The Laurentide ice sheet removed soil fauna across a wide band of the continent. Forests that regrew after the ice did so without deep-burrowing earthworms, and they developed a thick, slowly decomposing organic layer known as the duff. Tree seedlings, ferns and woodland wildflowers root in that layer.
European lumbricids arrived in ships' ballast, in soil around imported plants, and later as discarded fishing bait. Where Lumbricus terrestris establishes, it pulls litter into its burrow and accelerates decomposition dramatically. The duff can disappear within a few years, exposing mineral soil, reducing seedling survival and altering nutrient cycling and the ground-dwelling invertebrate community.
The common framing that earthworms are simply good for soil is where this goes wrong. In tilled agricultural ground their burrowing and casting genuinely improve structure and drainage. In a previously worm-free northern forest, the same behaviour is a disturbance. Which native species survived in unglaciated refugia, and how far they are recolonising, is still being mapped.