A large tree hollow with some plants grown near. Hollows are an important habitat for many wildlife species, especially where the use of hollows is obligate, as this means no other resource would be a feasible substitute. A tree hollow is a cavity in a living tree. Tree holes can be caused when an injury to the tree, such as breakage of a limb, creates an opening through the bark and exposes the sapwood. The sapwood is attacked by fungi and bacteria, which form a cavity in the bole of the tree. Animals will select a hollow based on factors including entrance size and shape, depth, and degree of insulation. Such factors greatly affect the frequency and seasonality of hollow use.