More about the Invasion
We have no evidence that earthworms ever inhabited the Great Lakes region before European settlement. Even if they did, the glaciers killed any native North American earthworms in our region. For the last 11,000 years since the glaciers receded, Great Lakes forest ecosystems developed without earthworms.
There are over 100 species of native North American earthworms in unglaciated areas of North America such as the southeastern U.S. and the Pacific Northwest. However, native species have either been too slow to move northwards on their own or they are not able to survive in more harsh northern climates.
For example, earthworm populations spread at a rate of about 5 meters (5.5 yards) a year. So, in the 11,000 years since glacial retreat…at full speed…they could probably have spread 55 kilometers (34.2 miles). The maximum extent of glaciers south into Iowa was more than 64 Km (40 miles) so even if we knew that native, North American earthworms were moving northward, they would not have had enough time to get here. But remember, that permafrost extended even further south beyond the ice sheets, so any expansion back into glaciated regions of North America would have started from even further south and the climate would have had to warm substantially before any northward expansion could have begun. The North American earthworm species really haven’t had much of a chance.
For more information on the science behind the ecology and biogeography of earthworms in North America, see these recommended resources.