Have you heard about the saguaro cactus?
When a saguaro cactus dies, its flesh decays, leaving behind the woody skeleton that supported the plant. What also remains are boot-like structures, called saguaro boots. It's the combined creation of the saguaro and a Gila woodpecker.
A bird's got to build a nest somewhere, and trees aren't only scarce in the desert, but stunted. Saguaros, on the other hand, can grow as high as 50 feet. The bird begins by burrowing its way through the leathery skin of the cactus. Once inside the plant, it digs downward, hollowing out a space for its nest.
As a result, the saguaros lose water through evaporation. Not only would such a wound cause the saguaro to continuously lose water, but it would leave the plant susceptible to infection. That's why the saguaro responds to the Gila woodpecker's intrusion by secreting a resinous sap all around the hollow. The sap hardens into a kind of permanent scab, which remains in tact long after the saguaro's death. The woodpecker actually abandons its new nest and returns once the sap has hardened.
What's more, Gila woodpeckers typically build new nests each year, which means the old nests are then taken up by other feathered residents, including desert owls. After a saguaro dies, its boots may provide shelter to still more creatures, such as snakes, scorpions, and spiders.