Lichen is not one organism. It’s at least two, in symbiotic mutualism: mycobionts, fungi that can flourish in all kinds of places, and photobionts, algae that can nourish by photosynthesizing chlorophyll. Sometimes there’s cyanobacteria, too. The fungi gets to eat sugar from sunlight, and the algal companions get to live on bark or rock or a fence like a boss, all and only because of each other. The feeling is: mutual!
Lichen are obviously, visibly, gloriously interdependent af. When these fungi and algae have each other, they make a gorgeous thing that’s nourished by sunlight and flourishes in otherwise inaccessible places.
And: they are sensitive. They don’t happen everywhere. They are indicator species because if the air is shitty, no dice. No lichen dice.