Worms, Pacific Northwest
It seems strange to think of earthworms as immigrants. After all, they seem to be everywhere, performing their lowly task of working the soil. It seems impossible that they could not always have been here and yet, in many parts of North America there were no worms until they were transported here from Europe and elsewhere by yet another invasive species, the humans.
At least since the end of the last ice age, for the most part, North America has been an earthworm free zone.
In North America, the ice age glaciers scraped away the topsoil along with most of the worms it contained. When the glaciers retreated about 10,000 years ago, the northern forests grew back without the help of worms.
Then came the Europeans, bringing with them plants, as well as the dirt those plants were growing in and the worms that dirt contained. Worms were also introduced intentionally, as a way to fertilize and improve the productivity of European style vegetable gardens, they were transported in the ballast of sailing ships too.
Now, anyone in North America who turns over a rich shovel full of garden dirt is likely to find that dirt literally crawling with at least one non native earthworm species. For instance, the dew worm that is so prized by fisherman are native to Europe, not North America, and so are the several species of red wigglers that are so common to our gardens and compost bins and in terms of sheer numbers, the migrants thoroughly overwhelm the natives in most places.
Despite their green card status however, most people view earthworms as beneficial, especially when they are tilling and fertilizing a garden or rendering compost into black gold.
But in ecosystems that developed without worms can actually cause harm. for instance the ecology of northern forests often depends on a thick layer of leaf litter remaining on the ground throughout the year. But earthworms remove that leaf litter by converting it to topsoil and that seemingly benign action has so completely change the chemistry and biology of the forest that some native plants and trees are no longer able to grow there.