The analogy given in this question describes ECOLOGICAL SUCCESSION, which is the series of changes that an ecosystem undergoes over a period of time. Ecological succession is of two types viz: Primary succession and Secondary succession.
Primary succession involves the series of changes that occurs on a new habitat that has never been previously colonized. This is the case of the colonization of bare rocks with no plant life by LICHENS. The action of the lichen, which involves secretion of acids that disintegrates the rocks into soil, paves way for the colonization of that area by new species.
Lichens, via the role they play in primary succession by helping alter a barren area into a fertile one that accommodates other organisms, makes them be regarded as PIONEERS of primary succession.
