Leaf litter plays a crucial role in a naturalistic or bioactive terrarium. These fallen leaves provide essential surface area for beneficial bacterial and fungal processes while serving as a food source for the cleanup crew. Small creatures like dart frogs, snakes, and micro geckos use the litter for camouflage or security. It's recommended to thoroughly mix leaves with the substrate in your terrarium, and as the leaf litter degrades, it's necessary to add more leaves to maintain a healthy environment.