Many species of birds and small mammals rely on the cavities that form naturally in dying and dead trees as places to raise young and shelter from harsh weather. The "nest boxes" and "roost boxes" we humans put up are an attempt to provide birds with the equivalent of cavities in dead trees, but most animals prefer the natural cavities if they can find them. Unfortunately, because humans often cut down dead and dying trees, these animals have a tough time finding natural homes.
If a big old tree falls in an area inhabited area, the result can be damage to life or property. (So it's important to get a consultation from a trusted arborist if you think a tree might present a hazard.) But these events are natural and beneficial in the forest. The roots of a fallen tree, along with soil and other vegetation that was attached to the roots, are known as the "rootwad" or "root plane." This big mass of soil and decaying plant matter is great habitat for many animals and a good location for new plants to germinate and grow. Photo by Tim Skelly.