Standing dead trees (often called snags) are common in our forests, and it’s hard to overstate just how vital a role they play in a healthy ecosystem. These gray ghosts provide food and shelter for a whole heap of forest critters; a total of 43 species of birds and mammals are specially adapted to nesting or denning inside tree cavities. But before a dead tree becomes a high-rise condo for a long list of species, it first undergoes a remarkable transformation. In fact, snags undergo a series of changes, from the time they begin to die until they finally collapse, and each stage of decay has particular value to a whole host of different animals with unique needs. First things first: decaying wood is perfect for fungi -- molds, mildews and mushrooms -- decomposers that soften wood enough for insects to start to gnaw their way in. Next, termites, beetles, and ants all begin to chew apart and break down the cellulose and lignin that gives wood its normally rigid structure. And once you