The natural leaf cover of the forests of the northeast and great lakes regions creates the perfect warmth for them to spread. By them eating and tearing up leaf cover they change the understory's diversity of plants by removing heat. After the last ice age and the lack of worms some plants and insects evolved to fill the need and won't live without leaf over winter cover, so worms are decreasing beneficial plant numbers and increasing fast growing nutrient hungry plants looking for bare soil, then the rain washes the dirt away and all the trees fall down