Those splotches on rocks, tree trunks and twigs? Lichens! Lichens are actually a symbiosis between two major divisions of life, neither of which is a plant. And you can find them everywhere, as long as the air is not polluted.
If it weren’t for fungi, dead trees might not decompose and nutrients would be locked up, unavailable for other plants and animals to use. You can easily find (and make art with) wood-rotting bracket fungi even in winter.
Fungi use mushrooms to disperse their offspring, much like plants use fruits. The spores of fungi can be spread by wind, by animals, or by a combination. The oddest fungi are stinkhorns, which do it by smelling like rotting meat, attracting flies to transport their spores.