Why are lichens called pioneer species?

The answer

Lichens are composite organisms formed by a symbiotic partnership between fungi and photosynthetic algae or cyanobacteria. They are among the first organisms to colonize bare rock, volcanic lava, and other lifeless surfaces, earning them the title of pioneer species. Lichens break down rock through both physical penetration of their hyphae into microcracks and chemical weathering via the organic acids they secrete, slowly converting stone into mineral particles. Combined with the organic matter from their own dead tissue, this creates a thin proto-soil layer that mosses, ferns, and eventually larger plants can establish in. This primary succession process may take centuries to produce enough soil for complex plant communities, but it is the essential first step in creating nearly all terrestrial ecosystems.