The researcher (Gundale) found sites in the forest with the larger earthworm (L. rubellus) that had depleted leaf litter, and also did a 'bucket' experiment showing that they reduced this layer. That's both lab experiment and field observation.
The main reason I posted the links was to see if there were other stories about this particular brand of invasive species. Invasive (non-native) species are a worldwide problem; these earthworms probably can't hold a candle to Asian carp, zebra mussels, kudzu, melaleuca trees, garlic mustard, nutria, and gypsy moths, to name a few off the top of my head. Here's a link to informatoin about more:
Animal and Plant Health Inspection Service's Role in Fighting Invasive Species
Since we're concerned about restricting immigration, why limit ourselves to humans?