Native earthworms did not survive the Pleistocene glaciations in Minnesota. The earthworms currently found in the state are mostly Eurasian, and result from a biological invasion that followed European settlement in the 1800s. Although many regions of the state are now infested, some areas remain earthworm-free. The extent of the earthworm invasion in west central Minnesota has remained undocumented. Recent studies have reported negative impacts of Eurasian earthworms on Minnesota forests, and research has linked invasive earthworms to the invasion of European buckthorn Rhamnus cathartica (European buckthorn). We surveyed for earthworms using the liquid extraction method at 144 sample sites spread across six forested study areas in west central Minnesota. Earthworms were measured and categorized to three functional groups: epigeic (litter dwelling), endogeic (shallow soil dwelling), and anecic (deep burrowing) species. We sampled vegetation and measured soil moisture and the depth of soil layers at each earthworm sample site. Worm abundance varied greatly among our sample sites. At the extreme, one of our study areas, Glacial Lake State Park, appears to be largely earthworm-free. Contrary to published studies suggesting a link between earthworms and invasive R. cathartica, our results show an inverse correlation between earthworm and R. cathartica abundance. Sites without worms have significantly more R. cathartica stems (p=0.03). As expected, earthworms are associated with decreased organic layer soil depths at sample sites within our study areas. Earthworms are also associated with higher soil moisture.
