A weed is an unwanted plant. A pest is an unwanted animal.
Garbage is anything else perceived as unwanted.
In the natural world, nothing is unwanted or unused. Everything, from the smallest pile of dung to the most insignificant dead insect, is a useful part of the system. Dead or alive, each plays its part in the balance of the whole. The insect eats the leaf and is eaten by the bird. Beetles burrow and eat the dung and in turn are eaten.