"If earthworms are an exotic species to the Great Lakes region, how do you explain predators whose diets rely heavily on earthworms, like the northern redbelly snake, star-nosed mole, woodcock, etc?"
This is a good question that I get frequently. Most of these species have simply made a prey shift based on the availability of an abundant and high quality food source. The woodcock is a great example.
Gut analysis studies done in the 1950’s (reference The Audubon North American Bird profiles) show that these birds were generalists on soil invertebrates with preference to aquatic and riparian invertebrates, of which there are native Oligochaets (aquatic earthworms). So woodcocks were already very well adapted to take advantage of a new upland food source (terrestrial earthworms) and have expanded into uplands from their traditional habitat of wetland shrub communities. Now we think of them as an upland species, but this appears to be a relatively modern phenomenon.