The only published study of Michigan earthworms I could find was done in the western Upper Peninsula, in the Ottawa National Forest, which is dominated by sugar maples. That study compared earthworm communities in wilderness and non-wilderness sites within the ONF. It found that all the non-wilderness sites contained one to five species of exotic earthworms, while only half the wilderness sites had exotic earthworms, all of them a single species.
I got in touch with Cindy Hale in Duluth and asked whether the same problems she identified in Minnesota--drastic declines in native hardwood forests--were likely to occur in Michigan. She said: "The short answer is that the same issues that are going on in Minnesota are going on elsewhere."
Seeking an expert closer to home, I found Jasmine Crumsey, a U-M grad student in ecology and evolutionary biology. Crumsey has researched earthworms at the U-M Biological Station in Pellston--they're less abundant there than in Minnesota, she told me, because Michigan's sandier soil is less hospitable--but she was not aware of any studies of earthworms closer to Ann Arbor.