  • 4: incorrect — A mushroom is the fruiting body of a fungus. Fungi cannot photosynthesise, so they must feed on organic matter such as humus-rich soil, decaying wood, leaf litter or dung. A bare surface with no organic matter cannot feed a mushroom.

How to eliminate

Ask one question of each organism: can it make its own food and hold water without roots in soil? A mushroom is a fungus that cannot photosynthesise and needs decaying organic matter, so statement 4 goes. That removes the options that include 4, leaving only '2 only' and '2 and 3'. Now you need one fact: mosses, like lichens, are rootless, absorb water over their whole surface and are the second stage of plant life on bare rock after lichens. Anyone who has seen moss on old walls or rocks can pick '2 and 3'. The words 'most likely' in the question matter. A few ferns can grow on rocks, but the question asks for the typical colonisers of a soil-less surface. Fact to remember: in succession on bare rock, lichens come first, then mosses.

Know more

Primary succession on bare rock (xerarch succession)

Primary succession starts where no soil exists yet, such as bare rock, a fresh lava flow or rock left behind by a retreating glacier. The first organisms to arrive are pioneer species. They must survive drying out, extreme temperatures and a lack of nutrients.

Lichens come first. A lichen is a symbiosis: a fungus (the mycobiont) gives structure and holds water, and an alga or cyanobacterium (the photobiont) photosynthesises. The acids lichens release, together with freezing and thawing, slowly break the rock surface down. Dead lichen adds organic matter, and this begins soil formation.

Mosses come next. They root in this thin layer, trap dust and water, and build up more humus. Then come herbs, shrubs and finally a climax forest.

Stage Typical organisms Needs soil?
Pioneer Crustose, then foliose lichens No
2 Mosses (bryophytes) No, rhizoids only
3 Annual herbs, grasses Thin soil
4 Shrubs Deeper soil
Climax Trees Mature soil

Why fungi alone cannot be pioneers: fungi are heterotrophs. Only a fungus paired with a photosynthetic partner, which is what a lichen is, can live on bare rock.

Other exam angles on lichens

  • Air-quality indicators: many lichens die when sulphur dioxide levels rise, so their absence near cities signals pollution.
  • Tundra: in the Koeppen ET (tundra) climate, vegetation is mostly mosses, lichens and low flowering plants.
  • India: the Western Ghats are a lichen hotspot. New species of the crustose lichen genus Allographa were described there in 2025 and 2026.

NCERT Concepts to Revise

  • Soil formation: The question turns on which organisms can live before soil exists and help start pedogenesis on bare rock.

Read more