As days get shorter, deciduous trees stop using their leaves to gather sunlight and enter survival mode. They shed their leaves in order to conserve energy and toughen themselves up. Having served their primary purpose, leaves simply die and begin to decay after shedding.
Fortunately (in most cases), life finds a way to make just about everything serve a purpose, even dying leaves. Unfortunately, the “other purpose” life finds for leaves often has to do with feeding and sheltering fall pests. Given half a chance, bugs, rodents, and other undesirables would love to leap right into that leaf pile right alongside you. Here’s why: