Name the organisms and their role in conversion of rock into soil.

Answer: Lichens (pioneer species) followed by mosses.

• Lichens: they colonise bare rock and secrete organic acids (e.g. lichenic acids) that dissolve and weather the rock; they also trap dust particles. Their dead remains add organic matter, forming a thin layer of soil.
• Mosses: they grow on this thin soil, hold more particles and moisture, and on decay add more humus, so soil depth increases and herbs and other plants can grow (xerarch succession).
• Bacteria and fungi (decomposers): they decompose the dead remains of these plants into humus, and their acids help further weathering; burrowing animals such as earthworms later mix and aerate the soil.