A paper published in the December issue of the journal Conservation Biology concludes that European earthworms (including Eisenia hortensis) are threatening the rare goblin fern (Botrychium mormo) in forests in Minnesota. North American earthworms are found mostly in more southerly climates, so if you live in the northern parts of the continent, it's likely that all the earthworms you see are introduced. The addition of the earthworms to the forest ecosystem causes a decrease in the thickness of the forest floor. The study, done by Michael Gundale at Michigan Technological University, showed that goblin ferns were missing from parts of the forest where the litter layer was thinner. You can read about a similar study here.
Thanks to ScienceDaily for posting information about this study.