One thing to realize about lichens is that they are not a single organism.
Lichens are a mutual symbiosis between algae and fungi, meaning the two work togehter.
The algae is able to survive making its own food by photosynthesis. It is able to survive on its own, but to spread, it needs special conditions that fungi are able to provide by anchoring it to the ground and providing nutrients.
The fungi can decompose dead organisms around it, but needs sugar from plants--such as algae--to survive.
Not much is needed for lichen to thrive, just sunlight, soil, and maybe some decaying organsims. This is why lichens are the pioneers of an ecological sucession.
